5.3 Class Magnoliopsida – flowering plants - Cambridge University ...
5.3 Class Magnoliopsida – flowering plants - Cambridge University ...
5.3 Class Magnoliopsida – flowering plants - Cambridge University ...
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<strong>5.3</strong> <strong>Class</strong> <strong>Magnoliopsida</strong> <strong>–</strong> <strong>flowering</strong> <strong>plants</strong><br />
There are more than 250 000 species of <strong>flowering</strong> plant. Alternative<br />
names for them include the angiosperms, Angiospermopsida.<br />
<strong>5.3</strong>.1 Distinguishing features<br />
DNA sequence data indicate that the <strong>flowering</strong> <strong>plants</strong> are a monophyletic<br />
group with a separate origin from all gymnosperms. Characteristics<br />
that most <strong>flowering</strong> <strong>plants</strong> share but which are rare or<br />
absent in other groups and which also support the view that they<br />
are monophyletic (synapomorphies) include the following:<br />
differentiated xylem tissue including fibres, parenchyma and usually<br />
vessel elements<br />
phloem sieve-tube elements and companion cells formed from a<br />
common mother cell<br />
reaction wood produced in branches as a response to tension is<br />
made up of gelatinous fibres in an adaxial part of the xylem (in<br />
contrast to the abaxial rounded tracheids produced as a response<br />
to compression in conifers)<br />
leaves broad and flat with a distinct petiole<br />
leaves with pinnate secondary veins and fine veins reticulate and<br />
veinlets ending blindly<br />
a high degree of plasticity in vegetative growth<br />
bisexual reproductive axis (-flowers) with male organs situated<br />
below the female<br />
insect pollination<br />
pollen wall chambered (tectate)<br />
male gametophyte with three nuclei only<br />
carpel or pistil enclosing the ovules and fruit surrounding the seed,<br />
although in primitive <strong>flowering</strong> <strong>plants</strong> the carpel is sealed by a<br />
secretion only<br />
female gametophyte normally with eight nuclei only<br />
double-fertilisation and formation of the triploid endosperm<br />
‘direct’ development of the embryo from the zygote, i.e. with no<br />
intervening free-nuclear proembryo phase<br />
Not all of these features are found in all <strong>flowering</strong> <strong>plants</strong> and some<br />
are found in a few other plant groups.<br />
<strong>5.3</strong>.2 Fossil record and origin<br />
The origin of <strong>flowering</strong> <strong>plants</strong>, Darwin’s ‘Abominable Mystery’, is conjectural.<br />
There is scarcely any hard evidence of their origin before<br />
the Cretaceous (135 million years ago) but molecular data indicate a<br />
much more ancient origin for the lineage that eventually gave rise<br />
to <strong>flowering</strong> <strong>plants</strong>. There are intriguing fossils of flower-like structureslikethoseofArchaefructus,<br />
an aquatic plant of 124 million years<br />
ago, but the first undoubted <strong>flowering</strong>-plant fossils are pollen from<br />
equatorial latitudes of the Late Early Cretaceous, 125 million years<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 223<br />
(a)<br />
(b)<br />
(c)<br />
Figure 5.53. Floral diversity:<br />
(a) Houttuynia; (b) Aristolochia;<br />
(c) Allium.
224 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.54. Models of the<br />
origin of the angiosperm flower.<br />
ago. The <strong>flowering</strong>-<strong>plants</strong> probably originated in the tropics; extant<br />
primitive families are tropical or subtropical and early fossils show<br />
no adaptations to temperate conditions.<br />
There have been a number of theories about the origin of flowers,<br />
homologising in different ways floral structures with the reproductive<br />
structures of other <strong>plants</strong>. One theory, the anthophyte theory,<br />
related flowers to the bisexual flower-like axes present in groups<br />
such as the extinct Bennettitales and living Gnetidae. Two contrasting<br />
theories are the euanthium theory, which derives a flower from<br />
a uniaxial cone bearing both micro- and megasporophylls, and the<br />
pseudanthium theory, which derives the hermaphrodite flower from<br />
a complex inflorescence of unisexual male and female flowers. However,<br />
these different theories place undue emphasis on a fundamental<br />
distinction between organ types. It has also been suggested<br />
that a flower-like structure could have arisen by a change of sex of<br />
some of the microsporophylls of a male cone, a process called gamoheterotropy.<br />
This process is not hard to imagine in <strong>plants</strong> that are<br />
hermaphrodite and where the determination of sex is a developmental<br />
phenomenon that is only rarely associated with sex chromosomes.<br />
A similar transfer of sex has been proposed in the more recent evolution<br />
of the maize cob. This theory has received recent support<br />
from a study of the genes of reproductive development. The gene<br />
determining the sex of floral organs in <strong>flowering</strong> <strong>plants</strong> is a homologue<br />
of a gene active in the male axis of conifers but not the female<br />
axis.<br />
The first known leaves of <strong>flowering</strong> <strong>plants</strong> appear in fossils dated<br />
at 125 Ma and the first fossil inflorescence has been dated at 120 Ma.<br />
fossil has a mosaic of characteristics that are found in a range of basal<br />
groups of <strong>flowering</strong> <strong>plants</strong>. This has a female inflorescence, with a<br />
bract and two bracteoles at the base of female flowers that lack a<br />
perianth (achlamydeous flowers). The flowers are effectively just tiny<br />
pistils, less than 1 mm in diameter.<br />
<strong>5.3</strong>.3 The evolutionary radiation of <strong>flowering</strong> <strong>plants</strong><br />
There are many threads to the evolutionary diversification of <strong>flowering</strong><br />
<strong>plants</strong>:<br />
specialisation for pollination, including a reversal to wind pollination<br />
as well as more and more bizarre adaptations to attract specialist<br />
animal pollinators<br />
specialisation in fruit and seed dispersal, such as fleshy or otherwise<br />
elaborated fruits, and the evolution of seed dormancy<br />
adaptations for seedling establishment and growth in competition<br />
with other <strong>plants</strong><br />
specialisation for growing in different habitats from aquatic to arid<br />
terrestrial habitats, surviving heat, cold and low light levels, or<br />
growing as epiphytes and parasites<br />
adaptations conferring resistance to or prevention of herbivory
Insect pollination is closely associated with the origin and subsequent<br />
diversification of flowers. However, it is important to remember<br />
that insect pollination is associated with several other groups of seed<br />
<strong>plants</strong>, both living and extinct: Bennettitales, Gnetales, Cheirolepidaceae<br />
(extinct conifers), Cycadales and Medullosales (seed ferns).<br />
Insects grew in diversity with the origin of seed <strong>plants</strong> in the Late<br />
Devonian and this increase of diversity with the origin of flowers<br />
is just part of a continuous trend. Indeed there is some evidence<br />
to indicate a temporary decline in insect diversity as <strong>flowering</strong> <strong>plants</strong><br />
became more abundant in the Cretaceous. Nevertheless, flowers diversified<br />
in parallel with particular groups of pollinator specialists,<br />
the bees (Apoidea/Apidae), the pollen wasps (Vespidae: Masarinae),<br />
brachyceran flies (Acroceridae, Apioceridae, Bombyliidae, Empididae,<br />
Nemestrinidae, Stratiomyidae and Syrphidae) and the moths and butterflies<br />
(Lepidoptera). The evolution of a bisexual reproductive axis<br />
was a crucial event.<br />
Several trends in floral evolution can be discerned. Primitive flowerseitherlackaperianthorhaveoneinwhichthereisasingle<br />
whorl of tepals. In the perianth there has been a trend from having<br />
a perianth in which distinct whorls are not clearly differentiated to<br />
clear specialisation of a distinct calyx and corolla. The calyx protects<br />
against drought, temperature shock and predatory insects and the<br />
corolla attracts and controls pollinators. Both calyx and corolla may<br />
have been derived from tepals, but it is likely that in some groups,<br />
such as the buttercups, the petals originated from stamens, to which<br />
they are anatomically similar. In the peony, Paeonia, there is a gradual<br />
transition from leaves, through modified leaves on the <strong>flowering</strong> stem<br />
called bracts, into the perianth with parts at first sepal-like and then<br />
petal-like. Generally there has been a trend for the greater integration<br />
of the floral parts with greater precision in number and placement as<br />
flowers have become specialised to particular patterns of pollination.<br />
An important aspect of the diversification of <strong>flowering</strong> <strong>plants</strong> and<br />
their evolutionary success has been their vegetative flexibility. They<br />
have evolved into a bewildering range of forms through the activity<br />
of sub-apical and intercalary meristems. Flowering <strong>plants</strong> also have<br />
a greater capacity for elongation of cells, including root hairs and<br />
trichomes. One of the most important vegetative specialisations has<br />
been their possession of vessels in their wood, permitting more efficient<br />
water transport and hence greater photosynthetic rates, fast<br />
growth rates and earlier maturation. There have been significant<br />
adaptations in seasonal habitats; for example, although a few non<strong>flowering</strong><br />
<strong>plants</strong> are deciduous this has been a particular <strong>flowering</strong>plant<br />
trait that has adapted them to high latitudes or seasonally-dry<br />
environments.<br />
One aspect to the burgeoning biotic diversity of <strong>flowering</strong> <strong>plants</strong><br />
from the Cretaceous onwards was their great expansion of chemical<br />
diversity. New ranges of what have been called secondary compounds<br />
provided attractants for pollinators or seed dispersers, and repellants<br />
and toxic compounds to inhibit herbivores.<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 225<br />
(a)<br />
(b)<br />
Figure 5.55. Specialisation in<br />
the perianth in the Asparagales:<br />
(a) Belamcanda; (b) Kniphofia.
226 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.56. Phylogeny of the<br />
<strong>flowering</strong> <strong>plants</strong> from the<br />
Angiosperm Phylogeny Group<br />
(APG). In some respects the basal<br />
clades of <strong>flowering</strong> <strong>plants</strong> are only<br />
linked together by what features<br />
they lack. These were called by<br />
some the ANITA group from the<br />
initial letters of their family names.<br />
The last families belong in the<br />
same order (the Schisandrales),<br />
and the first two perhaps ought to<br />
be recognised as orders on their<br />
own (the Amborellales and<br />
Nympheales).<br />
<strong>5.3</strong>.4 The phylogeny of <strong>flowering</strong> <strong>plants</strong><br />
Ideas about the phylogenetic history of the <strong>flowering</strong> <strong>plants</strong> have<br />
been revolutionised in recent years by the cladistic analysis of DNA<br />
sequence data (Figure 5.56). What is presented here is as up to date as<br />
at time of press. <strong>Class</strong>ifications are achieved by consensus and gain<br />
currencybyuse.
<strong>5.3</strong>.5 Basal <strong>flowering</strong> <strong>plants</strong> and Eumagnoliids<br />
At the base of the <strong>flowering</strong>-plant phylogenetic tree there is a diverse<br />
group of families a ‘grade rather than a clade’ with a high proportion<br />
of primitive or unspecialised character states, fascinating<br />
because they represent the relicts of an early stage of <strong>flowering</strong> plant<br />
evolution.<br />
Table 5.4 The distribution of features of <strong>flowering</strong> <strong>plants</strong><br />
In basal groups<br />
(primitive/plesiomorphic/unspecialised)<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 227<br />
Features in high frequency<br />
In derived groups<br />
(advanced/apomorphic/specialised)<br />
Small shrubs, lianes or rhizomatous perennial<br />
herbs, or aquatics and semi-aquatics<br />
Various but including annuals and tall trees and herbs<br />
Vesselless Vessels present<br />
Parts in whorls or spirals of variable numbers Parts in whorls of three, four or five<br />
Actinomorphic Actinomorphic or Zygomorphic<br />
Parts free Parts connate or adnate<br />
Stamens broad with poor differentiation between<br />
filament and anther<br />
Stamens with well-differentiated filament<br />
Apocarpous Syncarpous<br />
Unsealed stigma and poorly differentiated style Sealed stigma with well-differentiated style<br />
Follicles Fruits (various)<br />
amborellales<br />
DNA sequence data places a plant called Amborella trichopoda from<br />
New Caledonia in a basal position as a sister to all other <strong>flowering</strong><br />
<strong>plants</strong>. This is not to say that it is the ancestor of all other <strong>flowering</strong><br />
<strong>plants</strong>. Rather that it is the closest living relative of the ancestor and<br />
like all other living <strong>flowering</strong> <strong>plants</strong> it exhibits a mixture of primitive<br />
and derived features. It is a shrubby evergreen plant with simple<br />
leaves that may be lobed. It has tracheids but no vessel elements. It is<br />
dioecious with flowers grouped in axillary cymose inflorescences. It<br />
has a perianth consisting of five to eight undifferentiated segments<br />
that are weakly joined at the base and are arranged in a spiral. The<br />
male flower has numerous (10--25) laminar stamens, the outer fused<br />
to the base of the perianth segments. Pollen is aperturate to nonaperturate<br />
and sulcate with a granulate outer wall and is possibly<br />
not tectate, a feature of possibly great significance. The female flower<br />
has five to six free carpels in a whorl. Carpels are open at the tip<br />
and have one ovule. Seeds are endospermic and the embryo has two<br />
cotyledons.<br />
(a)<br />
(c)<br />
(b)<br />
Figure 5.57. Amborella: (a) plant;<br />
(b) male flower; (c) female flower.
228 ORDERING THE PATHS OF DIVERSITY<br />
(a)<br />
(b)<br />
Figure 5.58. Nymphaeales: (a)<br />
Nymphaea; (b) Cabomba.<br />
Figure 5.59. Schisandra.<br />
nymphaeales (water-lilies)<br />
The next most-basal <strong>flowering</strong> <strong>plants</strong> are either six genera and about<br />
40 species of water-lilies, with large flattened floating leaves and large<br />
bowl-shaped flowers in the Nymphaeaceae, or two genera (Brasenia<br />
and Cabomba) of waterweeds with floating stems and relatively small<br />
simple and unspecialised flowers in the Cabombaceae (Figure 5.58).<br />
The Nymphaeales have a mixture of unspecialised, probably primitive<br />
features, and specialised features such as abundant aerenchyma<br />
tissue, adapting them to their aquatic habitat. Vessel elements have<br />
been recorded in some species but these are not like those of other<br />
<strong>flowering</strong>-<strong>plants</strong>. The pollen has a tectum of sorts but its inner layer,<br />
the endexine, is compact and lacks the columellate appearance of all<br />
other <strong>flowering</strong>-<strong>plants</strong>. Some features are shared with the monocots:<br />
the primary root is soon aborted and the stem has scattered closed<br />
bundles. The showy petals have originated as sterile stamens (staminodes).<br />
The families differ in the degree of fusion of the carpels; laterally<br />
fused in Nymphaeaceae and free in Cabombaceae. Flowers are<br />
normally hermaphrodite with only a weak distinction between sepals<br />
and petals. Sepals and petals are arranged in a spiral. The largeflowered<br />
water-lilies Nymphaea, Victoria and Nuphar are specialised for<br />
beetle pollination. It is remarkable that similar looking aquatic <strong>plants</strong><br />
have evolved convergently in the distantly related Nelumbo (Proteales)<br />
and Nymphoides (Asterales).<br />
schisandrales (including illiciales and<br />
austrobaileyales)<br />
The Schisandrales include four families of small trees and scrambling<br />
shrubs or lianes. austrobaileyaceae: These are lianes. Austrobaileya<br />
scandens, one of only two species in this family, grows in<br />
NE Australia, and has flowers that smell of rotting fish. The flowers<br />
have 12 perianth segments, 6--11 laminar stamens, with sterile stamens<br />
(staminodes) inside surrounding the 6--9 free carpels. trimeniaceae:<br />
There are only two genera, Trimenia and Piptocalyx, with a<br />
total of five species, of small trees and scrambling shrubs found from<br />
SE Asia to Australia. They are monoecious with small wind-pollinated<br />
flowers. The female flower has a single carpel and the male flower<br />
6--25 stamens in pairs. illiciaceae: There is only one genus, Illicium,<br />
with 42 species of trees and shrubs found in SE Asia and USA,<br />
Mexico and the Caribbean. Flowers may have 12 or more perianth<br />
segments, and 7--15 carpels. Illicium has peppery tasting leaves, and<br />
produces a star-shaped unripe fruit called star-anise that is used as a<br />
spice. schisandraceae: There are only two genera, Schisandra and<br />
Kadsura, and a total of 47 species of lianes and twining shrubs, found<br />
in East Asia and eastern North America. The Illiciaceae and Schisandraceae<br />
share some chemical features, a primary vascular cylinder<br />
and tricolpate pollen. Unlike the previous two orders some members<br />
have clearly formed vessel elements although of a primitive sort, with<br />
sclariform (ladder-like) perforations.
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 229<br />
eumagnoliids<br />
The eumagnoliids include several dicot orders as well as the monocots<br />
(see below). They include many species that exhibit some primitive<br />
features they share with basal monocotyledons: whorls of floral parts<br />
in threes (trimerous), monosulcate/uniaperturate pollen, apocarpous<br />
flowers.<br />
Like the ANITA grade of <strong>flowering</strong> <strong>plants</strong> many of these eumagnoliids<br />
are aromatic and include, for example, Lindera -- allspice, Piper<br />
-- pepper, Cinnamomum -- cinnamon, Aniba -- bois-de-rose oil, and Sassafras<br />
with its scented insecticidal oil. Another useful plant in this<br />
group is Persea, the avocado. Figure 5.60. Chloranthus.<br />
chloranthales<br />
The Chloranthaceae (Figure 5.60) is the only family. It has about 75<br />
species, most in the genus Hedyosmum. They are shrubs, lacking vessel<br />
elements. Their wood is soft and their swollen internodes sometimes<br />
collapse on drying. Flowers are very small and unisexual, with a single<br />
stamen or carpel and with or without a single whorl of three perianth<br />
segments.<br />
piperales (peppers and birthworts)<br />
It is hard to believe that Aristolochia, with its extraordinary tubular<br />
trap blossom belongs in the same order as Piper with its spikes of tiny<br />
flowers that lack a perianth altogether, along with Hydnora, which<br />
has flowers that arise from buds in the roots. The Aristolochiaceae<br />
are woody vines, the Piperaceae are herbs and the Hydnoraceae are<br />
root parasites that lack chlorophyll.<br />
laurales (laurels)<br />
There are seven families, most of which are trees, shrubs or lianes<br />
but Cassytha, in the Lauraceae, is a genus of twining, almost leafless<br />
plant parasites like the dodders (Cuscuta). They have flowers that vary<br />
from small to large (Figure 5.61).<br />
magnoliales (magnolias)<br />
Generally small trees or shrubs and lianes, most having relatively<br />
large showy flowers (Figure 5.62). The large flowers have a poorly<br />
differentiated perianth and quite often a variable number of segments.<br />
In addition some have partially sealed carpels and diverse but<br />
commonly broad stamens with a weakly distinguished filament and<br />
valvate anthers.<br />
canellales<br />
There are only two families of evergreen shrubs and trees, the Canellaceae<br />
and Winteraceae, in the order. Drimys, in the Winteraceae, has<br />
a peppery taste. The flower has variable numbers of perianth segments,<br />
flat stamens with a poorly differentiated filament and only<br />
weakly sealed carpels. The Canellaceae have the stamens fused in a<br />
ring.<br />
Figure 5.61. Laurus.<br />
Figure 5.62. Magnolia.<br />
Figure 5.63. Drimys.
230 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.64. Acorus.<br />
(a)<br />
(b)<br />
Figure 5.65. Alismatales:<br />
(a) Butomus; (b) Lysichiton.<br />
<strong>5.3</strong>.6 Monocots<br />
The monocots represent by far the most evolutionarily successful<br />
lineage of the eumagnoliids and represent about 25% of all <strong>flowering</strong><br />
<strong>plants</strong>, about 50 000 species. They are very diverse ranging from<br />
tall palm trees to tiny aquatic <strong>plants</strong> like Lemna (duckweed). Numerically<br />
they might be considered less important than the eudicots,<br />
but they include the grasses that provide the great majority of food<br />
for humans, either directly (wheat, rice, millet, etc.) or by feeding<br />
domestic grazing animals. They are clearly a monophyletic group and<br />
most have:<br />
single cotyledon<br />
sympodial growth (the palms Arecales are monopodial)<br />
linear, parallel-veined leaves in which the leaf base surrounds the<br />
stem<br />
primary root soon aborts and a wholly adventitious root system<br />
develops<br />
closed vascular bundles and lack of interfascicular cambium<br />
flower parts in threes<br />
pollen development (microsporogenesis) successive<br />
monosulcate pollen<br />
The basal monocots are aquatic or semi-aquatic <strong>plants</strong>. As the<br />
seedling germinates the single cotyledon elongates to push the primary<br />
embryonic root, the radicle, out of the seed and down into<br />
the wet mud. It is because of their lack of a vascular cambium that<br />
monocots are well able to undertake elongating growth but poor at<br />
thickening (secondary) growth. Elongating growth adapts them for<br />
the aquatic, climbing and epiphytic niches where they predominate,<br />
and also permits them to re-grow rapidly after grazing. The parallel<br />
venation of their leaves is a consequence of the extension of the<br />
leaves from a basal meristem. Indeed it is likely that parallel-veined<br />
monocot leaves are homologous to the petiole of leaves in other <strong>flowering</strong><br />
<strong>plants</strong>. Monocot trees and shrubs undergo various different and<br />
amorphous kinds of stem thickening, which is sometimes described<br />
as anomalous, and they are usually either unbranched or only weakly<br />
branched.<br />
acorus (sweet-flag or calamus)<br />
At the base of the monocot lineage are the two species of Acorus, a<br />
rooted aquatic. It has a tiny bisexual flowers crowded in a spadix.<br />
The carpels are primitive, intermediate between the ascidiate of the<br />
ANITA group and folded ones of other <strong>flowering</strong> <strong>plants</strong>. It has been<br />
utilised for centuries as a rush for floor covering because of its spicy<br />
scented properties.
alismatales (water weeds and aroids)<br />
There are several distinct aquatic families in this order linked by many<br />
adaptations to the aquatic habitat (floating stems, aerenchyma) as<br />
well as tricolpate pollen, and several have stamens produced in pairs,<br />
but even among these there are distinct rush-like forms (Butomaceae)<br />
as well as free-floating and submerged ones (especially the Alismataceae,<br />
Potamogetonaceae and Hydocharitaceae). Unlike most monocots,<br />
several of these aquatic families have a leaf that is clearly petiolate,<br />
a condition that is also present in the main terrestrial family of<br />
the order, the Araceae. Here also are found the few <strong>flowering</strong> <strong>plants</strong><br />
to have entered the marine habitat, in the Zosteraceae and Posidoniaceae.<br />
The Alismataceae and Limnocharitaceae produce latex. The<br />
Juncaginaceae have a fascinating reversal of floral whorls with an<br />
inner perianth whorl inside the outer stamens. It is likely that the<br />
Araceae have an aquatic origin but they are mainly terrestrial now.<br />
The few aquatic Araceae are very specialised, free-floating ones in<br />
the subfamily Lemnoideae, including the smallest <strong>flowering</strong> plant<br />
Wolffia, aswellasLemna and Pistia. Most Araceae are vines and epiphytes<br />
and form an important component of tropical and subtropical<br />
forests. Like Acorus the Araceae have a spadix associated with a large<br />
spathe.<br />
asparagoids dioscoreales (yams)<br />
The Burmanniaceae are mycotrophic, effectively saprophytic by utilising<br />
fungi to garner nutrients and energy. Some even lack chlorophyll<br />
and have only scale-like leaves. The Dioscoreaceae are yams, from<br />
the African word nyami, twiners with thick rhizomes or tuber-like<br />
swellings, sometimes many kilograms in size, and net-veined leaves.<br />
They are exceptional in some having secondary thickening, although<br />
it occurs in the tubers! There are only four genera but about 900<br />
species.<br />
pandanales (screw pines)<br />
The Pandanaceae, increase their girth as each node is produced so<br />
that the ‘trunk’ is balanced on a conical base, but supported by<br />
profuse, thick, strut-like roots. Because of the spiral way they producetheirleavestheyseemtoscrewtheirwayuptowardstheforest<br />
canopy. The young fibrous leaves of Carludovica are divided into strips<br />
and bleached with lemon juice to be made into Panama hats. Pentastemona<br />
in the Stemonaceae stands out as a monocot with three whorls<br />
of five parts. They also have petiolate leaves. The Pandanales, like several<br />
other monocot orders, have their own family (the Triuridaceae)<br />
of echlorophyllous mycotrophs.<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 231<br />
(a) (b)<br />
Figure 5.66. Dioscorea:<br />
(a) twining stem; (b) root tubers.<br />
Figure 5.67. Pandanus.
232 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.68. Lilium.<br />
Figure 5.69. Veltheimia<br />
bracteata (Eastern Cape Province,<br />
South Africa).<br />
liliales (lilies)<br />
In contrast to the superficially similar Asparagales, the Liliales tend<br />
to have nectaries at the base of the floral parts, spots on the petals<br />
and stamens with anthers opening to the outside (Figure 5.68). Other<br />
features like the cellular structure of the seed coat also link them.<br />
Like the amaryllids they include many geophytes, producing bulbs<br />
(Liliaceae, Melanthiaceae) or corms or rhizomes (Colchicaceae). Some<br />
are shrubby or vines (Philesiaceae, Smilacaceae). They also include<br />
chlorophyll-lacking mycotrophs in the Corsiaceae. There are four<br />
main lineages: (1) Smilacaceae with Liliaceae; (2) Alstroemeriaceae<br />
with Luzuriagaceae and Colchicaceae; (3) Campynemataceae; and<br />
(4) Melanthiaceae.<br />
asparagales (amaryllis, irises and orchids)<br />
The Aparagales is the largest order of monocots and includes many<br />
beautiful flowers (Figure 5.69). The Asparagales have a seed coat in<br />
which cellular structure has become obliterated and there is a black<br />
crust of phytomelan. Most Asparagales are rhizomatous herbs but a<br />
few such as the Ruscaceae are shrubs (Ruscus) oreventrees(Dracaena).<br />
Many of the most beautiful flowers are in a lineage of bulb-formers<br />
that includes the Amaryllidaceae, Agapanthaceae and Alliaceae. Many<br />
of these produce flowers in an umbel with a spathe at its base. The<br />
Iridaceae (irises and crocus) are in a distinct lineage and are distinguished<br />
by their divided style. Another interesting feature is that<br />
they do not produce root-hairs and rely entirely on mycorrhizae for<br />
garnering nutrients from the soil. The irises have a flower that provides<br />
three distinct entry points for pollinators, and a petaloid stigma<br />
overarching each of the three stamens. Another interesting lineage<br />
is one including tussock formers and trees in the Xanthorhoeaceae<br />
(grass-trees) and Asphodellaceae the aloes (Aloe) and red-hot pokers<br />
(Kniphofia).<br />
(a) (b) (c) (d)<br />
Figure 5.70. (a) Watsonia, (b) Tulipa, (c) Xanthorhoea, (d) Moraea.
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 233<br />
The orchids are the largest family of the order and comprise the<br />
most diverse and remarkable of all <strong>flowering</strong> plant families with<br />
between 800 and 1000 genera and up to 20 000 species, rivalled in<br />
numbers only by the Asteraceae. Orchid flowers have a complex architecture<br />
and are exceedingly diverse in the structure and arrangement<br />
of the various parts of the column, rostellum and pollinaria and in<br />
the shape, colour, and scent of the perianth. Figure 5.71. Floral diagrams of<br />
the three main patterns of orchid<br />
architecture. The Apostasioideae<br />
are a small SE Asian subfamily with<br />
three stamens. The<br />
Cyprepediodeae are the slipper<br />
orchids. The monandroid orchids<br />
are by far the most numerous and<br />
include several families of both<br />
terrestrial orchids (mainly<br />
Orchidoideae and Spiranthoideae),<br />
and epiphytic orchids (mainly<br />
Epidendroideae) and even lianes<br />
(Vanilloideae).<br />
(a) (b) (c)<br />
Figure 5.72. Oncidium sp. are<br />
epiphytic orchids commonly<br />
cultivated for their beautiful<br />
flowers.<br />
Figure 5.73. Three kinds of<br />
terrestrial orchids: (a) Anacamptis<br />
laxiflora; (b) Neottia nidus-avis lacks<br />
chlorophyll and is mycotrophic; (c)<br />
a slipper orchid, Phragmipedium sp.
234 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.74. Palm<br />
inflorescences.<br />
Figure 5.75. Bromeliad<br />
inflorescence, showing the<br />
disticous arrangement of bracts<br />
and flowers common in the Poales.<br />
Commelinids<br />
arecales (palms)<br />
The Arecaceae palms are the most important group of monocot trees<br />
but, like other monocot trees, they are unbranched or only weakly<br />
branched and lack a vascular cambium (Figure <strong>5.3</strong>0). Rather, secondary<br />
growth occurs by the expansion, and occasional dichotomous<br />
splitting, of a large apical meristem. The leaves of palms are large and<br />
complex, often palmate or pinnately lobed, and highly folded. Their<br />
flowers are relatively simple, following a standard monocot pattern<br />
with parts in threes, but often they are grouped together in massive,<br />
profusely branched inflorescences. About six subfamilies have been<br />
distinguished based on the form of the leaf (fan or feather) and its<br />
folding (rib down -- induplicate, or rib up -- reduplicate).<br />
poales (grasses, sedges and rushes)<br />
The Poales dominate most ecosystems where the growth of trees is<br />
limited. The Typhaceae, Juncaceae and Cyperaceae are common dominants<br />
of semi-aquatic and water-logged conditions. In contrast the<br />
Poaceae dominate moist or dry habitats in many climates, everywhere<br />
tree growth is restricted for any reason. They tolerate fire and grazing,<br />
even mowing, regrowing rapidly after they have been damaged.<br />
Grasses have a diverse of photosynthetic mechanism; C4 photosynthesis<br />
is common and has evolved many times over, permitting them to<br />
grow rapidly in the tropics.<br />
The Xyridaceae have showy flowers. The Bromeliaceae have showiness,<br />
but it is mostly provided by colourful bracts. The bromeliads are<br />
very important epiphytes in the Americas as tank-<strong>plants</strong> or air-<strong>plants</strong>.<br />
Their flowers are relatively unspecialised, but commonly each flower<br />
is produced at the base of a brightly coloured bract. Most are epiphytic<br />
and show many adaptations for their epiphytic life-style such<br />
as water storage tissue or water-absorbing peltate scales. The air-<strong>plants</strong><br />
Tillandsia, including Spanish moss, are perhaps the most remarkable<br />
in their ability to absorb moisture from the air. Most of the ground<br />
bromeliads have adaptations for arid environments.
Most other families in the order have reduced and relatively inconspicuous,<br />
often unisexual, flowers adapted for wind pollination by<br />
having an inflorescence held on a long leafless scape above the leafy<br />
rosettes. This trend has occurred in several of the constituent lineages.<br />
For example, sister to the Xyridaceae, are the mostly wind<br />
pollinated Eriocaulaceae, with similar dense heads of flowers. A few<br />
species in the Eriocaulaceae are insect pollinated with nectariferous<br />
petals. Some species are monoecious with female only marginal flowers<br />
in the head and central ones male. Other species are dioecious. In<br />
the Typhaceae the male part of the spadix is above the female. The<br />
Juncaceae have a typical monocot perfect flower, but it has a green<br />
or brown and papery perianth. They are wind pollinated, and a few<br />
are dioecious, but some have become secondarily insect-pollinated<br />
although they lack nectaries. In their sister family the sedges, Cyperaceae,<br />
the perianth is reduced to scales or bristles or is absent. Some<br />
genera, such as Scirpus, have hermaphrodite flowers. Others, including<br />
the largest genus Carex, have unisexual flowers with male and<br />
female flowers in different parts of the inflorescence. The male flowers<br />
consist only of three stamens with a bract called the glume on the<br />
abaxial side. A glume is also present in the female flower. In addition,<br />
two inner glumes have become fused to form a bottle-shaped perigynium<br />
or utricle surrounding the pistil. The style protrudes through<br />
the opening in the utricle.<br />
In the Poaceae, the grasses, the flowers are similarly reduced<br />
and adapted for wind pollination (Figure 5.78). They are grouped<br />
alternately side-by-side in spikelets. The primitive floral condition<br />
for grasses is retained by the bamboos (Bambusoideae). They have<br />
simple, often trimerous spikelets and they may have three lodicules<br />
and three stigmas. Progressive reduction has given rise to the standard<br />
grass floret pattern. Ampelodesmus, a primitive member of the<br />
advanced subfamily Pooidae also has three lodicules. At the base of<br />
the spikelet are two glumes that protect the spikelet in development.<br />
Each floret in the spikelet is enclosed by two other bracts; the lower<br />
is the lemma and the upper is the palea. Within the floret there<br />
are usually three stamens and a pistil with two feathery stigmas. At<br />
the base of the pistil there are two tiny lodicules, remnants of the<br />
perianth, which swell to push open the floret or shrink to allow it<br />
to shut. Cross-pollination is ensured not by separation of the sexes<br />
but by the different time of pollen release and stigma receptivity<br />
and, most importantly, by a unique form of self-incompatibility. The<br />
fruit is an achene with the seed fused to the fruit wall (a caryopsis).<br />
It is usually shed enclosed within the lemma and palea, which<br />
may be modified to aid dispersal. Threshing releases the grain from<br />
this chaff. The Poaceae is one of the most successful of all <strong>flowering</strong><br />
plant families because of its ability to spread laterally by rhizomes<br />
and stolons or the production of herbivory, drought- and fire-resistant<br />
tussocks.<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 235<br />
Figure 5.76. Xyridaceae.<br />
(a) (b)<br />
Figure 5.77. Cyperaceae<br />
florets: (a) female; (b) male.<br />
(c)<br />
(d)<br />
(b)<br />
(a)<br />
Figure 5.78. Poaceae: grass<br />
inflorescence: (a) plant with<br />
paniculate inflorescence,<br />
(b) spikelet, (c) distichous<br />
arrangement of florets in spikelet,<br />
(d) single floret with three dangling<br />
stamens and two feathery stigmas.
236 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.79. Commelinaceae:<br />
Tradescantia gigantia, an upright<br />
form (Enchanted Rock, Texas).<br />
Figure 5.80. Zingiberaceae: (a)<br />
Zingiber; (b) Heliconia.<br />
commelinales (spiderworts)<br />
The Commelinales are mainly herbs like the Haemodoraceae and<br />
Commelinaceae (Figure 5.79) but include the aquatic Pontederiaceae.<br />
Frequently the flowers are zygomorphic (monosymmetric). Heteromorphic<br />
flowers are found in the Commelinaceae (enantiostyly) and<br />
Pontederiaceae (heterostyly).<br />
zingiberales (gingers)<br />
The Zingiberales may be regarded as <strong>plants</strong> that do not produce an<br />
aerial stem except when <strong>flowering</strong> (Figure 5.80). They have showy,<br />
often strongly zygomorphic flowers especially adapted to bird pollination.<br />
The form of the flower with a large lip (labellum) is only<br />
exceeded in complexity in the monocots by the orchids. Sepals<br />
and petals are fused (connate) and sterile stamens (staminodes) are<br />
petaloid. The different families illustrate a great variety of specialisations<br />
for pollination in the tropics. The bananas, Musaceae, are<br />
the largest of all herbs, though they look like trees. The gingers,<br />
Zingiberaceae, include more than a thousand species of tropical<br />
herbs. In some ways the flower is analogous to that of the orchids<br />
with its pronounced zygomorphy and adnation of a single stamen to<br />
the perianth with other stamens converted into tepals and the anther<br />
supporting a slender style. However, the filaments and style are long<br />
and the anthers and stigma are exposed so that the pollinator is not<br />
as effectively ‘controlled’ as in the orchids. The Costaceae have five<br />
staminoids connate as a labellum and the stamen is broadly petaloid.<br />
Pollinators include hummingbirds and large bees. As in the gingers,<br />
the anther supports the slender style. The arrowroots (Marantaceae)<br />
have an asymmetrical flower with a single median stamen, which is<br />
half petaloid and all others are staminoidal and petaloid. The style is<br />
under tension and triggered to scoop pollen from the bee pollinator.<br />
Canna (Cannaceae), or Indian shot, has two whorls of three tepals, and<br />
up to five petaloid staminoids, which are showier than the perianth.<br />
In addition the fertile stamen and style are petaloid.<br />
(a) (b)
Basal Eudicots<br />
Basal eudicots such as the Proteales and Ranunculales have a pattern<br />
of leaf venation in which the lateral veins terminate at the margin in<br />
a small tooth (craspedodromus). They have a well-developed perianth<br />
but this is poorly differentiated into a calyx and corolla, and has a<br />
variable number of tepals spirally arranged or in whorls of three. Stamens<br />
and carpels are numerous and varying in number. The carpels<br />
are free to connate and have a sessile stigma.<br />
ranunculales (buttercups, poppies and barberries)<br />
Evolutionary trends in the Ranunculales include changes in the symmetry<br />
and the increasing complexity of the flower. Floral parts,<br />
especially the numerous stamens and carpels are commonly spirally<br />
arranged and the fruit is usually a follicle or an achene. The<br />
flower is apocarpous with superior pistils. The poppies, Papaveraceae<br />
(∼660 species) are derived from the buttercups from which they<br />
differ by having a syncarpous gynoecium and only two to three<br />
sepals. There are two subfamilies, the actinomorphic Papaveroideae,<br />
which produce latex, and the strongly zygomorphic Fumarioideae,<br />
which have a clear sap. Another large family in the order, the<br />
Berberidaceae (∼570 species) is distinguished from the Ranunculaceae<br />
by having stamens opposite the petals and a single pistil<br />
which becomes a berry. They have their flower parts in whorls rather<br />
than a spiral with two whorls of stamens and one to two whorls of<br />
nectaries.<br />
proteales (proteas, banksia and grevilleas)<br />
The Proteales are well represented in the Southern Hemisphere. The<br />
family Proteaceae in particular shows a distribution which records<br />
the old Gondwanan supercontinent. The Proteaceae are one important<br />
family (Figure 5.82) where brush blossoms have evolved. Some<br />
relationships discovered by the analysis of DNA sequence data are<br />
distinctly odd. For example, in the Proteales, Nelumbo, the sacred<br />
lotus, and Platanus, the plane tree, are sister groups. If this sister<br />
relationship is true it provides an astonishing reminder of how little<br />
we know about the evolutionary history that connects living plant<br />
groups. What kind of shared ancestor did these two lineages have<br />
and what were the circumstances that led to one becoming aquatic<br />
and the other a tree? The existence of such differences in sister lineages<br />
demonstrate the potential evolutionary fluidity of morphological<br />
characters, and emphasises the fact that living <strong>plants</strong> are only<br />
the tips of a highly branched phylogenetic bush. Within the bush<br />
many branches end blindly and do not reach the surface so that<br />
intermediate linking kinds between living plant groups do not now<br />
exist.<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 237<br />
Figure 5.81. Fumarioideae<br />
(Corydalis): (a) plant; (b) detail of a<br />
single flower.<br />
Figure 5.82. Protea: the long<br />
slender flowers are grouped in<br />
heads surrounded by showy<br />
bracts. Each flower has a single<br />
pistil surrounded by four petals<br />
(three fused and one free) with<br />
anthers adnate to the petals.
238 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.83. Trochodendron.<br />
Figure 5.84. Gunnera.<br />
Figure 5.85. Polygonaceae:<br />
Reynoutria.<br />
Figure 5.86. Amaranthaceae:<br />
Ptilotus.<br />
trochodendrales<br />
This order has only two species of evergreen trees from east and South<br />
East Asia each in its own family, Tetracentron sinense and Trochodendron<br />
aralioides (Figure 5.83).<br />
gunnerales<br />
There are only two genera in the order. Gunnera has the familiar, massive,<br />
palmate and deeply ribbed leaves. Usually grown beside water it<br />
is also a colonist of land-slips. One advantage it has is the fixed nitrogen<br />
it gets from the symbiotic blue-green bacteria (Nostoc) that live<br />
in its exposed roots and rhizomes. It produces large strobiloid inflorescences<br />
of tiny flowers, either bisexual or unisexual. Myrothamnus,<br />
from tropical Africa and Madagscar, is a resurrection plant, appearing<br />
to dry out but able to revive and start growing again when water<br />
becomes available.<br />
Core Eudicots<br />
Core eudicots have predominantly flowers with parts in fives (pentamerous)<br />
with a clear distinction between calyx and corolla. There<br />
are two main lineages, the Rosids and Asterids, three large basal lineages,<br />
the Caryophyllales, the Santalales and the Saxifragales, and a<br />
number of others of uncertain relationship like the Berberidopsidales<br />
and Vitales. These basal orders are crassinucellate (see below).<br />
caryophyllales (catchflies, stonecrops and cacti)<br />
The Caryophyllales is a large, and an interesting group of about 4%<br />
of all <strong>flowering</strong> <strong>plants</strong> that exhibits a unique set of characters (see<br />
Figure 5.86). For example it seems to lack mycorrhizae. It comprises<br />
mostly herbs, but others are lianes or twiners and shrubs. They have a<br />
peculiar pattern of secondary growth with the production of diffuse<br />
or successive cambia, which is commonly associated with succulence<br />
and CAM photosynthesis. Many families that have a high frequency<br />
of succulence have species that are either xerophytic like the cacti<br />
(Cactaceae), stonecrops (Aizoaceae) or halophytic like the sea-lavenders<br />
and thrifts (Plumbaginaceae).<br />
Most have a campylotropous ovule in which the inner integument<br />
protrudes, and a peripheral embryo surrounding a nutritive central<br />
perisperm tissue and so they were previously called the Centrospermae.<br />
Another shared character is a peculiar type of sieve-tube plastid,<br />
and their chemistry is distinct. The core caryophyllid families are<br />
Amaranthaceae, Aizoaceae, Cactaceae, Caryophyllaceae, Didiereaceae,<br />
Molluginaceae, Nyctaginaceae, and Phytolaccaceae. Most of these have<br />
a shikimic acid biosynthetic pathway as a starting point for the synthesis<br />
of nitrogen-containing benzylisoquinoline alkaloids and the<br />
betalain pigments. The latter are utilised instead of the anthocyanins<br />
used in other <strong>flowering</strong> <strong>plants</strong>.<br />
Non-core caryophyllids are the Plumbaginaceae, Polygonaceae,<br />
Tamaricaceae, and Frankeniaceae in one clade and four families<br />
of insectivorous <strong>plants</strong> in a sister clade. The former include many<br />
halophytic <strong>plants</strong>, which are also found in core caryophyllids such<br />
as the Amaranthaceae and Chenopodiaceae (Salicornia). Many have
epidermal glands but in different families the glands are adapted<br />
to produce either mucilage (in Polygonaceae), excrete salt as in the<br />
halophytes, or digestive enzymes in the insectivorous Droseraceae and<br />
Nepenthaceae. In the latter, the gland-type is shared but the subsidiary<br />
insect-trapping apparatus is quite diverse ranging from pitchers<br />
(Nepenthes), sticky traps (Drosera, Drosophyllum) to spring traps<br />
(Dionaea, Aldovandra).<br />
Several families in the order have a peculiar flower development<br />
that starts in a polymerous way but becomes organised into a pseudodiplostemonous<br />
way. Stamens appear to be produced in pairs. In<br />
the Caryophyllaceae and Plumbaginaceae stamens are antipetalous<br />
and arise with the petal as a unit.<br />
Some families in the Caryophyllalaes (Caryophyllaceae, Chenopodiaceae)<br />
generally lack mycorrhizae perhaps because they tend to<br />
occupy nutrient rich fresh soils. In contrast, the insectivores in the<br />
Nepenthaceae, Droseraceae and Drosophyllaceae can inhabit nutrientpoor<br />
soils.<br />
santalales (sandlewoods)<br />
All five families of the Santalales (Santalaceae ∼500 species, Olacaceae<br />
∼200 species, Opiliaceae 28 species, Misodendraceae 8 species, and<br />
the Loranthaceae ∼940 species) include tropical parasitic species. The<br />
Loranthaceae are the most specialised parasites. The shrubby, liane<br />
or twining habit is common throughout the order.<br />
saxifragales (saxifrages, currants and stonecrops)<br />
The Saxifragales is a very diverse order and includes: trees<br />
(Hamamelidaceae ∼100 species of witch hazel and sweet gum, Cercidiphyllaceae<br />
-- katsura, Altingiaceae), shrubs (Grossulariaceae ∼325<br />
species of currants and gooseberies) and showy ornamentals like the<br />
Paeoniaceae (∼34 species of peony), but the two largest families are<br />
herbs, mainly rosette-forming Saxifragaceae (∼475 species of Astilbe<br />
and saxifrage) or leafy succulents, the Crassulaceae (∼1280 species of<br />
Sempervivum, Echeveria, Sedum and Kalanchoe). Many of these rosette<br />
formers in the Crassulaceae, Saxifragaceae and some other small<br />
families, are linked by a similar look to their flowers and a number<br />
of characters such as a persistent scarious calyx and cellular<br />
endosperm.<br />
vitales (vines)<br />
This order is of supreme importance to us as the source of wine from<br />
the grape vine Vitis. Many other genera are twining vines with or<br />
without tendrils (Rhoiocissus, Cissus). Cyphostemma is a caudiciform and<br />
Leea a shrub and small tree.<br />
berberidopsidales<br />
This tiny order of two families, one with only one species Aextoxicon<br />
from Chile and the other with only two genera Berberidopsis and<br />
Streptothamnus from Chile and eastern Australia.<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 239<br />
Figure 5.87. Aizoaceae:<br />
Mesembryanthemum.<br />
Figure 5.88. Grossulariaceea:<br />
Ribes.<br />
Figure 5.89. Paeoniaceae:<br />
Paeonia.<br />
Figure 5.90. Berberidopsidales:<br />
Berberidopsis.
240 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.91. Geraniales:<br />
Geranium.<br />
Rosids and Asterids<br />
The remaining eudicots form two great lineages, the crassinucellate<br />
(thick nucellus) Rosids and the tenuinucellate Asterids, each<br />
split into two sister lineages (Eurosid 1 and 2, Asterid 1 and 2).<br />
The tenuinucellate condition is a derived feature of the ovule where<br />
the tissue layer, the nucellus, surrounding the developing megasporangium/embryosac<br />
is thin. There are various kinds of crassinucellate<br />
condition. Each main and sub-lineage is very variable but exhibits<br />
particular evolutionary tendencies. For example, all the nitrogenfixing<br />
families are found in the Eurosid I group: Casuarinaceae,<br />
Myricaceae (Fagales), Eleagnaceae, Rhamnaceae, Ulmaceae (Rosales),<br />
Fabaceae (Fabales) and Coriariaceae (Cucurbitales). There are well<br />
defined families like the Fabaceae, almost stereotypical with its characteristic<br />
fruit, the legume, and characteristic forms of flowers and<br />
inflorescence, or large and diverse families, like the Rosaceae, that<br />
have proved evolutionarily flexible.<br />
<strong>5.3</strong>.7 Rosids<br />
Rosid orders or even families are very diverse in their floral structure.<br />
Most of the ecologically (and economically) important trees from<br />
forests and savannas around the world are Rosids. They include the<br />
tallest <strong>flowering</strong> <strong>plants</strong> such as the eucalypts, the mahoganies in the<br />
tropics, the savanna acacias, and oaks, maples and beech from temperate<br />
forests.<br />
Percentage of families Rosids Asterids<br />
stipules 66% with 16% with<br />
stamens 58% with two whorls 79% single whorl<br />
corolla 97% free 75% fused<br />
integument(s) 94% two 87% one<br />
nucellus 92% crassinucellate 91% tenuinucellate<br />
endosperm 96% nuclear 75% cellular<br />
iridoids 5% present 61% present<br />
Basal Rosids<br />
geraniales (geraniums)<br />
The Geraniales include one large family, the Geraniaceae, and several<br />
very small ones. One interesting feature they share is the presence<br />
of glands on the margin of the leaf. They have pentamerous<br />
obdiplostemonous flowers with a persistent calyx. The two largest<br />
genera Pelargonium and Geranium produce similar beaked fruits but<br />
differ in the former having monosymmetric flowers and a nectariferous<br />
pedicel (Figure 5.91). The Geraniaceae are commonly herbs with<br />
jointed stems but the other families in the order include shrubs and<br />
trees.
crossosomatales<br />
These are a small order of shrubs and small trees adapted to dry<br />
habitats.<br />
myrtales (eucalypts and myrtles)<br />
The Myrtales have an uncertain relationship to either main Eurosid<br />
clade. Many have flowers with a large number of stamens (Figure 5.92).<br />
However, not all show this pattern. In the Melastomataceae (4750<br />
species) the stamens are dimorphic with showy colourful outer stamens<br />
and short pollen-producing inner ones. The Onagraceae (650<br />
species) have only four or two stamens and a long hypanthium tube.<br />
These families are significant as herbs or shrubs but the Myrtales<br />
includes the Myrtaceae and Combretaceae, which are highly significant<br />
as trees in the tropics especially in semi-arid areas of Australia<br />
(Eucalyptus ∼450 species) and Africa (Combretum ∼250 species, and<br />
Terminalia ∼150 species).<br />
Eurosid I<br />
zygophyllales (creosote-bush and lignum-vitae)<br />
There are several species important in arid and saline soils like Larrea,<br />
Balanites (thorny) and Zygophyllum. Larrea divaricata, the Creosote bush<br />
of the deserts of USA and Mexico is strongly allelopathic.<br />
celastrales (spindle-tree and ebony)<br />
A member of the Celastraceae familiar to us is the widely planted garden<br />
plant Euonymus, the spindle-tree, with its characteristically angled<br />
fruit. Maytenus (ebony) is an important tree in warmer areas throughout<br />
the world. The flowers generally have a broad disk with the ovary<br />
submerged in it.<br />
malpighiales (spurges, violets, willows and<br />
passion-flowers)<br />
The diversity of the order is illustrated by a comparison of the families<br />
Violaceae (pansies), Passifloraceae (passion flowers), Linaceae (flax),<br />
Salicaceae (willows) and Clusiaceae (Hypericum), each with a very different<br />
kind of flower. Perhaps its most interesting family is the latexproducing<br />
Euphorbiaceae that includes such important genera as<br />
Ricinus, Euphorbia, Manihot and Hevea (rubber) (see Figure 5.94).<br />
oxalidales (bermuda buttercup and wood sorrel)<br />
The order includes trees, shrubs, lianes and herbs (Oxalis) and also the<br />
insectivore Cephalotus (Cephalotaceae).<br />
(a) (b) (c) (d)<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 241<br />
Figure 5.92. Myrtales:<br />
Eucalyptus.<br />
(a)<br />
(b)<br />
Figure 5.93. (a) Viola;<br />
(b) Linum.<br />
Figure 5.94. Euphorbiaceae: the<br />
inflorescence (a cyathium) mimics<br />
a flower: (a) succulent euphorb<br />
with a crown of leaves and cyathia;<br />
(b) a cyathium showing the<br />
arrangement of a central female<br />
floret surrounded by male florets<br />
and an involucre of bracts; (c) a<br />
single female floret; (d) a single<br />
male floret.
242 ORDERING THE PATHS OF DIVERSITY<br />
(a)<br />
(b)<br />
(c)<br />
Figure 5.95. Cesalpinioideae:<br />
(a) Delonix; (b) Cassia, half-flower;<br />
and (c) side and front view.<br />
(a)<br />
Figure 5.96. Faboideae: (a)<br />
Trifolium (keeled flowers in a head);<br />
(b) Vicia (half-flower and dissected<br />
flower).<br />
Figure 5.97. Mimosoideae;<br />
Acacia: (a) florets in spherical<br />
heads; (b) single floret and a pistil.<br />
fabales (legumes)<br />
The Fabales include the Fabaceae, sometimes recognised as a single<br />
family, with three subfamilies, or as three families (Figure 5.95). They<br />
all share the characteristic of a kind of fruit, the legume that gives<br />
them their alternative name, the Leguminosae. The Caesalpinioideae<br />
(∼2000 species) have large showy, more or less regular flowers. They<br />
are basal in the family and the other two families show different<br />
patterns of specialisation.<br />
The Mimosoideae (∼3100 species) have small regular flowers in<br />
dense spikes or heads, brush blossoms with numerous exserted stamens.<br />
Two evolutionary trends are observed in the Mimosoideae. One<br />
trend is for an increase of the number of stamens although each<br />
has a tiny anther. The filaments are long and the stigma is small<br />
and cup-shaped. Pollen is released as a polyad. In Acacia only one<br />
polyad can fit on each stigma and the number of seeds produced in<br />
the legume is directly related to the number of pollen grains in the<br />
polyad. Another evolutionary trend shows a reduction of the number<br />
of stamens but specialisation of the flowers in the head to form a<br />
kind of pseudanthemum. For example, in Parkia the lower florets are<br />
showy and scent-producing but sterile, the intermediate ones sterile<br />
but nectar producing and the upper fertile.<br />
The Caesalpinioideae and Mimosoideae are mainly trees and<br />
shrubs but most of the third subfamily, the Faboideae (∼11 300<br />
species) are herbs. They have the zygomorphic flower that gives them<br />
their alternative name (Papilionoideae). The flag blossom is pollinated<br />
by large bees which land on the keel. The nectary is at the base of<br />
a staminal tube. In forcing its proboscis into the staminal tube the<br />
heavy insect pushes the keel petals (alae) down and the stamens and<br />
stigma rub against its ventral surface. A similar type of papilionate<br />
flower has evolved in parallel in the Polygalaceae, also in the Fabales.<br />
The Faboideae includes many agriculturally important species, peas<br />
and beans of all sorts. Clovers are especially important in pasture.<br />
(b) 5.97 (a) (b)<br />
The stereotypical nature of the Fabales/Fabaceae is emphasised<br />
because about one third of all species in the order/family belong<br />
tooneofafewverylargegenera:Acacia (1200 species), Mimosa (400<br />
species) (both Mimosoideae), Cassia (540 species) (Caesalpinioideae),<br />
Astragalus (2000 species), Crotalaria (600 species), Infigofera (700 species)
(all Faboideae/Popilionoideae). All these large genera are important in<br />
open habitats, especially in the arid and semi-arid tropics.<br />
rosales (roses and allies)<br />
The Rosales include several interesting families such as the figs<br />
and mulberries (Moraceae ∼1200 species), the elms (Ulmaceae ∼140<br />
species), the nettles (Urticaceae ∼1050 species) and buckthorns<br />
(Rhamnaceae ∼880 species) but Rosaceae (∼3000 species) is the largest<br />
family. It is central to the evolution of many other families in the<br />
Rosales but relatively difficult to circumscribe. For example the ovary<br />
may be hypogynous, perigynous or epigynous and perhaps the only<br />
widespread feature is the possession of a hypanthium, a floral cup<br />
that is primitively small and saucer- or cup-shaped, or has evolved to<br />
become large and ultimately connate with the carpels. Traditionally<br />
the family has been divided into four subfamiles differing in the form<br />
of the fruit (the Spiroideae -- an aggregate of follicles, the Rosoideae -an<br />
aggregate of achenes, the Prunoideae -- a drupe, the Maloideae -a<br />
pome). One of the smallest families in the order, with only three<br />
species, but certainly not the least significant is the Cannabaceae<br />
because it contains both hops and cannabis.<br />
cucurbitales (gourds and begonias)<br />
The Cucurbitales include three families interesting for different reasons.<br />
The Coriariaceae have only one widely distributed genus, Coriaria<br />
(Mexico to Chile; Mediterranean to Himalayas/Japan; New Guinea<br />
to New Zealand and W Pacific) but it is interesting because it has a<br />
nitrogen-fixing association with Frankia in root nodules. The Begoniaceae<br />
have only two genera, Symbegonia with 12 species from New<br />
Guinea and Begonia with over a thousand. Symbegonia differs from<br />
Begonia in having a corolla fused in a tube. Begonia are succulent<br />
herbs and shrubs. Water conservation is aided by Crassulacean Acid<br />
Metabolism (CAM) and they have stomata in clusters. The Cucurbitaceae<br />
are climbers with tendrils and are important commercially<br />
as gourds, squash, melons, etc. These families share a tendency to<br />
fleshy or juicy tissues.<br />
fagales (oaks and beeches)<br />
The Fagales include many familiar wind-pollinated trees of temperate<br />
regions. The related families of trees, Fagaceae (beech, hornbeam<br />
and oak), Betulaceae (birch and alder), Casuarinaceae (she-oaks) and<br />
Juglandaceae (walnuts) exhibit a wide range of adaptations for windpollination.<br />
Different genera show different degrees of reduction of<br />
the flower and its aggregation into unisexual catkins. Wind-pollinated<br />
species are concentrated in the temperate regions but some genera<br />
like the oaks and hornbeams have insect-pollinated species in the<br />
tropics and here have stiff erect catkins. Walnut (Juglans) and wingnut<br />
(Pterocarya) have a tiny but well-formed perianth. In Oak (Quercus robur)<br />
male florets have a six-lobed perianth and seven to eight stamens but<br />
female florets have a minute perianth but a scaly cupule. Hazel (Corylus)<br />
has male florets consisting of two bracteoles and four stamens<br />
only with a bract at the base and female flowers that are surrounded<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 243<br />
Figure 5.98. Rosaceae: Rosa.<br />
Figure 5.99. Cucurbitaceae:<br />
Cucurbita.<br />
Figure 5.100. Fagaceae: Alnus.
244 ORDERING THE PATHS OF DIVERSITY<br />
(a)<br />
(b)<br />
Figure 5.101. Brassicaceae:<br />
(a) half flower; (b) floral diagram.<br />
Figure 5.102. Malvaceae:<br />
Hibiscus.<br />
Figure 5.103. Anarcardiaceae:<br />
Pistacia.<br />
by scales and have a minute perianth. The strange she-oak (Casuarina)<br />
of South-East Asia and Australia was once considered to be very primitive<br />
because of its very simple inflorescence. The flower consists only<br />
of a bract with two scale-like bracteoles with, in the male, a single<br />
stamen and, in the female, a single pistil. Male flowers are aggregated<br />
into catkins and the hard bracteoles of the female flowers form part<br />
of a woody ‘cone’.<br />
Eurosid II<br />
The Eurosid II clade includes these important orders: Brassicales,<br />
Malvales, and Sapindales.<br />
brassicales (crucifers)<br />
The Brassicales include sister lineages so distinct and without intermediates<br />
that one wouldn’t guess their close relationship. The families<br />
Brassicaceae (crucifers), Resedaceae (mignonette), Limnanthaceae<br />
(poached-egg flower), Batidaceae (saltwort), Koeberliniaceae (allthorn),<br />
Setchellanthaceae, Moringaceae (Bennut), Caricaceae (papaya) and<br />
Tropaeolaceae (nasturtium) are very distinct in their floral morphology.<br />
For example, the Brassicaceae is also called the Cruciferae because<br />
of its cross-shaped flowers of four petals and usually six stamens. In<br />
contrast the Resedaceae usually has six fringed petals and the Tropaeolaceae<br />
has five and also has a long hairy claw. One floral feature that<br />
is present in several families of the order is a nectariferous portion<br />
of the axis below the stamens (androgynophore) or pistil (gynophore).<br />
One of the most significant features these families share is the possession<br />
of mustard oils (glucosinolates). This seemingly obscure chemical<br />
character provides protection against herbivory and fungal attack.<br />
Another interesting feature is the usual lack of mycorrhizae in the<br />
Brassicaceae, perhaps because they tend to occupy relatively nutrient<br />
rich early successional situations.<br />
malvales (hibiscus and mallows)<br />
The Malvales are linked by a chemical characteristic of obscure significance,<br />
the presence of mucilage cells, or canals and cavities. The<br />
Malvales include the important family of tropical trees, the Dipterocarpaceae.<br />
Many exhibit a common rosid trait of showy polypetalous<br />
flowers with many stamens. The sequence in which the stamens<br />
mature is centrifugal, a pattern of development that was thought<br />
to be significant enough to warrant separating them from the rosids<br />
(centripetal development) in a group called the Dillenidae, but this<br />
pattern of development is difficult to see in the Malvaceae where the<br />
stamens are united to form a tube around the style.<br />
sapindales (mahoganies)<br />
The Sapindales also include several very important tropical and subtropical<br />
families of trees and shrubs such as the Meliaceae (∼575<br />
species), Sapindaceae (∼1350 species), Anacardiaceae (∼850 species)<br />
and Burseraceae (∼540 species). They frequently have pinnate or
tri-foliolate leaves. Many of these families are highly resinous. The<br />
resin is toxic and protects them to a degree from leaf-browsing animals<br />
and also protects the wood from wood-boring insects. Azadirachta<br />
in the Meliaceae is a source of insecticide. The Sapindaceae often have<br />
saponins present. We are familiar with the Sapindales as the source<br />
of fruits and seeds (litchi, longan or rambutan -- Sapindaceae; mango,<br />
cashew, pistacia -- Anacardiaceae) and as timber trees (mahogany<br />
Khaya, Swietenia -- Meliaceae).<br />
<strong>5.3</strong>.8 Asterids<br />
The Asterids are tenuinucellate. Most also have a pentamerous sympetalous<br />
corolla, and most have an equal number of epipetalous stamens,<br />
alternating with the five corolla lobes. This set of attributes<br />
has long been recognized as those of a group called ‘Sympetalae’<br />
(for a tubular corolla of connate (fused) petals). The Asterids contain<br />
the most advanced members of the Eudicots, and the most<br />
recently evolved. They have diversified especially in having specialised<br />
pollination mechanisms. Floral architecture and behaviour show<br />
many individual adaptations to particular kinds of pollinator.<br />
Basal Asterids<br />
cornales (dogwoods)<br />
The Cornales exhibit a tendency, seen more fully developed elsewhere<br />
in the euasterids, towards the possession of a pseudanthium, a compound<br />
inflorescence of small flowers grouped together in a flat head<br />
and made showy in different ways. In Hydrangea, for example, flowers<br />
are in a cymose inflorescence with marginal ones sterile and showy,<br />
and fertile central ones. An alternative pattern is seen in Cornus (Cornaceae),<br />
which has large, showy, outer bracts like petals around the<br />
inflorescence. Although some Cornales have a synsepalous calyx most<br />
have free petals. There are three large families in the order, the Cornaceae,<br />
Hydrangeaceae and Loasaceae, ranging from trees and shrubs<br />
to robust herbs. The Loasaceae have barbed stinging hairs.<br />
ericales (heathers)<br />
The Ericales are a diverse order and include, as well as the heathers<br />
(Ericaceae), other very distinct families such as the balsams (Balsaminaceae<br />
-- fleshy herbs), the Marcgraviaceae (lianes), the Polemoniaceae<br />
(mainly herbs, especially of arid areas, but some shrubs and<br />
lianes), the camellias (Theaceae -- shrubs and trees with thick leaves),<br />
and primulas (Primulaceae -- herbs). The brazil-nut family Lecythidaceae,<br />
and the Sapotaceae, another important tropical family, are<br />
sister families in the order. The latter produces latex and gums, and<br />
includes species such as the chewing-gum plant Manilkara and guttapercha<br />
plant Palaquium. One lineage of Ericales includes the insectivorous<br />
pitcher-plant family Sarraceniaceae (Sarracenia, Darlingtonia,<br />
Heliamphora) andRoridula with sticky resin secreting hairs (but not<br />
insectivorous) and sensitive stamens, as well as the Actinidiaceae (the<br />
kiwi-fruit or Chinese gooseberry).<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 245<br />
Figure 5.104. Cornaceae:<br />
Cornus.<br />
Figure 5.105. Theaceae:<br />
Camellia.<br />
Figure 5.106. Primulaceae:<br />
Dodecatheon.
246 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.107. Ericaceae:<br />
Rhododendron.<br />
Figure 5.108. Garryaceae:<br />
Garrya.<br />
Figure 5.109. Gentianaceae:<br />
Gentiana.<br />
Figure 5.110. Apocyancaeae:<br />
Nerium.<br />
The Clethraceae and Cyrillaceae, two small families sister to the<br />
Ericaceae, are all trees or shrubs with tough, spirally-arranged leaves<br />
and pendulous flowers.<br />
There are about 4000 species of Ericaceae. They are strongly<br />
mycorrhizal, with different subgroups having different forms of<br />
mycorrhizae: broadly identified as arbutoid, ericoid and monotropoid<br />
types. Erica and Rhododendron are by far the largest genera, each with<br />
about 800 species. The flowers of Ericaceae range from small and<br />
relatively inconspicuous Calluna type to large and highly colourful<br />
Rhododendron blossoms, and from radial to bilateral symmetry, representing<br />
a great diversity of pollination mechanisms involving wind,<br />
thrips, bees and other insects, birds and other pollinators.<br />
Euasterid I<br />
The Euasterid I group shows a trend towards large zygomorphic<br />
flowers, held laterally, exemplified by snapdragon, salvias and deadnettles.<br />
garryales (silk-tassel)<br />
Aucuba is the commonly grown yellow spotted evergreen ‘laurel’.<br />
Garrya (Silk-tassel Bush) is another commonly cultivated shrub with<br />
showy catkins. It produces highly toxic alkaloids. One rather peculiar<br />
feature of this order is the presence of petroselenic acid as a major<br />
fatty acid in seeds.<br />
gentianales (gentians and bedstraws)<br />
The order Gentianales has the most generalised flowers in the Euasterids<br />
I with five epipetalous stamens, but they also exhibit secondary<br />
pollen presentation mechanisms, which in the asclepiads,<br />
have resulted in one of the most peculiar floral morphologies and<br />
pollination mechanisms of any <strong>flowering</strong> <strong>plants</strong>. The Gentianaceae<br />
(∼1200 species) are regular, sympetalous and actinomorphic with five<br />
normal epipetalous stamens. The Rubiaceae (∼11 000 species) are similar,<br />
although frequently they have floral parts in fours and much<br />
smaller flowers in cymes. Many shed their pollen onto a club-shaped<br />
stigma while in bud. When the bud opens the pollen is presented<br />
to the pollinator and only later does the stigma mature. The Gentianaceae<br />
are mainly herbs and are especially common in temperate<br />
conditions, while the Rubiaceae are important as tropical trees,<br />
shrubs and lianes.<br />
The Apocynaceae (∼5000 species) ranges from genera such as Vinca<br />
(completely united carpels, and anthers, which are distinct and fully<br />
fertile) to those like Nerium (carpels separated up from the base and<br />
united only by their style and stigma, and anthers in which only the<br />
top part produces pollen and grouped closely together in depressions<br />
around the top of the expanded style). The asclepiads were formerly<br />
recognised as a separate family. In Asclepias the anthers are adnate to<br />
the style to form a structure called the gynostegium and the pollen<br />
in each theca is a compact mass, a pollinium. Pollinia from adjacent<br />
thecae are united by an acellular yoke called the translator and<br />
the whole structure is released as a pollinarium. Evolution of the<br />
pollinium has been accompanied by a merging of the pollen sacs of
each theca so that each anther is bisporangiate. The translator clips<br />
the pollinarium to the pollinator and it is later pulled off by being<br />
caught in a groove in the stigma of another flower.<br />
solanales (potato and morning glory)<br />
The Solanales have two large families, the Solanaceae (∼2600 species)<br />
(potato, tomato, tobacco, petunia) and the Convolvulaceae (morning<br />
glory, bindweed). Most species are polysymmetric with five equal<br />
corolla lobes but some genera of the Solanaceae are zygomorphic.<br />
Schizanthus has even lost one stamen and, of the remaining four, only<br />
two are fertile. The small family Nolanaceae has five carpels, the<br />
primitive condition, but other families in the Solanales have fewer<br />
carpels; either two- or pseudo-four-locular. In some of the Convolvulaceae<br />
(Dichondra) there are two carpels and the ovary is deeply twoor<br />
four-lobed with a gynobasic style, which resembles the pattern of<br />
the Boraginaceae and Lamiaceae.<br />
lamiales (dead-nettles and gesners)<br />
The Lamiales are mainly characterised by their monosymmetric tubular<br />
flowers. From an ancestral pattern of five more or less equal corolla<br />
lobes and five stamens there has been a shift to either four (with the<br />
loss of the posterior stamen) or two stamens (with in addition a loss<br />
of one lateral pair) and a strongly lipped flower. Patterns of floral<br />
evolution are complex though. For example the Oleaceae (olive, ash,<br />
privet, jasmine), close to the base of the order, are polysymmetric in<br />
their corolla but usually have four corolla lobes and only two stamens.<br />
Calceolaria (Calceolariaceae), also in a basal family, is one of the most<br />
strongly monosymmetric with two stamens and a corolla having a<br />
pouch-like lower lip.<br />
The order includes <strong>plants</strong> of many different life-forms. The Gesneriaceae<br />
are mainly herbs or shrubs with many epiphytes and lianes<br />
and have many evolutionary novelties (see Chapter 3). Dispersal of<br />
the seed in this epiphytic niche has been accompanied by the evolution<br />
of unilocular ovaries with many seeds and parietal placentation.<br />
An alternative adaptation is exhibited by the Acanthaceae (∼4350<br />
species). They have the funiculus (the stalk that attaches the seed to<br />
the fruit) modified into a hook-shaped jaculator, which flings the seed<br />
out. Another family with many lianes but also some large tropical<br />
trees are the Bignoniaceae. They have large flowers and fruits variously<br />
adapted for pollination and seed dispersal especially by birds<br />
and bats.<br />
The Scrophulariaceae was a large diverse family of many showy<br />
monosymmetric flowers that has been split and circumscribed in new<br />
ways as a result of molecular information (see Chapter 8). Less controversially<br />
and perhaps preserving features more representative of<br />
an earlier stage in the evolution of the order are the Verbenaceae<br />
(∼1900 species) (verbena, teak). The two-carpellate pistil has each<br />
carpel divided by an extra wall. Each locule has a single ovule. The<br />
corolla is only slightly zygomorphic but bilateral symmetry is emphasised<br />
by the loss of one stamen in most species. The Lamiaceae (∼5600<br />
species), the labiates, so-named because of their two-lipped flower,<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 247<br />
Figure 5.111. Solanaceae:<br />
Brugmansia.<br />
Figure 5.112. Acanthaceae:<br />
Justicia.<br />
Figure 5.113. Lamiaceae:<br />
Thymus.
248 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.114. Scrophulariaceae:<br />
(a) Digitalis; (b) floral diagram.<br />
Figure 5.115. Aquifoliaceae:<br />
Ilex.<br />
Figure 5.116. Dipsacaceae:<br />
Scabiosa.<br />
have flowers that are usually very strongly zygomorphic and have<br />
either four or two (Salvia) stamens. The four-loculate ovary is divided<br />
into four segments from the top. The style reaches down to the base<br />
of each locule (gynobasic). Each segment is dispersed as a separate<br />
nutlet when the seed is mature. In the Lamiaceae the inflorescence<br />
is a verticillaster with cymose verticals on a main raceme.<br />
The Orobanchaceae, contains most of the parasitic, while the Lentibulariaceae<br />
and Byblidaceae are insectivorous and have sticky and<br />
digestive glands.<br />
boraginales (borages)<br />
Parallel evolutionary trends can be seen in the related family, the<br />
Boraginaceae. Some borages have an entire ovary like the Verbenaceae<br />
whereas others are like the Lamiaceae with a gynobasic style and four<br />
nutlets. Most have polysymmetric flowers arranged in cymes. Echium<br />
is exceptional with its zygomorphic flower.<br />
Euasterid II<br />
The Euasterid II lineage shows varying degrees of aggregation of flowers<br />
into a head in which different flowers may become specialised for<br />
showiness (pseudanthium). They tend to have small epigynous flowers<br />
grouped together in a flat head, exemplified by the umbel of the Apiales<br />
(umbellifers) and the capitulum of the Asterales (daisies). Some<br />
are woody shrubs or climbers but the majority are herbs.<br />
They have a remarkable chemical diversity especially in compounds<br />
that act as herbivore deterrants.<br />
aquifoliales (hollies)<br />
The Aquifoliaceae is by far the biggest family in the Aquifoliales<br />
because of the genus llex (holly) with 400 species of small evergreen<br />
trees or shrubs. Flowers are normally small and unisexual. Most<br />
species are dioecious. Helwingia (three species) and Phyllonoma (four<br />
species) have epiphyllous inflorescences.<br />
dipsacales (teasels and honey-suckles)<br />
The Dipsacales exhibit various stages in the evolution of a pseudanthium.<br />
The valerians (Valerianaceae ∼400 species) have cymose inflorescences<br />
of small florets. The honey-suckles and elders (Caprifoliaceae<br />
∼365 species) have cymose umbels. The teasels and scabiouses (Dipsacaceae<br />
∼250 species), have the most highlly developed capitulate<br />
heads with an involucre of bracts surrounding the head that in some<br />
species have the outer florets modified to be more showy (Scabiosa,<br />
Knautia). In the Dipsacales the head is not bracteate and is basically<br />
cymose.
apiales (umbels and ivy)<br />
There are two families in the order (Apiaceae ∼3100 species and Araliaceae<br />
∼800 species). The Apiaceae is a family that has many species<br />
cultivated for food (carrot) or more frequently for spice (coriander,<br />
cumin, etc.). The compound umbel is so characteristic of the Apiaceae<br />
that the family was one of the first <strong>flowering</strong> plant families to be<br />
clearly recognised as the Umbelliferae (Figure 5.117). However, the<br />
compound umbel is confined to subfamily Apoideae. In some genera<br />
like Hydrocotyle, in subfamily Hydrocotyloideae, the compound umbel<br />
is reduced to a single floret; and in Eryngium, subfamily Saniculoideae,<br />
the inflorescence is rounded rather than flat. The umbel is normally<br />
visited by a range of different insects, which move freely over the<br />
platform of the umbel. In each floret, the ovary has a disc at the<br />
base of the paired styles. The styles are swollen, together forming a<br />
nectariferous stylopodium. Flowers in different parts of the umbel<br />
may be specialised. Those in lateral umbels are sometimes female<br />
and sterile with abortive ovaries and shorter stylopodia. The ovary<br />
has two carpels and matures into a dry fruit, a schizocarp, which<br />
splits into two mericarps joined by a carpophore. Some species have<br />
fruits with thick corky walls so that they float, have wings for wind<br />
dispersal, or have hooks for animal dispersal. The Apiaceae have a<br />
particularly effective combination of anti-herbivore chemical repellants<br />
such as polycetylenes and sesquiterpene lactones. The Araliaceae<br />
share many similarities with the Apiaceae but are mainly tropical and<br />
rarely form regular compound umbels. They also produce a fleshy<br />
fruit.<br />
asterales (daisies, lobelias and bellflowers)<br />
Most species of Asterales are in one family, the Asteraceae, which is<br />
widely recognised as one of the most advanced families of <strong>flowering</strong><br />
<strong>plants</strong> (Figure 5.121). Its origin is relatively recent but it has 1100 genera<br />
and 20 000 species. All species have a capitulum, a head with many<br />
florets on a common and usually flattened receptacle, and surrounded<br />
by an involucre of bracts. The involucre is particularly obvious in<br />
‘everlasting’ flowers like Helichrysum, where it is showy. Florets are<br />
tubular and epigynous with a single ovule. They are protandrous and<br />
mature centripetally (i.e. the capitulum is racemose). The florets are<br />
of different sorts, either polysymmetric or monosymmetric. The latter<br />
have either three corolla lobes that are connate and greatly expanded<br />
to form a strap, or all five corolla lobes extending in one direction.<br />
Florets may be pistillate, hermaphrodite or functionally staminate.<br />
Heads are made of a single kind of floret or a combination of kinds,<br />
commonly with central disk of hermaphrodite, polysymmetric florets<br />
and a margin of showy, monosymmetric ray florets, with the latter<br />
pistillate or sterile, as in the daisy and sunflower. One group, which<br />
includes lettuce and dandelion, have all florets with all five corolla<br />
lobes forming the strap.<br />
Florets are protandrous. Anthers form a tube. Pollen is shed into<br />
the anther tube and then the immature style elongates and pushes<br />
<strong>5.3</strong> CLASS MAGNOLIOPSIDA <strong>–</strong> FLOWERING PLANTS 249<br />
(a)<br />
(b)<br />
Figure 5.117. Apiaceae: (a)<br />
Heracleum; (b) floret in section<br />
showing stylidium and schizocarp.<br />
(a)<br />
(b)<br />
Figure 5.118. Structure of a<br />
capitulum (a) capitulum t.s.<br />
showing central and marginal<br />
florets; (b) florets of diverse sorts.
250 ORDERING THE PATHS OF DIVERSITY<br />
Figure 5.119. Tubular central<br />
floret showing inferior ovary and<br />
the calyx converted into a bristly<br />
pappus.<br />
Figure 5.120. Pollen<br />
presentation mechanism. Florets<br />
are protandrous and the piston<br />
like style pushes pollen on to the<br />
surface before the stigma lobes<br />
open to become receptive.<br />
Figure 5.121. Diverse<br />
Asteraceae: (a) Bidens with<br />
marginal strap florets mimicking a<br />
5-petalled flower; (b) Centaurea<br />
with marginal expanded disk<br />
florets; (c) Galactites with a showy<br />
involucre of bracts; (d) Cynara with<br />
a head of disk florets and a spiny<br />
involcre; (e) Solidago spikes of small<br />
ligulate heads; (f) Echinops with a<br />
head of capitula each with a single<br />
floret.<br />
the pollen onto the surface of the capitulum. Later the stigmatic lobes<br />
open. The fruit of the Asteraceae is usually crowned by a pappus<br />
derived from the calyx. The pappus in Asteraceae is very variable,<br />
either absent or cup-like, or with scales, bristles, simple or feathery<br />
hairs, which are barbed, or glandular. The fruit, called a cypsela, is a<br />
kind of achene of an inferior ovary. The dispersal adaptations of the<br />
fruit contribute to the success of many species as weeds.<br />
The features described above have evolved in many groups outside<br />
the Asteraceae. Many of the structures of a capitulate infloresecence<br />
found in the Asteraceae are paralleled in other families in the Asterales<br />
such as the Goodeniaceae (∼300 species), which has an indusium,<br />
the Calyceraceae (∼55 species), Lobeliaceae (∼1200 species) and<br />
Campanulaceae (∼600 species). Phyteuma and Jasione, in the Campanulaceae,<br />
both have capitulate inflorescences surrounded by an involucre<br />
of bracts. Jasione has a kind of primitive ‘pseudo-indusium’ formed<br />
by swollen stigmatic lobes. The one species of the monotypic Brunoniaceae,<br />
which is remarkably similar in appearance to Jasione and is<br />
sometimes put in the Goodeniaceae, shows a further parallel in having<br />
an involucrate head, though the head is cymose and the florets<br />
are hypogynous. The piston-like mode of pollen presentation in the<br />
Asteraceae also has parallels with that in the Campanulaceae (Physoplexis)<br />
and Lobeliaceae.<br />
The repeated evolution of these features argues strongly that<br />
they are adaptive. One advantage of having a capitulum, for example,<br />
is the protection given to the ovule and seed. Functionally it<br />
provides a large showy target for pollinators and yet each ovule is<br />
packaged separately, as a defence against predators and for dispersal.<br />
There is a lot of diversity in the size and number of florets that<br />
capitula contain. There are the familiar huge capitula of the sunflowers,<br />
which have been selected by plant breeders. At the other end of<br />
the spectrum many species have capitula containing very few florets.<br />
(a) (b) (c)<br />
(d) (e) (f)<br />
(c)
Frequently species with small capitula have the capitula grouped in<br />
some way. In Solidago the capitula are arranged in spikes. In Achillea<br />
they are grouped in a corymb. In Echinops each capitulum only has<br />
a single floret and they are grouped in a globular head with its own<br />
involucre.<br />
However, because these features are also present outside the Asteraceae<br />
they do not explain the particular evolutionary success of the<br />
Asteraceae. The Asteraceae have an unusual multiallelic, homomorphic,<br />
sporophytic, self-incompatibility that has maintained high levels<br />
of genetic diversity among individuals and populations. However, they<br />
are also flexible in their breeding system and many weedy species are<br />
secondarily self-compatible and self-pollinating. One example is the<br />
ubiquitous groundsel, Senecio vulgaris, which, no longer needing to<br />
attract pollinators, lacks the ray florets of its relatives. Polyploidy,<br />
sometimes following hybridisation, seems to play a significant role in<br />
the evolution of the weedy species, especially since it can destroy the<br />
self-incompatibility. Another aspect of their evolution is the chemical<br />
diversity associated with the deterrence of herbivores, especially in<br />
alkaloids. One of the largest genera of all <strong>plants</strong>, the ragworts (Senecio),<br />
with over 1500 species, has its own peculiar type of alkaloid.<br />
Further reading for Chapter 5<br />
Beck,C.B.Origin and Evolution of Gymnosperms (New York: Columbia<br />
<strong>University</strong> Press, 1988).<br />
Bold, H. C., Alexopoulos, C. J. and Delevoryas, T. Morphology of Plants and<br />
Fungi, 5th edition (New York: Harper and Row, 1987).<br />
Heywood, V. J. (ed.) Flowering Plants of the World. (Oxford: Oxford <strong>University</strong><br />
Press, 1978).<br />
Mabberley, D. J. The Plant Book (<strong>Cambridge</strong>: <strong>Cambridge</strong> <strong>University</strong> Press, 1987).<br />
Schofield, W. B. Introduction to Bryology (New York: Macmillan, 1985).<br />
Shaw, A. J. and Goffinet, B. Bryophyte Biology (<strong>Cambridge</strong>: <strong>Cambridge</strong><br />
<strong>University</strong> Press, 2000).<br />
Stewart, W. N. Paleobotany and the Evolution of Plants (<strong>Cambridge</strong>: <strong>Cambridge</strong><br />
<strong>University</strong> Press, 1983).<br />
Takhtajan, A. Evolutionary Trends in Flowering Plants (New York: Columbia<br />
<strong>University</strong> Press, 1991).<br />
Thomas, B. A. and Spicer, R. A. (1987) The Evolution and Palaeobiology of Land<br />
Plants (London: Croom Helm, 1987).<br />
Willis, K. J. and McElwain, J. C. The Evolution of Plants (Oxford: Oxford<br />
<strong>University</strong> Press, 2002).<br />
FURTHER READING FOR CHAPTER 5 251<br />
Figure 5.122. Campanulaceae<br />
(a) Campanula; (b) Jasione.