5.3 Class Magnoliopsida – flowering plants - Cambridge University ...

5.3 Class Magnoliopsida – flowering plants 

There are more than 250 000 species of flowering plant. Alternative 

names for them include the angiosperms, Angiospermopsida. 

5.3.1 Distinguishing features 

DNA sequence data indicate that the flowering plants are a monophyletic 

group with a separate origin from all gymnosperms. Characteristics 

that most flowering plants share but which are rare or 

absent in other groups and which also support the view that they 

are monophyletic (synapomorphies) include the following: 

differentiated xylem tissue including fibres, parenchyma and usually 

vessel elements 

phloem sieve-tube elements and companion cells formed from a 

common mother cell 

reaction wood produced in branches as a response to tension is 

made up of gelatinous fibres in an adaxial part of the xylem (in 

contrast to the abaxial rounded tracheids produced as a response 

to compression in conifers) 

leaves broad and flat with a distinct petiole 

leaves with pinnate secondary veins and fine veins reticulate and 

veinlets ending blindly 

a high degree of plasticity in vegetative growth 

bisexual reproductive axis (-flowers) with male organs situated 

below the female 

insect pollination 

pollen wall chambered (tectate) 

male gametophyte with three nuclei only 

carpel or pistil enclosing the ovules and fruit surrounding the seed, 

although in primitive flowering plants the carpel is sealed by a 

secretion only 

female gametophyte normally with eight nuclei only 

double-fertilisation and formation of the triploid endosperm 

‘direct’ development of the embryo from the zygote, i.e. with no 

intervening free-nuclear proembryo phase 

Not all of these features are found in all flowering plants and some 

are found in a few other plant groups. 

5.3.2 Fossil record and origin 

The origin of flowering plants, Darwin’s ‘Abominable Mystery’, is conjectural. 

There is scarcely any hard evidence of their origin before 

the Cretaceous (135 million years ago) but molecular data indicate a 

much more ancient origin for the lineage that eventually gave rise 

to flowering plants. There are intriguing fossils of flower-like structureslikethoseofArchaefructus, 

an aquatic plant of 124 million years 

ago, but the first undoubted flowering-plant fossils are pollen from 

equatorial latitudes of the Late Early Cretaceous, 125 million years 

5.3 CLASS MAGNOLIOPSIDA – FLOWERING PLANTS 223 

(a) 

(b) 

(c) 

Figure 5.53. Floral diversity: 

(a) Houttuynia; (b) Aristolochia; 

(c) Allium.

224 ORDERING THE PATHS OF DIVERSITY 

Figure 5.54. Models of the 

origin of the angiosperm flower. 

ago. The flowering-plants probably originated in the tropics; extant 

primitive families are tropical or subtropical and early fossils show 

no adaptations to temperate conditions. 

There have been a number of theories about the origin of flowers, 

homologising in different ways floral structures with the reproductive 

structures of other plants. One theory, the anthophyte theory, 

related flowers to the bisexual flower-like axes present in groups 

such as the extinct Bennettitales and living Gnetidae. Two contrasting 

theories are the euanthium theory, which derives a flower from 

a uniaxial cone bearing both micro- and megasporophylls, and the 

pseudanthium theory, which derives the hermaphrodite flower from 

a complex inflorescence of unisexual male and female flowers. However, 

these different theories place undue emphasis on a fundamental 

distinction between organ types. It has also been suggested 

that a flower-like structure could have arisen by a change of sex of 

some of the microsporophylls of a male cone, a process called gamoheterotropy. 

This process is not hard to imagine in plants that are 

hermaphrodite and where the determination of sex is a developmental 

phenomenon that is only rarely associated with sex chromosomes. 

A similar transfer of sex has been proposed in the more recent evolution 

of the maize cob. This theory has received recent support 

from a study of the genes of reproductive development. The gene 

determining the sex of floral organs in flowering plants is a homologue 

of a gene active in the male axis of conifers but not the female 

axis. 

The first known leaves of flowering plants appear in fossils dated 

at 125 Ma and the first fossil inflorescence has been dated at 120 Ma. 

fossil has a mosaic of characteristics that are found in a range of basal 

groups of flowering plants. This has a female inflorescence, with a 

bract and two bracteoles at the base of female flowers that lack a 

perianth (achlamydeous flowers). The flowers are effectively just tiny 

pistils, less than 1 mm in diameter. 

5.3.3 The evolutionary radiation of flowering plants 

There are many threads to the evolutionary diversification of flowering 

plants: 

specialisation for pollination, including a reversal to wind pollination 

as well as more and more bizarre adaptations to attract specialist 

animal pollinators 

specialisation in fruit and seed dispersal, such as fleshy or otherwise 

elaborated fruits, and the evolution of seed dormancy 

adaptations for seedling establishment and growth in competition 

with other plants 

specialisation for growing in different habitats from aquatic to arid 

terrestrial habitats, surviving heat, cold and low light levels, or 

growing as epiphytes and parasites 

adaptations conferring resistance to or prevention of herbivory

Insect pollination is closely associated with the origin and subsequent 

diversification of flowers. However, it is important to remember 

that insect pollination is associated with several other groups of seed 

plants, both living and extinct: Bennettitales, Gnetales, Cheirolepidaceae 

(extinct conifers), Cycadales and Medullosales (seed ferns). 

Insects grew in diversity with the origin of seed plants in the Late 

Devonian and this increase of diversity with the origin of flowers 

is just part of a continuous trend. Indeed there is some evidence 

to indicate a temporary decline in insect diversity as flowering plants 

became more abundant in the Cretaceous. Nevertheless, flowers diversified 

in parallel with particular groups of pollinator specialists, 

the bees (Apoidea/Apidae), the pollen wasps (Vespidae: Masarinae), 

brachyceran flies (Acroceridae, Apioceridae, Bombyliidae, Empididae, 

Nemestrinidae, Stratiomyidae and Syrphidae) and the moths and butterflies 

(Lepidoptera). The evolution of a bisexual reproductive axis 

was a crucial event. 

Several trends in floral evolution can be discerned. Primitive flowerseitherlackaperianthorhaveoneinwhichthereisasingle 

whorl of tepals. In the perianth there has been a trend from having 

a perianth in which distinct whorls are not clearly differentiated to 

clear specialisation of a distinct calyx and corolla. The calyx protects 

against drought, temperature shock and predatory insects and the 

corolla attracts and controls pollinators. Both calyx and corolla may 

have been derived from tepals, but it is likely that in some groups, 

such as the buttercups, the petals originated from stamens, to which 

they are anatomically similar. In the peony, Paeonia, there is a gradual 

transition from leaves, through modified leaves on the flowering stem 

called bracts, into the perianth with parts at first sepal-like and then 

petal-like. Generally there has been a trend for the greater integration 

of the floral parts with greater precision in number and placement as 

flowers have become specialised to particular patterns of pollination. 

An important aspect of the diversification of flowering plants and 

their evolutionary success has been their vegetative flexibility. They 

have evolved into a bewildering range of forms through the activity 

of sub-apical and intercalary meristems. Flowering plants also have 

a greater capacity for elongation of cells, including root hairs and 

trichomes. One of the most important vegetative specialisations has 

been their possession of vessels in their wood, permitting more efficient 

water transport and hence greater photosynthetic rates, fast 

growth rates and earlier maturation. There have been significant 

adaptations in seasonal habitats; for example, although a few nonflowering 

plants are deciduous this has been a particular floweringplant 

trait that has adapted them to high latitudes or seasonally-dry 

environments. 

One aspect to the burgeoning biotic diversity of flowering plants 

from the Cretaceous onwards was their great expansion of chemical 

diversity. New ranges of what have been called secondary compounds 

provided attractants for pollinators or seed dispersers, and repellants 

and toxic compounds to inhibit herbivores. 


(a) 

(b) 

Figure 5.55. Specialisation in 

the perianth in the Asparagales: 

(a) Belamcanda; (b) Kniphofia.


Figure 5.56. Phylogeny of the 

flowering plants from the 

Angiosperm Phylogeny Group 

(APG). In some respects the basal 

clades of flowering plants are only 

linked together by what features 

they lack. These were called by 

some the ANITA group from the 

initial letters of their family names. 

The last families belong in the 

same order (the Schisandrales), 

and the first two perhaps ought to 

be recognised as orders on their 

own (the Amborellales and 

Nympheales). 

5.3.4 The phylogeny of flowering plants 

Ideas about the phylogenetic history of the flowering plants have 

been revolutionised in recent years by the cladistic analysis of DNA 

sequence data (Figure 5.56). What is presented here is as up to date as 

at time of press. Classifications are achieved by consensus and gain 

currencybyuse.

5.3.5 Basal flowering plants and Eumagnoliids 

At the base of the flowering-plant phylogenetic tree there is a diverse 

group of families a ‘grade rather than a clade’ with a high proportion 

of primitive or unspecialised character states, fascinating 

because they represent the relicts of an early stage of flowering plant 

evolution. 

Table 5.4 The distribution of features of flowering plants 

In basal groups 

(primitive/plesiomorphic/unspecialised) 


Features in high frequency 

In derived groups 

(advanced/apomorphic/specialised) 

Small shrubs, lianes or rhizomatous perennial 

herbs, or aquatics and semi-aquatics 

Various but including annuals and tall trees and herbs 

Vesselless Vessels present 

Parts in whorls or spirals of variable numbers Parts in whorls of three, four or five 

Actinomorphic Actinomorphic or Zygomorphic 

Parts free Parts connate or adnate 

Stamens broad with poor differentiation between 

filament and anther 

Stamens with well-differentiated filament 

Apocarpous Syncarpous 

Unsealed stigma and poorly differentiated style Sealed stigma with well-differentiated style 

Follicles Fruits (various) 

amborellales 

DNA sequence data places a plant called Amborella trichopoda from 

New Caledonia in a basal position as a sister to all other flowering 

plants. This is not to say that it is the ancestor of all other flowering 

plants. Rather that it is the closest living relative of the ancestor and 

like all other living flowering plants it exhibits a mixture of primitive 

and derived features. It is a shrubby evergreen plant with simple 

leaves that may be lobed. It has tracheids but no vessel elements. It is 

dioecious with flowers grouped in axillary cymose inflorescences. It 

has a perianth consisting of five to eight undifferentiated segments 

that are weakly joined at the base and are arranged in a spiral. The 

male flower has numerous (10--25) laminar stamens, the outer fused 

to the base of the perianth segments. Pollen is aperturate to nonaperturate 

and sulcate with a granulate outer wall and is possibly 

not tectate, a feature of possibly great significance. The female flower 

has five to six free carpels in a whorl. Carpels are open at the tip 

and have one ovule. Seeds are endospermic and the embryo has two 

cotyledons. 

(a) 

(c) 

(b) 

Figure 5.57. Amborella: (a) plant; 

(b) male flower; (c) female flower.


(a) 

(b) 

Figure 5.58. Nymphaeales: (a) 

Nymphaea; (b) Cabomba. 

Figure 5.59. Schisandra. 

nymphaeales (water-lilies) 

The next most-basal flowering plants are either six genera and about 

40 species of water-lilies, with large flattened floating leaves and large 

bowl-shaped flowers in the Nymphaeaceae, or two genera (Brasenia 

and Cabomba) of waterweeds with floating stems and relatively small 

simple and unspecialised flowers in the Cabombaceae (Figure 5.58). 

The Nymphaeales have a mixture of unspecialised, probably primitive 

features, and specialised features such as abundant aerenchyma 

tissue, adapting them to their aquatic habitat. Vessel elements have 

been recorded in some species but these are not like those of other 

flowering-plants. The pollen has a tectum of sorts but its inner layer, 

the endexine, is compact and lacks the columellate appearance of all 

other flowering-plants. Some features are shared with the monocots: 

the primary root is soon aborted and the stem has scattered closed 

bundles. The showy petals have originated as sterile stamens (staminodes). 

The families differ in the degree of fusion of the carpels; laterally 

fused in Nymphaeaceae and free in Cabombaceae. Flowers are 

normally hermaphrodite with only a weak distinction between sepals 

and petals. Sepals and petals are arranged in a spiral. The largeflowered 

water-lilies Nymphaea, Victoria and Nuphar are specialised for 

beetle pollination. It is remarkable that similar looking aquatic plants 

have evolved convergently in the distantly related Nelumbo (Proteales) 

and Nymphoides (Asterales). 

schisandrales (including illiciales and 

austrobaileyales) 

The Schisandrales include four families of small trees and scrambling 

shrubs or lianes. austrobaileyaceae: These are lianes. Austrobaileya 

scandens, one of only two species in this family, grows in 

NE Australia, and has flowers that smell of rotting fish. The flowers 

have 12 perianth segments, 6--11 laminar stamens, with sterile stamens 

(staminodes) inside surrounding the 6--9 free carpels. trimeniaceae: 

There are only two genera, Trimenia and Piptocalyx, with a 

total of five species, of small trees and scrambling shrubs found from 

SE Asia to Australia. They are monoecious with small wind-pollinated 

flowers. The female flower has a single carpel and the male flower 

6--25 stamens in pairs. illiciaceae: There is only one genus, Illicium, 

with 42 species of trees and shrubs found in SE Asia and USA, 

Mexico and the Caribbean. Flowers may have 12 or more perianth 

segments, and 7--15 carpels. Illicium has peppery tasting leaves, and 

produces a star-shaped unripe fruit called star-anise that is used as a 

spice. schisandraceae: There are only two genera, Schisandra and 

Kadsura, and a total of 47 species of lianes and twining shrubs, found 

in East Asia and eastern North America. The Illiciaceae and Schisandraceae 

share some chemical features, a primary vascular cylinder 

and tricolpate pollen. Unlike the previous two orders some members 

have clearly formed vessel elements although of a primitive sort, with 

sclariform (ladder-like) perforations.


eumagnoliids 

The eumagnoliids include several dicot orders as well as the monocots 

(see below). They include many species that exhibit some primitive 

features they share with basal monocotyledons: whorls of floral parts 

in threes (trimerous), monosulcate/uniaperturate pollen, apocarpous 

flowers. 

Like the ANITA grade of flowering plants many of these eumagnoliids 

are aromatic and include, for example, Lindera -- allspice, Piper 

-- pepper, Cinnamomum -- cinnamon, Aniba -- bois-de-rose oil, and Sassafras 

with its scented insecticidal oil. Another useful plant in this 

group is Persea, the avocado. Figure 5.60. Chloranthus. 

chloranthales 

The Chloranthaceae (Figure 5.60) is the only family. It has about 75 

species, most in the genus Hedyosmum. They are shrubs, lacking vessel 

elements. Their wood is soft and their swollen internodes sometimes 

collapse on drying. Flowers are very small and unisexual, with a single 

stamen or carpel and with or without a single whorl of three perianth 

segments. 

piperales (peppers and birthworts) 

It is hard to believe that Aristolochia, with its extraordinary tubular 

trap blossom belongs in the same order as Piper with its spikes of tiny 

flowers that lack a perianth altogether, along with Hydnora, which 

has flowers that arise from buds in the roots. The Aristolochiaceae 

are woody vines, the Piperaceae are herbs and the Hydnoraceae are 

root parasites that lack chlorophyll. 

laurales (laurels) 

There are seven families, most of which are trees, shrubs or lianes 

but Cassytha, in the Lauraceae, is a genus of twining, almost leafless 

plant parasites like the dodders (Cuscuta). They have flowers that vary 

from small to large (Figure 5.61). 

magnoliales (magnolias) 

Generally small trees or shrubs and lianes, most having relatively 

large showy flowers (Figure 5.62). The large flowers have a poorly 

differentiated perianth and quite often a variable number of segments. 

In addition some have partially sealed carpels and diverse but 

commonly broad stamens with a weakly distinguished filament and 

valvate anthers. 

canellales 

There are only two families of evergreen shrubs and trees, the Canellaceae 

and Winteraceae, in the order. Drimys, in the Winteraceae, has 

a peppery taste. The flower has variable numbers of perianth segments, 

flat stamens with a poorly differentiated filament and only 

weakly sealed carpels. The Canellaceae have the stamens fused in a 

ring. 

Figure 5.61. Laurus. 

Figure 5.62. Magnolia. 

Figure 5.63. Drimys.


Figure 5.64. Acorus. 

(a) 

(b) 

Figure 5.65. Alismatales: 

(a) Butomus; (b) Lysichiton. 

5.3.6 Monocots 

The monocots represent by far the most evolutionarily successful 

lineage of the eumagnoliids and represent about 25% of all flowering 

plants, about 50 000 species. They are very diverse ranging from 

tall palm trees to tiny aquatic plants like Lemna (duckweed). Numerically 

they might be considered less important than the eudicots, 

but they include the grasses that provide the great majority of food 

for humans, either directly (wheat, rice, millet, etc.) or by feeding 

domestic grazing animals. They are clearly a monophyletic group and 

most have: 

single cotyledon 

sympodial growth (the palms Arecales are monopodial) 

linear, parallel-veined leaves in which the leaf base surrounds the 

stem 

primary root soon aborts and a wholly adventitious root system 

develops 

closed vascular bundles and lack of interfascicular cambium 

flower parts in threes 

pollen development (microsporogenesis) successive 

monosulcate pollen 

The basal monocots are aquatic or semi-aquatic plants. As the 

seedling germinates the single cotyledon elongates to push the primary 

embryonic root, the radicle, out of the seed and down into 

the wet mud. It is because of their lack of a vascular cambium that 

monocots are well able to undertake elongating growth but poor at 

thickening (secondary) growth. Elongating growth adapts them for 

the aquatic, climbing and epiphytic niches where they predominate, 

and also permits them to re-grow rapidly after grazing. The parallel 

venation of their leaves is a consequence of the extension of the 

leaves from a basal meristem. Indeed it is likely that parallel-veined 

monocot leaves are homologous to the petiole of leaves in other flowering 

plants. Monocot trees and shrubs undergo various different and 

amorphous kinds of stem thickening, which is sometimes described 

as anomalous, and they are usually either unbranched or only weakly 

branched. 

acorus (sweet-flag or calamus) 

At the base of the monocot lineage are the two species of Acorus, a 

rooted aquatic. It has a tiny bisexual flowers crowded in a spadix. 

The carpels are primitive, intermediate between the ascidiate of the 

ANITA group and folded ones of other flowering plants. It has been 

utilised for centuries as a rush for floor covering because of its spicy 

scented properties.

alismatales (water weeds and aroids) 

There are several distinct aquatic families in this order linked by many 

adaptations to the aquatic habitat (floating stems, aerenchyma) as 

well as tricolpate pollen, and several have stamens produced in pairs, 

but even among these there are distinct rush-like forms (Butomaceae) 

as well as free-floating and submerged ones (especially the Alismataceae, 

Potamogetonaceae and Hydocharitaceae). Unlike most monocots, 

several of these aquatic families have a leaf that is clearly petiolate, 

a condition that is also present in the main terrestrial family of 

the order, the Araceae. Here also are found the few flowering plants 

to have entered the marine habitat, in the Zosteraceae and Posidoniaceae. 

The Alismataceae and Limnocharitaceae produce latex. The 

Juncaginaceae have a fascinating reversal of floral whorls with an 

inner perianth whorl inside the outer stamens. It is likely that the 

Araceae have an aquatic origin but they are mainly terrestrial now. 

The few aquatic Araceae are very specialised, free-floating ones in 

the subfamily Lemnoideae, including the smallest flowering plant 

Wolffia, aswellasLemna and Pistia. Most Araceae are vines and epiphytes 

and form an important component of tropical and subtropical 

forests. Like Acorus the Araceae have a spadix associated with a large 

spathe. 

asparagoids dioscoreales (yams) 

The Burmanniaceae are mycotrophic, effectively saprophytic by utilising 

fungi to garner nutrients and energy. Some even lack chlorophyll 

and have only scale-like leaves. The Dioscoreaceae are yams, from 

the African word nyami, twiners with thick rhizomes or tuber-like 

swellings, sometimes many kilograms in size, and net-veined leaves. 

They are exceptional in some having secondary thickening, although 

it occurs in the tubers! There are only four genera but about 900 

species. 

pandanales (screw pines) 

The Pandanaceae, increase their girth as each node is produced so 

that the ‘trunk’ is balanced on a conical base, but supported by 

profuse, thick, strut-like roots. Because of the spiral way they producetheirleavestheyseemtoscrewtheirwayuptowardstheforest 

canopy. The young fibrous leaves of Carludovica are divided into strips 

and bleached with lemon juice to be made into Panama hats. Pentastemona 

in the Stemonaceae stands out as a monocot with three whorls 

of five parts. They also have petiolate leaves. The Pandanales, like several 

other monocot orders, have their own family (the Triuridaceae) 

of echlorophyllous mycotrophs. 


(a) (b) 

Figure 5.66. Dioscorea: 

(a) twining stem; (b) root tubers. 

Figure 5.67. Pandanus.


Figure 5.68. Lilium. 

Figure 5.69. Veltheimia 

bracteata (Eastern Cape Province, 

South Africa). 

liliales (lilies) 

In contrast to the superficially similar Asparagales, the Liliales tend 

to have nectaries at the base of the floral parts, spots on the petals 

and stamens with anthers opening to the outside (Figure 5.68). Other 

features like the cellular structure of the seed coat also link them. 

Like the amaryllids they include many geophytes, producing bulbs 

(Liliaceae, Melanthiaceae) or corms or rhizomes (Colchicaceae). Some 

are shrubby or vines (Philesiaceae, Smilacaceae). They also include 

chlorophyll-lacking mycotrophs in the Corsiaceae. There are four 

main lineages: (1) Smilacaceae with Liliaceae; (2) Alstroemeriaceae 

with Luzuriagaceae and Colchicaceae; (3) Campynemataceae; and 

(4) Melanthiaceae. 

asparagales (amaryllis, irises and orchids) 

The Aparagales is the largest order of monocots and includes many 

beautiful flowers (Figure 5.69). The Asparagales have a seed coat in 

which cellular structure has become obliterated and there is a black 

crust of phytomelan. Most Asparagales are rhizomatous herbs but a 

few such as the Ruscaceae are shrubs (Ruscus) oreventrees(Dracaena). 

Many of the most beautiful flowers are in a lineage of bulb-formers 

that includes the Amaryllidaceae, Agapanthaceae and Alliaceae. Many 

of these produce flowers in an umbel with a spathe at its base. The 

Iridaceae (irises and crocus) are in a distinct lineage and are distinguished 

by their divided style. Another interesting feature is that 

they do not produce root-hairs and rely entirely on mycorrhizae for 

garnering nutrients from the soil. The irises have a flower that provides 

three distinct entry points for pollinators, and a petaloid stigma 

overarching each of the three stamens. Another interesting lineage 

is one including tussock formers and trees in the Xanthorhoeaceae 

(grass-trees) and Asphodellaceae the aloes (Aloe) and red-hot pokers 

(Kniphofia). 

(a) (b) (c) (d) 

Figure 5.70. (a) Watsonia, (b) Tulipa, (c) Xanthorhoea, (d) Moraea.


The orchids are the largest family of the order and comprise the 

most diverse and remarkable of all flowering plant families with 

between 800 and 1000 genera and up to 20 000 species, rivalled in 

numbers only by the Asteraceae. Orchid flowers have a complex architecture 

and are exceedingly diverse in the structure and arrangement 

of the various parts of the column, rostellum and pollinaria and in 

the shape, colour, and scent of the perianth. Figure 5.71. Floral diagrams of 

the three main patterns of orchid 

architecture. The Apostasioideae 

are a small SE Asian subfamily with 

three stamens. The 

Cyprepediodeae are the slipper 

orchids. The monandroid orchids 

are by far the most numerous and 

include several families of both 

terrestrial orchids (mainly 

Orchidoideae and Spiranthoideae), 

and epiphytic orchids (mainly 

Epidendroideae) and even lianes 

(Vanilloideae). 

(a) (b) (c) 

Figure 5.72. Oncidium sp. are 

epiphytic orchids commonly 

cultivated for their beautiful 

flowers. 

Figure 5.73. Three kinds of 

terrestrial orchids: (a) Anacamptis 

laxiflora; (b) Neottia nidus-avis lacks 

chlorophyll and is mycotrophic; (c) 

a slipper orchid, Phragmipedium sp.


Figure 5.74. Palm 

inflorescences. 

Figure 5.75. Bromeliad 

inflorescence, showing the 

disticous arrangement of bracts 

and flowers common in the Poales. 

Commelinids 

arecales (palms) 

The Arecaceae palms are the most important group of monocot trees 

but, like other monocot trees, they are unbranched or only weakly 

branched and lack a vascular cambium (Figure 5.30). Rather, secondary 

growth occurs by the expansion, and occasional dichotomous 

splitting, of a large apical meristem. The leaves of palms are large and 

complex, often palmate or pinnately lobed, and highly folded. Their 

flowers are relatively simple, following a standard monocot pattern 

with parts in threes, but often they are grouped together in massive, 

profusely branched inflorescences. About six subfamilies have been 

distinguished based on the form of the leaf (fan or feather) and its 

folding (rib down -- induplicate, or rib up -- reduplicate). 

poales (grasses, sedges and rushes) 

The Poales dominate most ecosystems where the growth of trees is 

limited. The Typhaceae, Juncaceae and Cyperaceae are common dominants 

of semi-aquatic and water-logged conditions. In contrast the 

Poaceae dominate moist or dry habitats in many climates, everywhere 

tree growth is restricted for any reason. They tolerate fire and grazing, 

even mowing, regrowing rapidly after they have been damaged. 

Grasses have a diverse of photosynthetic mechanism; C4 photosynthesis 

is common and has evolved many times over, permitting them to 

grow rapidly in the tropics. 

The Xyridaceae have showy flowers. The Bromeliaceae have showiness, 

but it is mostly provided by colourful bracts. The bromeliads are 

very important epiphytes in the Americas as tank-plants or air-plants. 

Their flowers are relatively unspecialised, but commonly each flower 

is produced at the base of a brightly coloured bract. Most are epiphytic 

and show many adaptations for their epiphytic life-style such 

as water storage tissue or water-absorbing peltate scales. The air-plants 

Tillandsia, including Spanish moss, are perhaps the most remarkable 

in their ability to absorb moisture from the air. Most of the ground 

bromeliads have adaptations for arid environments.

Most other families in the order have reduced and relatively inconspicuous, 

often unisexual, flowers adapted for wind pollination by 

having an inflorescence held on a long leafless scape above the leafy 

rosettes. This trend has occurred in several of the constituent lineages. 

For example, sister to the Xyridaceae, are the mostly wind 

pollinated Eriocaulaceae, with similar dense heads of flowers. A few 

species in the Eriocaulaceae are insect pollinated with nectariferous 

petals. Some species are monoecious with female only marginal flowers 

in the head and central ones male. Other species are dioecious. In 

the Typhaceae the male part of the spadix is above the female. The 

Juncaceae have a typical monocot perfect flower, but it has a green 

or brown and papery perianth. They are wind pollinated, and a few 

are dioecious, but some have become secondarily insect-pollinated 

although they lack nectaries. In their sister family the sedges, Cyperaceae, 

the perianth is reduced to scales or bristles or is absent. Some 

genera, such as Scirpus, have hermaphrodite flowers. Others, including 

the largest genus Carex, have unisexual flowers with male and 

female flowers in different parts of the inflorescence. The male flowers 

consist only of three stamens with a bract called the glume on the 

abaxial side. A glume is also present in the female flower. In addition, 

two inner glumes have become fused to form a bottle-shaped perigynium 

or utricle surrounding the pistil. The style protrudes through 

the opening in the utricle. 

In the Poaceae, the grasses, the flowers are similarly reduced 

and adapted for wind pollination (Figure 5.78). They are grouped 

alternately side-by-side in spikelets. The primitive floral condition 

for grasses is retained by the bamboos (Bambusoideae). They have 

simple, often trimerous spikelets and they may have three lodicules 

and three stigmas. Progressive reduction has given rise to the standard 

grass floret pattern. Ampelodesmus, a primitive member of the 

advanced subfamily Pooidae also has three lodicules. At the base of 

the spikelet are two glumes that protect the spikelet in development. 

Each floret in the spikelet is enclosed by two other bracts; the lower 

is the lemma and the upper is the palea. Within the floret there 

are usually three stamens and a pistil with two feathery stigmas. At 

the base of the pistil there are two tiny lodicules, remnants of the 

perianth, which swell to push open the floret or shrink to allow it 

to shut. Cross-pollination is ensured not by separation of the sexes 

but by the different time of pollen release and stigma receptivity 

and, most importantly, by a unique form of self-incompatibility. The 

fruit is an achene with the seed fused to the fruit wall (a caryopsis). 

It is usually shed enclosed within the lemma and palea, which 

may be modified to aid dispersal. Threshing releases the grain from 

this chaff. The Poaceae is one of the most successful of all flowering 

plant families because of its ability to spread laterally by rhizomes 

and stolons or the production of herbivory, drought- and fire-resistant 

tussocks. 


Figure 5.76. Xyridaceae. 

(a) (b) 

Figure 5.77. Cyperaceae 

florets: (a) female; (b) male. 

(c) 

(d) 

(b) 

(a) 

Figure 5.78. Poaceae: grass 

inflorescence: (a) plant with 

paniculate inflorescence, 

(b) spikelet, (c) distichous 

arrangement of florets in spikelet, 

(d) single floret with three dangling 

stamens and two feathery stigmas.


Figure 5.79. Commelinaceae: 

Tradescantia gigantia, an upright 

form (Enchanted Rock, Texas). 

Figure 5.80. Zingiberaceae: (a) 

Zingiber; (b) Heliconia. 

commelinales (spiderworts) 

The Commelinales are mainly herbs like the Haemodoraceae and 

Commelinaceae (Figure 5.79) but include the aquatic Pontederiaceae. 

Frequently the flowers are zygomorphic (monosymmetric). Heteromorphic 

flowers are found in the Commelinaceae (enantiostyly) and 

Pontederiaceae (heterostyly). 

zingiberales (gingers) 

The Zingiberales may be regarded as plants that do not produce an 

aerial stem except when flowering (Figure 5.80). They have showy, 

often strongly zygomorphic flowers especially adapted to bird pollination. 

The form of the flower with a large lip (labellum) is only 

exceeded in complexity in the monocots by the orchids. Sepals 

and petals are fused (connate) and sterile stamens (staminodes) are 

petaloid. The different families illustrate a great variety of specialisations 

for pollination in the tropics. The bananas, Musaceae, are 

the largest of all herbs, though they look like trees. The gingers, 

Zingiberaceae, include more than a thousand species of tropical 

herbs. In some ways the flower is analogous to that of the orchids 

with its pronounced zygomorphy and adnation of a single stamen to 

the perianth with other stamens converted into tepals and the anther 

supporting a slender style. However, the filaments and style are long 

and the anthers and stigma are exposed so that the pollinator is not 

as effectively ‘controlled’ as in the orchids. The Costaceae have five 

staminoids connate as a labellum and the stamen is broadly petaloid. 

Pollinators include hummingbirds and large bees. As in the gingers, 

the anther supports the slender style. The arrowroots (Marantaceae) 

have an asymmetrical flower with a single median stamen, which is 

half petaloid and all others are staminoidal and petaloid. The style is 

under tension and triggered to scoop pollen from the bee pollinator. 

Canna (Cannaceae), or Indian shot, has two whorls of three tepals, and 

up to five petaloid staminoids, which are showier than the perianth. 

In addition the fertile stamen and style are petaloid. 

(a) (b)

Basal Eudicots 

Basal eudicots such as the Proteales and Ranunculales have a pattern 

of leaf venation in which the lateral veins terminate at the margin in 

a small tooth (craspedodromus). They have a well-developed perianth 

but this is poorly differentiated into a calyx and corolla, and has a 

variable number of tepals spirally arranged or in whorls of three. Stamens 

and carpels are numerous and varying in number. The carpels 

are free to connate and have a sessile stigma. 

ranunculales (buttercups, poppies and barberries) 

Evolutionary trends in the Ranunculales include changes in the symmetry 

and the increasing complexity of the flower. Floral parts, 

especially the numerous stamens and carpels are commonly spirally 

arranged and the fruit is usually a follicle or an achene. The 

flower is apocarpous with superior pistils. The poppies, Papaveraceae 

(∼660 species) are derived from the buttercups from which they 

differ by having a syncarpous gynoecium and only two to three 

sepals. There are two subfamilies, the actinomorphic Papaveroideae, 

which produce latex, and the strongly zygomorphic Fumarioideae, 

which have a clear sap. Another large family in the order, the 

Berberidaceae (∼570 species) is distinguished from the Ranunculaceae 

by having stamens opposite the petals and a single pistil 

which becomes a berry. They have their flower parts in whorls rather 

than a spiral with two whorls of stamens and one to two whorls of 

nectaries. 

proteales (proteas, banksia and grevilleas) 

The Proteales are well represented in the Southern Hemisphere. The 

family Proteaceae in particular shows a distribution which records 

the old Gondwanan supercontinent. The Proteaceae are one important 

family (Figure 5.82) where brush blossoms have evolved. Some 

relationships discovered by the analysis of DNA sequence data are 

distinctly odd. For example, in the Proteales, Nelumbo, the sacred 

lotus, and Platanus, the plane tree, are sister groups. If this sister 

relationship is true it provides an astonishing reminder of how little 

we know about the evolutionary history that connects living plant 

groups. What kind of shared ancestor did these two lineages have 

and what were the circumstances that led to one becoming aquatic 

and the other a tree? The existence of such differences in sister lineages 

demonstrate the potential evolutionary fluidity of morphological 

characters, and emphasises the fact that living plants are only 

the tips of a highly branched phylogenetic bush. Within the bush 

many branches end blindly and do not reach the surface so that 

intermediate linking kinds between living plant groups do not now 

exist. 


Figure 5.81. Fumarioideae 

(Corydalis): (a) plant; (b) detail of a 

single flower. 

Figure 5.82. Protea: the long 

slender flowers are grouped in 

heads surrounded by showy 

bracts. Each flower has a single 

pistil surrounded by four petals 

(three fused and one free) with 

anthers adnate to the petals.


Figure 5.83. Trochodendron. 

Figure 5.84. Gunnera. 

Figure 5.85. Polygonaceae: 

Reynoutria. 

Figure 5.86. Amaranthaceae: 

Ptilotus. 

trochodendrales 

This order has only two species of evergreen trees from east and South 

East Asia each in its own family, Tetracentron sinense and Trochodendron 

aralioides (Figure 5.83). 

gunnerales 

There are only two genera in the order. Gunnera has the familiar, massive, 

palmate and deeply ribbed leaves. Usually grown beside water it 

is also a colonist of land-slips. One advantage it has is the fixed nitrogen 

it gets from the symbiotic blue-green bacteria (Nostoc) that live 

in its exposed roots and rhizomes. It produces large strobiloid inflorescences 

of tiny flowers, either bisexual or unisexual. Myrothamnus, 

from tropical Africa and Madagscar, is a resurrection plant, appearing 

to dry out but able to revive and start growing again when water 

becomes available. 

Core Eudicots 

Core eudicots have predominantly flowers with parts in fives (pentamerous) 

with a clear distinction between calyx and corolla. There 

are two main lineages, the Rosids and Asterids, three large basal lineages, 

the Caryophyllales, the Santalales and the Saxifragales, and a 

number of others of uncertain relationship like the Berberidopsidales 

and Vitales. These basal orders are crassinucellate (see below). 

caryophyllales (catchflies, stonecrops and cacti) 

The Caryophyllales is a large, and an interesting group of about 4% 

of all flowering plants that exhibits a unique set of characters (see 

Figure 5.86). For example it seems to lack mycorrhizae. It comprises 

mostly herbs, but others are lianes or twiners and shrubs. They have a 

peculiar pattern of secondary growth with the production of diffuse 

or successive cambia, which is commonly associated with succulence 

and CAM photosynthesis. Many families that have a high frequency 

of succulence have species that are either xerophytic like the cacti 

(Cactaceae), stonecrops (Aizoaceae) or halophytic like the sea-lavenders 

and thrifts (Plumbaginaceae). 

Most have a campylotropous ovule in which the inner integument 

protrudes, and a peripheral embryo surrounding a nutritive central 

perisperm tissue and so they were previously called the Centrospermae. 

Another shared character is a peculiar type of sieve-tube plastid, 

and their chemistry is distinct. The core caryophyllid families are 

Amaranthaceae, Aizoaceae, Cactaceae, Caryophyllaceae, Didiereaceae, 

Molluginaceae, Nyctaginaceae, and Phytolaccaceae. Most of these have 

a shikimic acid biosynthetic pathway as a starting point for the synthesis 

of nitrogen-containing benzylisoquinoline alkaloids and the 

betalain pigments. The latter are utilised instead of the anthocyanins 

used in other flowering plants. 

Non-core caryophyllids are the Plumbaginaceae, Polygonaceae, 

Tamaricaceae, and Frankeniaceae in one clade and four families 

of insectivorous plants in a sister clade. The former include many 

halophytic plants, which are also found in core caryophyllids such 

as the Amaranthaceae and Chenopodiaceae (Salicornia). Many have

epidermal glands but in different families the glands are adapted 

to produce either mucilage (in Polygonaceae), excrete salt as in the 

halophytes, or digestive enzymes in the insectivorous Droseraceae and 

Nepenthaceae. In the latter, the gland-type is shared but the subsidiary 

insect-trapping apparatus is quite diverse ranging from pitchers 

(Nepenthes), sticky traps (Drosera, Drosophyllum) to spring traps 

(Dionaea, Aldovandra). 

Several families in the order have a peculiar flower development 

that starts in a polymerous way but becomes organised into a pseudodiplostemonous 

way. Stamens appear to be produced in pairs. In 

the Caryophyllaceae and Plumbaginaceae stamens are antipetalous 

and arise with the petal as a unit. 

Some families in the Caryophyllalaes (Caryophyllaceae, Chenopodiaceae) 

generally lack mycorrhizae perhaps because they tend to 

occupy nutrient rich fresh soils. In contrast, the insectivores in the 

Nepenthaceae, Droseraceae and Drosophyllaceae can inhabit nutrientpoor 

soils. 

santalales (sandlewoods) 

All five families of the Santalales (Santalaceae ∼500 species, Olacaceae 

∼200 species, Opiliaceae 28 species, Misodendraceae 8 species, and 

the Loranthaceae ∼940 species) include tropical parasitic species. The 

Loranthaceae are the most specialised parasites. The shrubby, liane 

or twining habit is common throughout the order. 

saxifragales (saxifrages, currants and stonecrops) 

The Saxifragales is a very diverse order and includes: trees 

(Hamamelidaceae ∼100 species of witch hazel and sweet gum, Cercidiphyllaceae 

-- katsura, Altingiaceae), shrubs (Grossulariaceae ∼325 

species of currants and gooseberies) and showy ornamentals like the 

Paeoniaceae (∼34 species of peony), but the two largest families are 

herbs, mainly rosette-forming Saxifragaceae (∼475 species of Astilbe 

and saxifrage) or leafy succulents, the Crassulaceae (∼1280 species of 

Sempervivum, Echeveria, Sedum and Kalanchoe). Many of these rosette 

formers in the Crassulaceae, Saxifragaceae and some other small 

families, are linked by a similar look to their flowers and a number 

of characters such as a persistent scarious calyx and cellular 

endosperm. 

vitales (vines) 

This order is of supreme importance to us as the source of wine from 

the grape vine Vitis. Many other genera are twining vines with or 

without tendrils (Rhoiocissus, Cissus). Cyphostemma is a caudiciform and 

Leea a shrub and small tree. 

berberidopsidales 

This tiny order of two families, one with only one species Aextoxicon 

from Chile and the other with only two genera Berberidopsis and 

Streptothamnus from Chile and eastern Australia. 


Figure 5.87. Aizoaceae: 

Mesembryanthemum. 

Figure 5.88. Grossulariaceea: 

Ribes. 

Figure 5.89. Paeoniaceae: 

Paeonia. 

Figure 5.90. Berberidopsidales: 

Berberidopsis.


Figure 5.91. Geraniales: 

Geranium. 

Rosids and Asterids 

The remaining eudicots form two great lineages, the crassinucellate 

(thick nucellus) Rosids and the tenuinucellate Asterids, each 

split into two sister lineages (Eurosid 1 and 2, Asterid 1 and 2). 

The tenuinucellate condition is a derived feature of the ovule where 

the tissue layer, the nucellus, surrounding the developing megasporangium/embryosac 

is thin. There are various kinds of crassinucellate 

condition. Each main and sub-lineage is very variable but exhibits 

particular evolutionary tendencies. For example, all the nitrogenfixing 

families are found in the Eurosid I group: Casuarinaceae, 

Myricaceae (Fagales), Eleagnaceae, Rhamnaceae, Ulmaceae (Rosales), 

Fabaceae (Fabales) and Coriariaceae (Cucurbitales). There are well 

defined families like the Fabaceae, almost stereotypical with its characteristic 

fruit, the legume, and characteristic forms of flowers and 

inflorescence, or large and diverse families, like the Rosaceae, that 

have proved evolutionarily flexible. 

5.3.7 Rosids 

Rosid orders or even families are very diverse in their floral structure. 

Most of the ecologically (and economically) important trees from 

forests and savannas around the world are Rosids. They include the 

tallest flowering plants such as the eucalypts, the mahoganies in the 

tropics, the savanna acacias, and oaks, maples and beech from temperate 

forests. 

Percentage of families Rosids Asterids 

stipules 66% with 16% with 

stamens 58% with two whorls 79% single whorl 

corolla 97% free 75% fused 

integument(s) 94% two 87% one 

nucellus 92% crassinucellate 91% tenuinucellate 

endosperm 96% nuclear 75% cellular 

iridoids 5% present 61% present 

Basal Rosids 

geraniales (geraniums) 

The Geraniales include one large family, the Geraniaceae, and several 

very small ones. One interesting feature they share is the presence 

of glands on the margin of the leaf. They have pentamerous 

obdiplostemonous flowers with a persistent calyx. The two largest 

genera Pelargonium and Geranium produce similar beaked fruits but 

differ in the former having monosymmetric flowers and a nectariferous 

pedicel (Figure 5.91). The Geraniaceae are commonly herbs with 

jointed stems but the other families in the order include shrubs and 

trees.

crossosomatales 

These are a small order of shrubs and small trees adapted to dry 

habitats. 

myrtales (eucalypts and myrtles) 

The Myrtales have an uncertain relationship to either main Eurosid 

clade. Many have flowers with a large number of stamens (Figure 5.92). 

However, not all show this pattern. In the Melastomataceae (4750 

species) the stamens are dimorphic with showy colourful outer stamens 

and short pollen-producing inner ones. The Onagraceae (650 

species) have only four or two stamens and a long hypanthium tube. 

These families are significant as herbs or shrubs but the Myrtales 

includes the Myrtaceae and Combretaceae, which are highly significant 

as trees in the tropics especially in semi-arid areas of Australia 

(Eucalyptus ∼450 species) and Africa (Combretum ∼250 species, and 

Terminalia ∼150 species). 

Eurosid I 

zygophyllales (creosote-bush and lignum-vitae) 

There are several species important in arid and saline soils like Larrea, 

Balanites (thorny) and Zygophyllum. Larrea divaricata, the Creosote bush 

of the deserts of USA and Mexico is strongly allelopathic. 

celastrales (spindle-tree and ebony) 

A member of the Celastraceae familiar to us is the widely planted garden 

plant Euonymus, the spindle-tree, with its characteristically angled 

fruit. Maytenus (ebony) is an important tree in warmer areas throughout 

the world. The flowers generally have a broad disk with the ovary 

submerged in it. 

malpighiales (spurges, violets, willows and 

passion-flowers) 

The diversity of the order is illustrated by a comparison of the families 

Violaceae (pansies), Passifloraceae (passion flowers), Linaceae (flax), 

Salicaceae (willows) and Clusiaceae (Hypericum), each with a very different 

kind of flower. Perhaps its most interesting family is the latexproducing 

Euphorbiaceae that includes such important genera as 

Ricinus, Euphorbia, Manihot and Hevea (rubber) (see Figure 5.94). 

oxalidales (bermuda buttercup and wood sorrel) 

The order includes trees, shrubs, lianes and herbs (Oxalis) and also the 

insectivore Cephalotus (Cephalotaceae). 

(a) (b) (c) (d) 


Figure 5.92. Myrtales: 

Eucalyptus. 

(a) 

(b) 

Figure 5.93. (a) Viola; 

(b) Linum. 

Figure 5.94. Euphorbiaceae: the 

inflorescence (a cyathium) mimics 

a flower: (a) succulent euphorb 

with a crown of leaves and cyathia; 

(b) a cyathium showing the 

arrangement of a central female 

floret surrounded by male florets 

and an involucre of bracts; (c) a 

single female floret; (d) a single 

male floret.


(a) 

(b) 

(c) 

Figure 5.95. Cesalpinioideae: 

(a) Delonix; (b) Cassia, half-flower; 

and (c) side and front view. 

(a) 

Figure 5.96. Faboideae: (a) 

Trifolium (keeled flowers in a head); 

(b) Vicia (half-flower and dissected 

flower). 

Figure 5.97. Mimosoideae; 

Acacia: (a) florets in spherical 

heads; (b) single floret and a pistil. 

fabales (legumes) 

The Fabales include the Fabaceae, sometimes recognised as a single 

family, with three subfamilies, or as three families (Figure 5.95). They 

all share the characteristic of a kind of fruit, the legume that gives 

them their alternative name, the Leguminosae. The Caesalpinioideae 

(∼2000 species) have large showy, more or less regular flowers. They 

are basal in the family and the other two families show different 

patterns of specialisation. 

The Mimosoideae (∼3100 species) have small regular flowers in 

dense spikes or heads, brush blossoms with numerous exserted stamens. 

Two evolutionary trends are observed in the Mimosoideae. One 

trend is for an increase of the number of stamens although each 

has a tiny anther. The filaments are long and the stigma is small 

and cup-shaped. Pollen is released as a polyad. In Acacia only one 

polyad can fit on each stigma and the number of seeds produced in 

the legume is directly related to the number of pollen grains in the 

polyad. Another evolutionary trend shows a reduction of the number 

of stamens but specialisation of the flowers in the head to form a 

kind of pseudanthemum. For example, in Parkia the lower florets are 

showy and scent-producing but sterile, the intermediate ones sterile 

but nectar producing and the upper fertile. 

The Caesalpinioideae and Mimosoideae are mainly trees and 

shrubs but most of the third subfamily, the Faboideae (∼11 300 

species) are herbs. They have the zygomorphic flower that gives them 

their alternative name (Papilionoideae). The flag blossom is pollinated 

by large bees which land on the keel. The nectary is at the base of 

a staminal tube. In forcing its proboscis into the staminal tube the 

heavy insect pushes the keel petals (alae) down and the stamens and 

stigma rub against its ventral surface. A similar type of papilionate 

flower has evolved in parallel in the Polygalaceae, also in the Fabales. 

The Faboideae includes many agriculturally important species, peas 

and beans of all sorts. Clovers are especially important in pasture. 

(b) 5.97 (a) (b) 

The stereotypical nature of the Fabales/Fabaceae is emphasised 

because about one third of all species in the order/family belong 

tooneofafewverylargegenera:Acacia (1200 species), Mimosa (400 

species) (both Mimosoideae), Cassia (540 species) (Caesalpinioideae), 

Astragalus (2000 species), Crotalaria (600 species), Infigofera (700 species)

(all Faboideae/Popilionoideae). All these large genera are important in 

open habitats, especially in the arid and semi-arid tropics. 

rosales (roses and allies) 

The Rosales include several interesting families such as the figs 

and mulberries (Moraceae ∼1200 species), the elms (Ulmaceae ∼140 

species), the nettles (Urticaceae ∼1050 species) and buckthorns 

(Rhamnaceae ∼880 species) but Rosaceae (∼3000 species) is the largest 

family. It is central to the evolution of many other families in the 

Rosales but relatively difficult to circumscribe. For example the ovary 

may be hypogynous, perigynous or epigynous and perhaps the only 

widespread feature is the possession of a hypanthium, a floral cup 

that is primitively small and saucer- or cup-shaped, or has evolved to 

become large and ultimately connate with the carpels. Traditionally 

the family has been divided into four subfamiles differing in the form 

of the fruit (the Spiroideae -- an aggregate of follicles, the Rosoideae -an 

aggregate of achenes, the Prunoideae -- a drupe, the Maloideae -a 

pome). One of the smallest families in the order, with only three 

species, but certainly not the least significant is the Cannabaceae 

because it contains both hops and cannabis. 

cucurbitales (gourds and begonias) 

The Cucurbitales include three families interesting for different reasons. 

The Coriariaceae have only one widely distributed genus, Coriaria 

(Mexico to Chile; Mediterranean to Himalayas/Japan; New Guinea 

to New Zealand and W Pacific) but it is interesting because it has a 

nitrogen-fixing association with Frankia in root nodules. The Begoniaceae 

have only two genera, Symbegonia with 12 species from New 

Guinea and Begonia with over a thousand. Symbegonia differs from 

Begonia in having a corolla fused in a tube. Begonia are succulent 

herbs and shrubs. Water conservation is aided by Crassulacean Acid 

Metabolism (CAM) and they have stomata in clusters. The Cucurbitaceae 

are climbers with tendrils and are important commercially 

as gourds, squash, melons, etc. These families share a tendency to 

fleshy or juicy tissues. 

fagales (oaks and beeches) 

The Fagales include many familiar wind-pollinated trees of temperate 

regions. The related families of trees, Fagaceae (beech, hornbeam 

and oak), Betulaceae (birch and alder), Casuarinaceae (she-oaks) and 

Juglandaceae (walnuts) exhibit a wide range of adaptations for windpollination. 

Different genera show different degrees of reduction of 

the flower and its aggregation into unisexual catkins. Wind-pollinated 

species are concentrated in the temperate regions but some genera 

like the oaks and hornbeams have insect-pollinated species in the 

tropics and here have stiff erect catkins. Walnut (Juglans) and wingnut 

(Pterocarya) have a tiny but well-formed perianth. In Oak (Quercus robur) 

male florets have a six-lobed perianth and seven to eight stamens but 

female florets have a minute perianth but a scaly cupule. Hazel (Corylus) 

has male florets consisting of two bracteoles and four stamens 

only with a bract at the base and female flowers that are surrounded 


Figure 5.98. Rosaceae: Rosa. 

Figure 5.99. Cucurbitaceae: 

Cucurbita. 

Figure 5.100. Fagaceae: Alnus.


(a) 

(b) 

Figure 5.101. Brassicaceae: 

(a) half flower; (b) floral diagram. 

Figure 5.102. Malvaceae: 

Hibiscus. 

Figure 5.103. Anarcardiaceae: 

Pistacia. 

by scales and have a minute perianth. The strange she-oak (Casuarina) 

of South-East Asia and Australia was once considered to be very primitive 

because of its very simple inflorescence. The flower consists only 

of a bract with two scale-like bracteoles with, in the male, a single 

stamen and, in the female, a single pistil. Male flowers are aggregated 

into catkins and the hard bracteoles of the female flowers form part 

of a woody ‘cone’. 

Eurosid II 

The Eurosid II clade includes these important orders: Brassicales, 

Malvales, and Sapindales. 

brassicales (crucifers) 

The Brassicales include sister lineages so distinct and without intermediates 

that one wouldn’t guess their close relationship. The families 

Brassicaceae (crucifers), Resedaceae (mignonette), Limnanthaceae 

(poached-egg flower), Batidaceae (saltwort), Koeberliniaceae (allthorn), 

Setchellanthaceae, Moringaceae (Bennut), Caricaceae (papaya) and 

Tropaeolaceae (nasturtium) are very distinct in their floral morphology. 

For example, the Brassicaceae is also called the Cruciferae because 

of its cross-shaped flowers of four petals and usually six stamens. In 

contrast the Resedaceae usually has six fringed petals and the Tropaeolaceae 

has five and also has a long hairy claw. One floral feature that 

is present in several families of the order is a nectariferous portion 

of the axis below the stamens (androgynophore) or pistil (gynophore). 

One of the most significant features these families share is the possession 

of mustard oils (glucosinolates). This seemingly obscure chemical 

character provides protection against herbivory and fungal attack. 

Another interesting feature is the usual lack of mycorrhizae in the 

Brassicaceae, perhaps because they tend to occupy relatively nutrient 

rich early successional situations. 

malvales (hibiscus and mallows) 

The Malvales are linked by a chemical characteristic of obscure significance, 

the presence of mucilage cells, or canals and cavities. The 

Malvales include the important family of tropical trees, the Dipterocarpaceae. 

Many exhibit a common rosid trait of showy polypetalous 

flowers with many stamens. The sequence in which the stamens 

mature is centrifugal, a pattern of development that was thought 

to be significant enough to warrant separating them from the rosids 

(centripetal development) in a group called the Dillenidae, but this 

pattern of development is difficult to see in the Malvaceae where the 

stamens are united to form a tube around the style. 

sapindales (mahoganies) 

The Sapindales also include several very important tropical and subtropical 

families of trees and shrubs such as the Meliaceae (∼575 

species), Sapindaceae (∼1350 species), Anacardiaceae (∼850 species) 

and Burseraceae (∼540 species). They frequently have pinnate or

tri-foliolate leaves. Many of these families are highly resinous. The 

resin is toxic and protects them to a degree from leaf-browsing animals 

and also protects the wood from wood-boring insects. Azadirachta 

in the Meliaceae is a source of insecticide. The Sapindaceae often have 

saponins present. We are familiar with the Sapindales as the source 

of fruits and seeds (litchi, longan or rambutan -- Sapindaceae; mango, 

cashew, pistacia -- Anacardiaceae) and as timber trees (mahogany 

Khaya, Swietenia -- Meliaceae). 

5.3.8 Asterids 

The Asterids are tenuinucellate. Most also have a pentamerous sympetalous 

corolla, and most have an equal number of epipetalous stamens, 

alternating with the five corolla lobes. This set of attributes 

has long been recognized as those of a group called ‘Sympetalae’ 

(for a tubular corolla of connate (fused) petals). The Asterids contain 

the most advanced members of the Eudicots, and the most 

recently evolved. They have diversified especially in having specialised 

pollination mechanisms. Floral architecture and behaviour show 

many individual adaptations to particular kinds of pollinator. 

Basal Asterids 

cornales (dogwoods) 

The Cornales exhibit a tendency, seen more fully developed elsewhere 

in the euasterids, towards the possession of a pseudanthium, a compound 

inflorescence of small flowers grouped together in a flat head 

and made showy in different ways. In Hydrangea, for example, flowers 

are in a cymose inflorescence with marginal ones sterile and showy, 

and fertile central ones. An alternative pattern is seen in Cornus (Cornaceae), 

which has large, showy, outer bracts like petals around the 

inflorescence. Although some Cornales have a synsepalous calyx most 

have free petals. There are three large families in the order, the Cornaceae, 

Hydrangeaceae and Loasaceae, ranging from trees and shrubs 

to robust herbs. The Loasaceae have barbed stinging hairs. 

ericales (heathers) 

The Ericales are a diverse order and include, as well as the heathers 

(Ericaceae), other very distinct families such as the balsams (Balsaminaceae 

-- fleshy herbs), the Marcgraviaceae (lianes), the Polemoniaceae 

(mainly herbs, especially of arid areas, but some shrubs and 

lianes), the camellias (Theaceae -- shrubs and trees with thick leaves), 

and primulas (Primulaceae -- herbs). The brazil-nut family Lecythidaceae, 

and the Sapotaceae, another important tropical family, are 

sister families in the order. The latter produces latex and gums, and 

includes species such as the chewing-gum plant Manilkara and guttapercha 

plant Palaquium. One lineage of Ericales includes the insectivorous 

pitcher-plant family Sarraceniaceae (Sarracenia, Darlingtonia, 

Heliamphora) andRoridula with sticky resin secreting hairs (but not 

insectivorous) and sensitive stamens, as well as the Actinidiaceae (the 

kiwi-fruit or Chinese gooseberry). 


Figure 5.104. Cornaceae: 

Cornus. 

Figure 5.105. Theaceae: 

Camellia. 

Figure 5.106. Primulaceae: 

Dodecatheon.


Figure 5.107. Ericaceae: 

Rhododendron. 

Figure 5.108. Garryaceae: 

Garrya. 

Figure 5.109. Gentianaceae: 

Gentiana. 

Figure 5.110. Apocyancaeae: 

Nerium. 

The Clethraceae and Cyrillaceae, two small families sister to the 

Ericaceae, are all trees or shrubs with tough, spirally-arranged leaves 

and pendulous flowers. 

There are about 4000 species of Ericaceae. They are strongly 

mycorrhizal, with different subgroups having different forms of 

mycorrhizae: broadly identified as arbutoid, ericoid and monotropoid 

types. Erica and Rhododendron are by far the largest genera, each with 

about 800 species. The flowers of Ericaceae range from small and 

relatively inconspicuous Calluna type to large and highly colourful 

Rhododendron blossoms, and from radial to bilateral symmetry, representing 

a great diversity of pollination mechanisms involving wind, 

thrips, bees and other insects, birds and other pollinators. 

Euasterid I 

The Euasterid I group shows a trend towards large zygomorphic 

flowers, held laterally, exemplified by snapdragon, salvias and deadnettles. 

garryales (silk-tassel) 

Aucuba is the commonly grown yellow spotted evergreen ‘laurel’. 

Garrya (Silk-tassel Bush) is another commonly cultivated shrub with 

showy catkins. It produces highly toxic alkaloids. One rather peculiar 

feature of this order is the presence of petroselenic acid as a major 

fatty acid in seeds. 

gentianales (gentians and bedstraws) 

The order Gentianales has the most generalised flowers in the Euasterids 

I with five epipetalous stamens, but they also exhibit secondary 

pollen presentation mechanisms, which in the asclepiads, 

have resulted in one of the most peculiar floral morphologies and 

pollination mechanisms of any flowering plants. The Gentianaceae 

(∼1200 species) are regular, sympetalous and actinomorphic with five 

normal epipetalous stamens. The Rubiaceae (∼11 000 species) are similar, 

although frequently they have floral parts in fours and much 

smaller flowers in cymes. Many shed their pollen onto a club-shaped 

stigma while in bud. When the bud opens the pollen is presented 

to the pollinator and only later does the stigma mature. The Gentianaceae 

are mainly herbs and are especially common in temperate 

conditions, while the Rubiaceae are important as tropical trees, 

shrubs and lianes. 

The Apocynaceae (∼5000 species) ranges from genera such as Vinca 

(completely united carpels, and anthers, which are distinct and fully 

fertile) to those like Nerium (carpels separated up from the base and 

united only by their style and stigma, and anthers in which only the 

top part produces pollen and grouped closely together in depressions 

around the top of the expanded style). The asclepiads were formerly 

recognised as a separate family. In Asclepias the anthers are adnate to 

the style to form a structure called the gynostegium and the pollen 

in each theca is a compact mass, a pollinium. Pollinia from adjacent 

thecae are united by an acellular yoke called the translator and 

the whole structure is released as a pollinarium. Evolution of the 

pollinium has been accompanied by a merging of the pollen sacs of

each theca so that each anther is bisporangiate. The translator clips 

the pollinarium to the pollinator and it is later pulled off by being 

caught in a groove in the stigma of another flower. 

solanales (potato and morning glory) 

The Solanales have two large families, the Solanaceae (∼2600 species) 

(potato, tomato, tobacco, petunia) and the Convolvulaceae (morning 

glory, bindweed). Most species are polysymmetric with five equal 

corolla lobes but some genera of the Solanaceae are zygomorphic. 

Schizanthus has even lost one stamen and, of the remaining four, only 

two are fertile. The small family Nolanaceae has five carpels, the 

primitive condition, but other families in the Solanales have fewer 

carpels; either two- or pseudo-four-locular. In some of the Convolvulaceae 

(Dichondra) there are two carpels and the ovary is deeply twoor 

four-lobed with a gynobasic style, which resembles the pattern of 

the Boraginaceae and Lamiaceae. 

lamiales (dead-nettles and gesners) 

The Lamiales are mainly characterised by their monosymmetric tubular 

flowers. From an ancestral pattern of five more or less equal corolla 

lobes and five stamens there has been a shift to either four (with the 

loss of the posterior stamen) or two stamens (with in addition a loss 

of one lateral pair) and a strongly lipped flower. Patterns of floral 

evolution are complex though. For example the Oleaceae (olive, ash, 

privet, jasmine), close to the base of the order, are polysymmetric in 

their corolla but usually have four corolla lobes and only two stamens. 

Calceolaria (Calceolariaceae), also in a basal family, is one of the most 

strongly monosymmetric with two stamens and a corolla having a 

pouch-like lower lip. 

The order includes plants of many different life-forms. The Gesneriaceae 

are mainly herbs or shrubs with many epiphytes and lianes 

and have many evolutionary novelties (see Chapter 3). Dispersal of 

the seed in this epiphytic niche has been accompanied by the evolution 

of unilocular ovaries with many seeds and parietal placentation. 

An alternative adaptation is exhibited by the Acanthaceae (∼4350 

species). They have the funiculus (the stalk that attaches the seed to 

the fruit) modified into a hook-shaped jaculator, which flings the seed 

out. Another family with many lianes but also some large tropical 

trees are the Bignoniaceae. They have large flowers and fruits variously 

adapted for pollination and seed dispersal especially by birds 

and bats. 

The Scrophulariaceae was a large diverse family of many showy 

monosymmetric flowers that has been split and circumscribed in new 

ways as a result of molecular information (see Chapter 8). Less controversially 

and perhaps preserving features more representative of 

an earlier stage in the evolution of the order are the Verbenaceae 

(∼1900 species) (verbena, teak). The two-carpellate pistil has each 

carpel divided by an extra wall. Each locule has a single ovule. The 

corolla is only slightly zygomorphic but bilateral symmetry is emphasised 

by the loss of one stamen in most species. The Lamiaceae (∼5600 

species), the labiates, so-named because of their two-lipped flower, 


Figure 5.111. Solanaceae: 

Brugmansia. 

Figure 5.112. Acanthaceae: 

Justicia. 

Figure 5.113. Lamiaceae: 

Thymus.


Figure 5.114. Scrophulariaceae: 

(a) Digitalis; (b) floral diagram. 

Figure 5.115. Aquifoliaceae: 

Ilex. 

Figure 5.116. Dipsacaceae: 

Scabiosa. 

have flowers that are usually very strongly zygomorphic and have 

either four or two (Salvia) stamens. The four-loculate ovary is divided 

into four segments from the top. The style reaches down to the base 

of each locule (gynobasic). Each segment is dispersed as a separate 

nutlet when the seed is mature. In the Lamiaceae the inflorescence 

is a verticillaster with cymose verticals on a main raceme. 

The Orobanchaceae, contains most of the parasitic, while the Lentibulariaceae 

and Byblidaceae are insectivorous and have sticky and 

digestive glands. 

boraginales (borages) 

Parallel evolutionary trends can be seen in the related family, the 

Boraginaceae. Some borages have an entire ovary like the Verbenaceae 

whereas others are like the Lamiaceae with a gynobasic style and four 

nutlets. Most have polysymmetric flowers arranged in cymes. Echium 

is exceptional with its zygomorphic flower. 

Euasterid II 

The Euasterid II lineage shows varying degrees of aggregation of flowers 

into a head in which different flowers may become specialised for 

showiness (pseudanthium). They tend to have small epigynous flowers 

grouped together in a flat head, exemplified by the umbel of the Apiales 

(umbellifers) and the capitulum of the Asterales (daisies). Some 

are woody shrubs or climbers but the majority are herbs. 

They have a remarkable chemical diversity especially in compounds 

that act as herbivore deterrants. 

aquifoliales (hollies) 

The Aquifoliaceae is by far the biggest family in the Aquifoliales 

because of the genus llex (holly) with 400 species of small evergreen 

trees or shrubs. Flowers are normally small and unisexual. Most 

species are dioecious. Helwingia (three species) and Phyllonoma (four 

species) have epiphyllous inflorescences. 

dipsacales (teasels and honey-suckles) 

The Dipsacales exhibit various stages in the evolution of a pseudanthium. 

The valerians (Valerianaceae ∼400 species) have cymose inflorescences 

of small florets. The honey-suckles and elders (Caprifoliaceae 

∼365 species) have cymose umbels. The teasels and scabiouses (Dipsacaceae 

∼250 species), have the most highlly developed capitulate 

heads with an involucre of bracts surrounding the head that in some 

species have the outer florets modified to be more showy (Scabiosa, 

Knautia). In the Dipsacales the head is not bracteate and is basically 

cymose.

apiales (umbels and ivy) 

There are two families in the order (Apiaceae ∼3100 species and Araliaceae 

∼800 species). The Apiaceae is a family that has many species 

cultivated for food (carrot) or more frequently for spice (coriander, 

cumin, etc.). The compound umbel is so characteristic of the Apiaceae 

that the family was one of the first flowering plant families to be 

clearly recognised as the Umbelliferae (Figure 5.117). However, the 

compound umbel is confined to subfamily Apoideae. In some genera 

like Hydrocotyle, in subfamily Hydrocotyloideae, the compound umbel 

is reduced to a single floret; and in Eryngium, subfamily Saniculoideae, 

the inflorescence is rounded rather than flat. The umbel is normally 

visited by a range of different insects, which move freely over the 

platform of the umbel. In each floret, the ovary has a disc at the 

base of the paired styles. The styles are swollen, together forming a 

nectariferous stylopodium. Flowers in different parts of the umbel 

may be specialised. Those in lateral umbels are sometimes female 

and sterile with abortive ovaries and shorter stylopodia. The ovary 

has two carpels and matures into a dry fruit, a schizocarp, which 

splits into two mericarps joined by a carpophore. Some species have 

fruits with thick corky walls so that they float, have wings for wind 

dispersal, or have hooks for animal dispersal. The Apiaceae have a 

particularly effective combination of anti-herbivore chemical repellants 

such as polycetylenes and sesquiterpene lactones. The Araliaceae 

share many similarities with the Apiaceae but are mainly tropical and 

rarely form regular compound umbels. They also produce a fleshy 

fruit. 

asterales (daisies, lobelias and bellflowers) 

Most species of Asterales are in one family, the Asteraceae, which is 

widely recognised as one of the most advanced families of flowering 

plants (Figure 5.121). Its origin is relatively recent but it has 1100 genera 

and 20 000 species. All species have a capitulum, a head with many 

florets on a common and usually flattened receptacle, and surrounded 

by an involucre of bracts. The involucre is particularly obvious in 

‘everlasting’ flowers like Helichrysum, where it is showy. Florets are 

tubular and epigynous with a single ovule. They are protandrous and 

mature centripetally (i.e. the capitulum is racemose). The florets are 

of different sorts, either polysymmetric or monosymmetric. The latter 

have either three corolla lobes that are connate and greatly expanded 

to form a strap, or all five corolla lobes extending in one direction. 

Florets may be pistillate, hermaphrodite or functionally staminate. 

Heads are made of a single kind of floret or a combination of kinds, 

commonly with central disk of hermaphrodite, polysymmetric florets 

and a margin of showy, monosymmetric ray florets, with the latter 

pistillate or sterile, as in the daisy and sunflower. One group, which 

includes lettuce and dandelion, have all florets with all five corolla 

lobes forming the strap. 

Florets are protandrous. Anthers form a tube. Pollen is shed into 

the anther tube and then the immature style elongates and pushes 


(a) 

(b) 

Figure 5.117. Apiaceae: (a) 

Heracleum; (b) floret in section 

showing stylidium and schizocarp. 

(a) 

(b) 

Figure 5.118. Structure of a 

capitulum (a) capitulum t.s. 

showing central and marginal 

florets; (b) florets of diverse sorts.


Figure 5.119. Tubular central 

floret showing inferior ovary and 

the calyx converted into a bristly 

pappus. 

Figure 5.120. Pollen 

presentation mechanism. Florets 

are protandrous and the piston 

like style pushes pollen on to the 

surface before the stigma lobes 

open to become receptive. 

Figure 5.121. Diverse 

Asteraceae: (a) Bidens with 

marginal strap florets mimicking a 

5-petalled flower; (b) Centaurea 

with marginal expanded disk 

florets; (c) Galactites with a showy 

involucre of bracts; (d) Cynara with 

a head of disk florets and a spiny 

involcre; (e) Solidago spikes of small 

ligulate heads; (f) Echinops with a 

head of capitula each with a single 

floret. 

the pollen onto the surface of the capitulum. Later the stigmatic lobes 

open. The fruit of the Asteraceae is usually crowned by a pappus 

derived from the calyx. The pappus in Asteraceae is very variable, 

either absent or cup-like, or with scales, bristles, simple or feathery 

hairs, which are barbed, or glandular. The fruit, called a cypsela, is a 

kind of achene of an inferior ovary. The dispersal adaptations of the 

fruit contribute to the success of many species as weeds. 

The features described above have evolved in many groups outside 

the Asteraceae. Many of the structures of a capitulate infloresecence 

found in the Asteraceae are paralleled in other families in the Asterales 

such as the Goodeniaceae (∼300 species), which has an indusium, 

the Calyceraceae (∼55 species), Lobeliaceae (∼1200 species) and 

Campanulaceae (∼600 species). Phyteuma and Jasione, in the Campanulaceae, 

both have capitulate inflorescences surrounded by an involucre 

of bracts. Jasione has a kind of primitive ‘pseudo-indusium’ formed 

by swollen stigmatic lobes. The one species of the monotypic Brunoniaceae, 

which is remarkably similar in appearance to Jasione and is 

sometimes put in the Goodeniaceae, shows a further parallel in having 

an involucrate head, though the head is cymose and the florets 

are hypogynous. The piston-like mode of pollen presentation in the 

Asteraceae also has parallels with that in the Campanulaceae (Physoplexis) 

and Lobeliaceae. 

The repeated evolution of these features argues strongly that 

they are adaptive. One advantage of having a capitulum, for example, 

is the protection given to the ovule and seed. Functionally it 

provides a large showy target for pollinators and yet each ovule is 

packaged separately, as a defence against predators and for dispersal. 

There is a lot of diversity in the size and number of florets that 

capitula contain. There are the familiar huge capitula of the sunflowers, 

which have been selected by plant breeders. At the other end of 

the spectrum many species have capitula containing very few florets. 

(a) (b) (c) 

(d) (e) (f) 

(c)

Frequently species with small capitula have the capitula grouped in 

some way. In Solidago the capitula are arranged in spikes. In Achillea 

they are grouped in a corymb. In Echinops each capitulum only has 

a single floret and they are grouped in a globular head with its own 

involucre. 

However, because these features are also present outside the Asteraceae 

they do not explain the particular evolutionary success of the 

Asteraceae. The Asteraceae have an unusual multiallelic, homomorphic, 

sporophytic, self-incompatibility that has maintained high levels 

of genetic diversity among individuals and populations. However, they 

are also flexible in their breeding system and many weedy species are 

secondarily self-compatible and self-pollinating. One example is the 

ubiquitous groundsel, Senecio vulgaris, which, no longer needing to 

attract pollinators, lacks the ray florets of its relatives. Polyploidy, 

sometimes following hybridisation, seems to play a significant role in 

the evolution of the weedy species, especially since it can destroy the 

self-incompatibility. Another aspect of their evolution is the chemical 

diversity associated with the deterrence of herbivores, especially in 

alkaloids. One of the largest genera of all plants, the ragworts (Senecio), 

with over 1500 species, has its own peculiar type of alkaloid. 

Further reading for Chapter 5 

Beck,C.B.Origin and Evolution of Gymnosperms (New York: Columbia 

University Press, 1988). 

Bold, H. C., Alexopoulos, C. J. and Delevoryas, T. Morphology of Plants and 

Fungi, 5th edition (New York: Harper and Row, 1987). 

Heywood, V. J. (ed.) Flowering Plants of the World. (Oxford: Oxford University 

Press, 1978). 

Mabberley, D. J. The Plant Book (Cambridge: Cambridge University Press, 1987). 

Schofield, W. B. Introduction to Bryology (New York: Macmillan, 1985). 

Shaw, A. J. and Goffinet, B. Bryophyte Biology (Cambridge: Cambridge 


Stewart, W. N. Paleobotany and the Evolution of Plants (Cambridge: Cambridge 


Takhtajan, A. Evolutionary Trends in Flowering Plants (New York: Columbia 


Thomas, B. A. and Spicer, R. A. (1987) The Evolution and Palaeobiology of Land 

Plants (London: Croom Helm, 1987). 

Willis, K. J. and McElwain, J. C. The Evolution of Plants (Oxford: Oxford 


FURTHER READING FOR CHAPTER 5 251 

Figure 5.122. Campanulaceae 

(a) Campanula; (b) Jasione.

5.3 Class Magnoliopsida – flowering plants - Cambridge University ...

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