Chapter 4 — Plant Kingdom

The previous chapter placed the algae, bryophytes, pteridophytes, gymnosperms and angiosperms in kingdom Plantae. Before describing these groups, it is useful to see how botanists have classified plants over the years, because the groups themselves are the product of that history.

Artificial systems were the earliest. They used only one or a few gross superficial morphological characters, such as habit, colour, number and shape of leaves, or, in the sexual system of Linnaeus, the number and arrangement of androecium and gynoecium. Such systems were easy to use but gave equal weight to vegetative and sexual characters, which is not acceptable because vegetative characters are more easily affected by the environment, and they separated closely related species and grouped unrelated ones. Linnaeus's classes Monandria (one stamen), Diandria (two stamens) and so on put the grass, the jasmine and the sage together merely because each has two stamens.

Natural systems came next, using natural affinities among organisms and considering many characters, external as well as internal features like ultrastructure, anatomy, embryology and phytochemistry. The classification of flowering plants by George Bentham and Joseph Dalton Hooker in their Genera Plantarum (1862–1883) is the best-known natural system; it describes 202 families of seed plants in three classes, Dicotyledons, Gymnosperms and Monocotyledons, and it is still the basis of the arrangement of most Indian herbaria and floras.

Phylogenetic systems, now in use, are based on evolutionary relationships between the various organisms. They assume that organisms belonging to the same taxa have a common ancestor, and they arrange groups from the primitive to the advanced. Engler and Prantl (Die Natürlichen Pflanzenfamilien, 1887–1915) placed the simple, wind-pollinated flowers first; John Hutchinson (1926–1934) regarded the magnolia-type flower with many free parts as primitive; and the Angiosperm Phylogeny Group (APG, 1998 onward) builds its classification directly on DNA sequences. Modern phylogenetic classifications are supported by evidence from various sources, and where fossils are lacking, as with most flowering plants, they lean on molecular data.

Three further sources of evidence are now standard:

• Numerical taxonomy (phenetics), which is easily carried out using computers, is based on all observable characteristics. Number and codes are assigned to all the characters and the data are then processed. In this way each character is given equal importance and at the same time hundreds of characters can be considered, giving an objective measure of overall similarity.
• Cytotaxonomy is based on cytological information like chromosome number, structure and behaviour; it separates polyploid series and detects hybrids.
• Chemotaxonomy uses the chemical constituents of the plant, such as alkaloids, flavonoids, proteins and DNA, to resolve confusions; the betalain pigments, for example, mark out one order of dicots, the Caryophyllales.

The groups that follow, algae, bryophytes, pteridophytes, gymnosperms and angiosperms, are arranged in phylogenetic order, from the simplest thalloid aquatic forms to the seed-bearing plants with flowers and fruits, and the sequence is also the order in which they appeared in the fossil record.

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms, found in both fresh water and marine habitats. They also occur in a variety of other habitats: moist stones, soils and wood. Some of them occur in association with fungi as lichens and with animals, for example on sloth bears. The form and size of algae is highly variable, ranging from colonial forms like Volvox and the filamentous forms like Ulothrix and Spirogyra to massive marine forms, the kelps, that are several metres long. The body is a thallus: it is not differentiated into true root, stem and leaves, and has no vascular tissue. The cells are eukaryotic, with a cell wall usually of cellulose, chloroplasts of characteristic shape (cup-shaped in Chlamydomonas, spiral in Spirogyra, girdle-shaped in Ulothrix) containing pyrenoids, the centres of starch formation.

Vegetative reproduction is by fragmentation: each fragment develops into a thallus. Asexual reproduction is by the production of different types of spores, the most common being zoospores, which are flagellated and motile and on germination give rise to new plants; aplanospores (non-motile), hypnospores and akinetes (thick-walled resting cells) also occur. Sexual reproduction takes place through fusion of two gametes. These gametes can be flagellated and similar in size, as in Ulothrix, or non-flagellated but similar in size, as in Spirogyra; such reproduction is called isogamous. The fusion of two gametes dissimilar in size, as in some species of Chlamydomonas (C. braunii), is termed anisogamous. Fusion between one large, non-motile female gamete and a smaller, motile male gamete is termed oogamous, as in Volvox and Fucus. Oogamy is the most advanced condition and leads on to the reproduction of land plants.

Economic importance. Algae are useful to human beings in a variety of ways. At least half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis; being photosynthetic they increase the level of dissolved oxygen in their immediate environment; and they are of paramount importance as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals. Many species of Porphyra, Laminaria and Sargassum are among the seventy species of marine algae used as food. Certain marine brown and red algae produce large amounts of hydrocolloids (water-holding substances), for example algin (brown algae) and carrageen (red algae), which are used commercially in ice cream, toothpaste and textiles. Agar, one of the commercial products obtained from Gelidium and Gracilaria, is used to grow microbes and in preparations of ice creams and jellies. Chlorella and Spirulina are unicellular algae rich in proteins and are used as food supplements even by space travellers. Iodine is obtained from Laminaria; and the diatoms and other algae are the source of much of the earth's petroleum.

The algae are divided into three main classes, Chlorophyceae, Phaeophyceae and Rhodophyceae, on the basis of pigments, stored food and cell wall composition.

The members of Chlorophyceae are commonly called green algae. The plant body may be unicellular (Chlamydomonas), colonial (Volvox) or filamentous (Ulothrix, Spirogyra); some are sheet-like (Ulva, the sea lettuce) or siphonaceous. They are usually grass green due to the dominance of the pigments chlorophyll a and b, the same pigments as in higher plants, together with carotenoids. The pigments are localised in definite chloroplasts, which may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species. Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts. Pyrenoids contain protein besides starch. Some algae may store food in the form of oil droplets. Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose. The motile cells and gametes bear two to eight equal, apical, whiplash flagella.

Reproduction. Vegetative reproduction usually takes place by fragmentation or by formation of different types of spores. Asexual reproduction is by flagellated zoospores produced in zoosporangia, which in the simpler forms are ordinary vegetative cells. The sexual reproduction shows considerable variation in the type and formation of sex cells and it may be isogamous, anisogamous or oogamous. Chlamydomonas alone shows all three conditions in its different species; Ulothrix is isogamous, Spirogyra isogamous by conjugation, Volvox oogamous. The zygote generally secretes a thick wall and rests, and meiosis occurs on its germination, so that the plant is haploid and the life cycle is haplontic.

Habitat. Green algae are mostly freshwater; about ten per cent are marine (Ulva, Caulerpa, Acetabularia). Some live on damp soil, tree bark and snow, and a few are symbionts in lichens (Trebouxia) and in Hydra and Paramecium (Chlorella).

Evolutionary importance. The green algae are of special interest because the land plants arose from them: the chlorophylls a and b, the storage of starch inside the chloroplast, the cellulose wall and the details of cell division are shared with bryophytes and vascular plants and with no other algae. The charophytes, stoneworts such as Chara, are the closest living relatives of land plants.

Commonly found green algae are Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara. Chlamydomonas, a unicellular biflagellate with a cup-shaped chloroplast, an eye spot and two contractile vacuoles, is the type most often drawn; Ulothrix is an unbranched filament attached by a holdfast cell, each cell with a girdle-shaped chloroplast; Spirogyra is the pond silk with spiral chloroplasts; Chara is a branched, whorled, calcified plant of ponds with complex sex organs, the globule and the nucule.

Phaeophyceae. The members of Phaeophyceae or brown algae are found primarily in marine habitats. They show great variation in size and form: they range from simple branched filamentous forms (Ectocarpus) to profusely branched forms as represented by kelps, which may reach a height of 100 metres (Macrocystis). They possess chlorophyll a, c, carotenoids and xanthophylls. They vary in colour from olive green to various shades of brown depending on the amount of the xanthophyll pigment fucoxanthin present in them. Food is stored as complex carbohydrates, which may be in the form of laminarin or mannitol. The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin. The protoplast contains, in addition to plastids, a centrally located vacuole and nucleus. The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe, and a leaf-like photosynthetic organ, the frond. Vegetative reproduction takes place by fragmentation. Asexual reproduction in most brown algae is by biflagellate zoospores that are pear-shaped and have two unequal, laterally attached flagella. Sexual reproduction may be isogamous, anisogamous or oogamous. Union of gametes may take place in water or within the oogonium (oogamous species). The gametes are pyriform (pear-shaped) and bear two laterally attached flagella. The common forms are Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus. Sargassum floats in vast masses in the Sargasso Sea; Laminaria is the source of iodine and algin; Fucus is the bladder wrack of rocky shores.

Rhodophyceae. The members of Rhodophyceae are commonly called red algae because of the predominance of the red pigment r-phycoerythrin in their body, together with chlorophyll a and d and phycocyanin. Majority of the red algae are marine, with greater concentrations found in the warmer areas. They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates, because phycoerythrin absorbs the blue-green light that reaches deep water. The red thalli of most of the red algae are multicellular; some of them have complex body organisation, and a few are unicellular (Porphyridium). The food is stored as floridean starch, which is very similar to amylopectin and glycogen in structure, and is stored outside the chloroplast. The cell wall is of cellulose with pectic compounds and the mucilaginous polysaccharides agar and carrageenan; in the coralline algae it is impregnated with calcium carbonate, and these algae help build coral reefs. The red algae usually reproduce vegetatively by fragmentation. They reproduce asexually by non-motile spores and sexually by non-motile gametes; at no stage in the life cycle is there a flagellated cell, which is unique among algae. Sexual reproduction is oogamous, the non-motile male gametes (spermatia) being carried by water currents to the female organ, the carpogonium, which bears a receptive hair, the trichogyne; the life cycle is accompanied by complex post-fertilisation developments, often with three generations, the gametophyte, the carposporophyte on it and a free tetrasporophyte. The common members are Polysiphonia, Porphyra, Gracilaria and Gelidium. Porphyra is eaten as nori; Gelidium and Gracilaria yield agar.

Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills. They are called the amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction; the sperms must swim to the egg. They usually occur in damp, humid and shaded localities, on wet rocks, tree trunks, walls and soil, and in bogs and stream banks. They play an important role in plant succession on bare rocks and soil.

Plant body. The plant body of bryophytes is more differentiated than that of algae. It is thallus-like and prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stem or leaves; they may possess root-like, leaf-like or stem-like structures, but these have no vascular tissue, and so the plants remain small. The main plant body of the bryophyte is haploid. It produces gametes, hence is called a gametophyte. The sex organs in bryophytes are multicellular, unlike the unicellular sex organs of algae, and the gametes are protected by a jacket of sterile cells. The male sex organ is called antheridium. It produces biflagellate antherozoids. The female sex organ, called archegonium, is flask-shaped and produces a single egg. The antherozoids are released into water, where they come in contact with the archegonium, swimming down its neck. An antherozoid fuses with the egg to produce the zygote. Zygotes do not undergo reduction division immediately. They produce a multicellular body called a sporophyte. The sporophyte is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it. Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores. These spores germinate to produce the gametophyte. Thus the bryophyte life cycle has two alternating generations: a dominant, independent gametophyte and a short-lived, dependent sporophyte.

Economic importance. Bryophytes in general are of little economic importance, but some mosses provide food for herbaceous mammals, birds and other animals. Species of Sphagnum, a moss, provide peat that has long been used as fuel, and because of their capacity to hold water as packing material for trans-shipment of living material. Mosses along with lichens are the first organisms to colonise rocks and hence are of great ecological importance. They decompose rocks, making the substrate suitable for the growth of higher plants. Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion.

The bryophytes are divided into liverworts (Hepaticopsida), hornworts (Anthocerotopsida) and mosses (Bryopsida); the syllabus treats liverworts and mosses.

Liverworts. The plant body of a liverwort is thalloid, for example Marchantia. The thallus is dorsiventral and closely appressed to the substrate; in the leafy members the leaf-like appendages are in two rows on the stem-like structures. Asexual reproduction in liverworts takes place by fragmentation of thalli, or by the formation of specialised structures called gemmae (singular gemma). Gemmae are green, multicellular, asexual buds, which develop in small receptacles called gemma cups located on the thalli. The gemmae become detached from the parent body and germinate to form new individuals. During sexual reproduction, male and female sex organs are produced either on the same or on different thalli. In Marchantia they are borne on stalked umbrella-like antheridiophores and archegoniophores. The sporophyte is differentiated into a foot, seta and capsule. After meiosis, spores are produced within the capsule, along with spiral elaters that help in their dispersal. These spores germinate to form free-living gametophytes. Marchantia and Riccia are the common liverworts.

Mosses. The predominant stage of the life cycle of a moss is the gametophyte, which consists of two stages. The first stage is the protonema, which develops directly from a spore. It is a creeping, green, branched and frequently filamentous stage, resembling a green alga and reminding us of the algal ancestry of the group. The second stage is the leafy stage, which develops from the secondary protonema as a lateral bud. It consists of upright, slender axes bearing spirally arranged leaves. It is attached to the soil through multicellular and branched rhizoids. This stage bears the sex organs.

Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema. In sexual reproduction, the sex organs, antheridia and archegonia, are produced at the apex of the leafy shoots, clustered among leaves. After fertilisation, the zygote develops into a sporophyte, consisting of a foot, seta and capsule. The sporophyte in mosses is more elaborate than that in liverworts: the seta is long and wiry, the capsule has a lid (operculum) and a ring of teeth (the peristome) that opens and closes with changes in humidity to release the spores, and the young capsule is covered by a hood, the calyptra, torn from the archegonium. The capsule contains spores, formed after meiosis, and mosses have an elaborate mechanism of spore dispersal by the peristome. Common examples of mosses are Funaria, Polytrichum and Sphagnum.

Comparison of liverworts and mosses. Liverworts have a thalloid or leafy dorsiventral gametophyte with unicellular rhizoids, no protonema stage, gemma cups for asexual reproduction, a simple sporophyte with a short seta, no peristome, and elaters in the capsule. Mosses have a radially symmetrical leafy gametophyte with multicellular branched rhizoids, a filamentous protonema, no gemma cups, an elaborate sporophyte with a long seta, an operculum and peristome, and no elaters.

The bryophyte life cycle illustrates alternation of generations in its bryophyte form. The haploid spore germinates into a gametophyte (in mosses through the protonema), which is the main, green, independent plant. It bears antheridia and archegonia; the biflagellate antherozoids swim in a film of rain or dew to the archegonium, attracted chemically by sugars secreted by the neck, and one fuses with the egg. The diploid zygote, retained in the archegonium, grows by mitosis into the sporophyte, which stays attached to the gametophyte by its foot and draws water and food from it, although the moss capsule is green and partly self-supporting. In the capsule, spore mother cells undergo meiosis to form haploid spores, which are shed and germinate to complete the cycle. The gametophyte is the dominant generation, the sporophyte dependent on it; the cycle is haplo-diplontic, but with the haploid phase in charge. This is the reverse of the vascular plants, where the sporophyte dominates, and the bryophytes are therefore seen as an early experiment in land life, successful within the limits set by the need for water for fertilisation and the absence of vascular tissue.

The pteridophytes include horsetails and ferns. Pteridophytes are used for medicinal purposes and as soil binders. They are also frequently grown as ornamentals. Evolutionarily, they are the first terrestrial plants to possess vascular tissues, xylem and phloem. The pteridophytes are found in cool, damp, shady places, though some may flourish well in sandy-soil conditions; they cannot colonise dry places because they still need water for fertilisation.

Plant body. In pteridophytes the main plant body is a sporophyte, which is differentiated into true root, stem and leaves. These organs possess well-differentiated vascular tissues, which allow the plant to conduct water and food over a distance and to grow tall; the tree ferns reach twenty metres, and in the Carboniferous period pteridophytes formed forests whose remains are coal. The leaves in pteridophytes are small (microphylls), as in Selaginella, or large (macrophylls), as in ferns, where the large compound frond unrolls from a coiled crozier.

Reproduction. The sporophytes bear sporangia, which are subtended by leaf-like appendages called sporophylls. In some cases sporophylls may form distinct compact structures called strobili or cones (Selaginella, Equisetum). In ferns the sporangia are clustered in sori on the undersurface of the fronds, often protected by a flap, the indusium. The sporangia produce spores by meiosis in spore mother cells. The spores germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus. These gametophytes require cool, damp, shady places to grow. Because of this specific restricted requirement and the need for water for fertilisation, the spread of living pteridophytes is limited and restricted to narrow geographical regions. The gametophytes bear male and female sex organs called antheridia and archegonia respectively. Water is required for transfer of antherozoids, the male gametes released from the antheridia, to the mouth of the archegonium. Fusion of the male gamete with the egg present in the archegonium results in the formation of a zygote. The zygote thereafter produces a multicellular well-differentiated sporophyte, which is the dominant phase of the pteridophytes.

Comparison with bryophytes. Both groups need water for fertilisation and have multicellular antheridia and archegonia, and both show alternation of generations. But in pteridophytes the sporophyte is dominant, independent and vascular, with true roots, stems and leaves, while the gametophyte is a small, short-lived prothallus; in bryophytes the gametophyte is dominant and the sporophyte is attached and dependent. The pteridophytes are thus the first plants in which the diploid generation took over the business of living on land.

The pteridophytes are further classified into four classes: Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum) and Pteropsida (Dryopteris, Pteris, Adiantum). Psilotum is a rootless, leafless living fossil; Lycopodium is the club moss with microphylls and a terminal cone; Equisetum the horsetail with jointed, ribbed, silica-rich stems and whorled scale leaves; and the true ferns bear large fronds with sori.

The majority of the pteridophytes produce spores that are all of one kind and size, a condition called homospory: each spore germinates into a prothallus that bears both antheridia and archegonia (monoecious gametophyte). Ferns such as Dryopteris, Pteris and Adiantum, and Lycopodium and Equisetum, are homosporous.

A few genera, such as Selaginella and Salvinia, produce two kinds of spores: macrospores (megaspores), which are large and few, and microspores, which are small and many. Such plants are called heterosporous. The two kinds of spores are formed in separate sporangia, megasporangia and microsporangia, borne on megasporophylls and microsporophylls, which in Selaginella are collected in the same strobilus. The megaspores and microspores germinate to give rise to female and male gametophytes respectively: the megaspore produces a small female gametophyte bearing only archegonia, and the microspore a still smaller male gametophyte consisting of little more than an antheridium. The gametophytes are therefore dioecious, and they are much reduced compared with the free-living prothallus of a fern.

The crucial feature is that the female gametophytes in these plants are retained on the parent sporophyte for variable periods. In Selaginella the megaspore begins to develop its gametophyte while still inside the megasporangium; the megaspore wall cracks open to expose the archegonia, the swimming antherozoids from nearby microspores fertilise the egg, and the development of the zygote into the young embryo takes place within the female gametophyte, which is still attached to, and nourished by, the parent plant. This retention of the female gametophyte and the embryo on the sporophyte is a precursor to the seed habit, considered an important step in evolution.

A seed, as found in gymnosperms and angiosperms, is a mature ovule; an ovule is a megasporangium (the nucellus) enclosed in integuments, in which a single functional megaspore is permanently retained, develops into a female gametophyte, and, after fertilisation, holds the embryo. Three steps separate Selaginella from a true seed: reduction to one megaspore per megasporangium, permanent retention of that megaspore, and the growth of protective integuments around the megasporangium. Heterospory is the first of these, and it is why Selaginella and its fossil relatives are studied so closely.

The significance of heterospory can be summed up: it separates the sexes on to different gametophytes and so encourages cross-fertilisation; it reduces the gametophyte and shortens the vulnerable haploid phase; it provides the megaspore with a large food store for the developing embryo; and it leads to the retention of the female gametophyte on the sporophyte and thence to the seed, the structure that freed plants from dependence on water for fertilisation and allowed them to conquer dry land. Heterospory appeared independently in several fossil lines of the Devonian and Carboniferous, and the seed plants arose from one of them.

This topic is a favourite for a short-answer question, 'What is heterospory? Explain its significance', and for the one-liner 'Name two heterosporous pteridophytes' (Selaginella, Salvinia; Marsilea, Azolla and Isoetes are also heterosporous).

The gymnosperms (Greek gymnos, naked; sperma, seed) are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds that develop post-fertilisation are not covered, that is, are naked, being borne on the surface of open scales. Gymnosperms include medium-sized trees or tall trees and shrubs. One of the gymnosperms, the giant redwood tree Sequoia, is one of the tallest tree species, exceeding 100 metres. The roots are generally tap roots. Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with nitrogen-fixing cyanobacteria. The stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leaves may be simple or compound. In Cycas the pinnate leaves persist for a few years. The leaves in gymnosperms are well adapted to withstand extremes of temperature, humidity and wind. In conifers the needle-like leaves reduce the surface area, and their thick cuticle and sunken stomata also help to reduce water loss. The wood is of tracheids without vessels (except in the Gnetales) and the phloem lacks companion cells.

Reproduction. The gymnosperms are heterosporous: they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls, which are arranged spirally along an axis to form lax or compact strobili or cones. The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili. The microspores develop into a male gametophytic generation that is highly reduced and is confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains takes place within the microsporangia. The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili. The male and the female cones or strobili may be borne on the same tree (Pinus, monoecious) or on different trees (Cycas, dioecious). The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes, the integument, and the composite structure is called an ovule. The ovules are borne on megasporophylls, which may be clustered to form the female cones. The megaspore mother cell divides meiotically to form four megaspores. One of the megaspores enclosed within the megasporangium develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs. The multicellular female gametophyte is also retained within the megasporangium.

Unlike bryophytes and pteridophytes, in gymnosperms the male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes. The pollen grain is released from the microsporangium. It is carried in air currents (wind pollination) and comes in contact with the opening of the ovules borne on megasporophylls. The pollen tube carrying the male gametes grows towards the archegonia in the ovules and discharges its contents near the mouth of the archegonia. Following fertilisation, the zygote develops into an embryo and the ovule into a seed. The female gametophyte serves as the endosperm, the nutritive tissue, and being formed before fertilisation it is haploid. The seeds are not covered, since there is no fruit. Water is no longer required for fertilisation, the pollen tube having replaced the swimming sperm, though Cycas still has large multiflagellate sperms that swim the last short distance inside the ovule.

Examples and uses. Cycas, a palm-like plant of Indian gardens with a crown of pinnate leaves, is a living fossil; Pinus, Cedrus (deodar) and Abies are the conifers of the Himalaya, giving timber, resin and turpentine; Ginkgo is the maidenhair tree; Ephedra, a desert shrub, yields the drug ephedrine; Gnetum is a climber of Indian forests; Sequoia is the redwood. Gymnosperms dominated the Mesozoic era and remain the great forest trees of cold and dry regions.

Unlike the gymnosperms, where the ovules are naked, in the angiosperms (Greek angeion, vessel; sperma, seed) or flowering plants the pollen grains and ovules are developed in specialised structures called flowers, and the seeds are enclosed in fruits. The angiosperms are an exceptionally large group of plants occurring in wide range of habitats: on land, in water, in deserts, on mountains, as epiphytes on other plants and as parasites. They range in size from tiny, almost microscopic Wolffia, the smallest flowering plant at a millimetre across, to tall trees of Eucalyptus over 100 metres. They provide us with food, fodder, fuel, medicines and several other commercially important products. There are about 2,50,000 to 3,00,000 species, making them the dominant plants of the earth since the Cretaceous period.

Distinguishing characters. The reproductive organs are borne in the flower, which is a modified shoot with four whorls: calyx, corolla, androecium and gynoecium. The ovules are enclosed in an ovary, the basal part of the carpel; the top of the carpel, the stigma, receives the pollen, so that pollen never touches the ovule directly. After fertilisation the ovary becomes the fruit and the ovules become seeds. The gametophytes are more reduced than in gymnosperms: the male gametophyte is a two- or three-celled pollen grain, the female gametophyte a seven-celled, eight-nucleate embryo sac, and there are no archegonia. Fertilisation is double, and the endosperm is triploid and formed after fertilisation. The xylem has vessels and the phloem has companion cells, which make conduction efficient. Pollination is by insects, wind, water, birds and bats, and the fruit protects and disperses the seed. Life spans range from a few weeks to thousands of years, and the habit from herbs to shrubs, trees and climbers.

Classification. The angiosperms are divided into two classes, the dicotyledons and the monocotyledons, which differ as follows.

The dicots, with about 200,000 species, include most trees and shrubs and the families Fabaceae, Solanaceae, Asteraceae, Brassicaceae and Malvaceae; the monocots, with about 60,000 species, include the grasses and cereals, palms, lilies, orchids, bananas and gingers, and are mostly herbaceous. Modern phylogenetic work shows the monocots to be a natural group that arose early from within the dicots, while the dicots as traditionally defined include a few primitive families (the magnolias, laurels and water lilies) that are now placed apart; but the two-class division remains the working framework of this course and of the chapter on the taxonomy of angiosperms.

The reproduction of angiosperms is described in detail in the chapters on sexual reproduction in flowering plants; here it is outlined to complete the comparison with gymnosperms and to explain the two features unique to the group, double fertilisation and the triploid endosperm.

The male sex organs in a flower are the stamens, each of which consists of a slender filament with an anther at the tip. The anthers, following meiosis in the microspore mother cells, produce pollen grains. The pollen grain is the male gametophyte, reduced to two cells at shedding, a vegetative cell and a generative cell; the generative cell later divides to give two male gametes, so that the mature male gametophyte is three-celled, and it has no antheridium.

The female sex organ is the pistil or carpel, made of the stigma at the top, the style and the ovary at the base, which encloses one to many ovules. Within an ovule, the megaspore mother cell in the nucellus undergoes meiosis to form four megaspores; three degenerate and one functional megaspore, after three mitotic divisions, develops into the female gametophyte, the embryo sac. The mature embryo sac has seven cells and eight nuclei: an egg apparatus at the micropylar end, consisting of the egg cell and two synergids; three antipodal cells at the opposite end; and a large central cell with two polar nuclei. There is no archegonium; the female gametophyte is the most reduced in the plant kingdom.

Pollination is the transfer of pollen grains from the anther to the stigma, by wind, water, insects, birds or bats. The pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of the stigma and style and reach the ovule, entering through the micropyle. The pollen tube discharges its two male gametes into one of the synergids of the embryo sac.

Double fertilisation. One of the male gametes fuses with the egg cell to form a zygote (syngamy). The other male gamete fuses with the two polar nuclei of the central cell to produce the triploid primary endosperm nucleus (PEN); this fusion of three nuclei is called triple fusion. Because of the involvement of two fusions, syngamy and triple fusion, this event is termed double fertilisation, an event unique to angiosperms, discovered by Nawaschin in 1898.

Post-fertilisation. The zygote develops into an embryo with one or two cotyledons, and the primary endosperm nucleus develops into the endosperm, the nutritive tissue for the developing embryo, which is triploid and forms only after fertilisation; in gymnosperms the endosperm is the haploid female gametophyte formed before fertilisation, and the difference is a standard examination point. The synergids and antipodals degenerate after fertilisation. The ovules develop into seeds and the ovary develops into the fruit, which protects the seeds and aids their dispersal.

The advantages of the angiosperm system are clear. The ovule is protected inside the ovary; the stigma can select compatible pollen; the pollen tube makes water unnecessary; the endosperm is formed only if fertilisation has occurred, so no food is wasted on unfertilised ovules; and the fruit spreads the seed. Together with the flower, which recruits insects and other animals as pollinators, these features explain why angiosperms became the dominant plants of the earth within a few tens of millions of years of their appearance.

In plants, both haploid and diploid cells can divide by mitosis. This ability leads to the formation of different plant bodies, haploid and diploid. The haploid plant body produces gametes by mitosis; this plant body represents a gametophyte. Following fertilisation the zygote also divides by mitosis to produce a diploid sporophytic plant body. Haploid spores are produced by this plant body by meiosis. These in turn divide by mitosis to form a haploid plant body once again. Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between the gamete-producing haploid gametophyte and the spore-producing diploid sporophyte. This is alternation of generations.

Different plant groups, as well as individuals representing them, differ in the following patterns.

1. Haplontic life cycle. The sporophytic generation is represented only by the one-celled zygote. There are no free-living sporophytes. Meiosis in the zygote (zygotic meiosis) results in the formation of haploid spores. The haploid spores divide mitotically and form the gametophyte. The dominant, photosynthetic phase in such plants is the free-living gametophyte. This kind of life cycle is termed haplontic. Many algae such as Volvox, Spirogyra and some species of Chlamydomonas represent this pattern. In Spirogyra, for instance, the filament is haploid; conjugation forms a zygospore, the only diploid cell, which undergoes meiosis on germination and produces one new haploid filament.

2. Diplontic life cycle. On the other extreme is the type wherein the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant. The gametophytic phase is represented by the single- to few-celled haploid gametophyte. Meiosis occurs at gamete formation (gametic meiosis). This kind of life cycle is termed diplontic. An alga, Fucus, represents this pattern. All seed-bearing plants, gymnosperms and angiosperms, follow this pattern with some variations, wherein the gametophytic phase is few- to multi-celled and retained on the sporophyte: the pollen grain and the embryo sac or female gametophyte. Strictly, the seed plants have a much-reduced haplo-diplontic cycle, but because the gametophyte is not free-living, the textbook treats them under the diplontic pattern.

3. Haplo-diplontic life cycle. Bryophytes and pteridophytes, interestingly, exhibit an intermediate condition; both phases are multicellular and often free-living. However, they differ in their dominant phases. A dominant, independent, photosynthetic, thalloid or erect phase is represented by a haploid gametophyte and it alternates with the short-lived multicellular sporophyte totally or partially dependent on the gametophyte for its anchorage and nutrition. All bryophytes represent this pattern. The diploid sporophyte is represented by a dominant, independent, photosynthetic, vascular plant body. It alternates with a multicellular, saprophytic or autotrophic, independent but short-lived haploid gametophyte. Such a pattern is known as haplo-diplontic life cycle. All pteridophytes exhibit this pattern. Meiosis occurs at spore formation (sporic meiosis). Many algae also show the haplo-diplontic pattern, with alternation between free-living gametophyte and sporophyte generations: in Ectocarpus, Polysiphonia and the kelps the two generations may be identical in appearance (isomorphic) or different (heteromorphic).

The evolutionary trend. Reading the three patterns in order, the sporophyte grows steadily in importance: a single cell in the haplontic algae, a dependent capsule in bryophytes, an independent vascular plant in pteridophytes, the whole visible plant in seed plants; and the gametophyte shrinks in step, from the whole plant to a few cells inside a pollen grain or ovule. The reason is that the diploid condition, with two copies of every gene, buffers mutations and allows greater size and complexity, and the retention of the gametophyte on the sporophyte protects the delicate haploid phase from the dryness of land. The plant kingdom, surveyed group by group in this chapter, is a record of that shift.