Plant Kingdom

In the previous chapter you met Whittaker's five kingdoms and saw that the cyanobacteria, once called blue-green algae, now sit in kingdom Monera, and that the fungi form their own kingdom. So when this chapter says plant kingdom it means only the true plants: algae, bryophytes, pteridophytes, gymnosperms and angiosperms, taken in that order because that is the order in which they appeared in evolution and grew in complexity.

NCERT opens with a short history of classification systems, and a 1-mark or 2-mark question on it is common. Artificial systems were the earliest. Linnaeus's sexual system was of this kind: it used only a few vegetative characters or the structure of the androecium (stamens). The trouble is that such systems give equal weight to vegetative and sexual characters, and vegetative characters are easily changed by the environment, so unrelated plants get grouped together. Natural systems (Bentham and Hooker for flowering plants) were based on natural affinities and used a much wider set of features: external morphology and also internal features such as ultrastructure, anatomy, embryology and phytochemistry. Phylogenetic systems, used today, arrange plants by evolutionary relationships, on the assumption that organisms in the same taxon share a common ancestor.

Three newer sources of evidence support phylogenetic classification. Numerical taxonomy uses computers: all observable characters are given numbers or codes, every character gets equal importance, and hundreds of characters are compared at once. Cytotaxonomy uses cytological information, chromosome number, structure and behaviour. Chemotaxonomy uses the chemical constituents of the plant. The examiner wants the pairing exact: Linnaeus and artificial, Bentham and Hooker and natural, chromosomes and cytotaxonomy, chemicals and chemotaxonomy, computers and numerical taxonomy.

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms. Learn those five words in that order; they are the standard 1-mark definition. They live in fresh and marine water but also on moist stones, soils and wood, and some live in association with fungi (as lichens) and with animals (on the fur of the sloth bear). The body ranges from unicellular forms to colonies (Volvox), filaments (Ulothrix, Spirogyra) and the massive bodies of marine kelps.

Reproduction happens in three ways. Vegetative reproduction is by fragmentation, each fragment growing into a thallus. Asexual reproduction is by spores, most commonly by zoospores, which are flagellated and motile and grow into new plants on release. Sexual reproduction is by fusion of gametes and is named by the gametes' size. Isogamy: the gametes are similar in size, either flagellated (Ulothrix) or non-flagellated (Spirogyra). Anisogamy: two gametes dissimilar in size, as in some species of Chlamydomonas. Oogamy: fusion between one large, non-motile female gamete and a smaller motile male gamete, as in Volvox and Fucus. The exemplar MCQ 'fusion of two motile gametes dissimilar in size' has the answer anisogamy.

Economic importance is a favourite 2-mark and 3-mark question. Algae carry out at least half of the total carbon dioxide fixation on earth by photosynthesis, and being photosynthetic they raise the level of dissolved oxygen in their surroundings. They are the primary producers of energy-rich compounds that form the basis of the food cycles of all aquatic animals. About 70 species of marine algae are used as food: Porphyra, Laminaria and Sargassum. Some marine brown and red algae produce hydrocolloids (water-holding substances): algin from brown algae and carrageen from red algae, both used commercially. Agar, from Gelidium and Gracilaria, is used to grow microbes and in the preparation of ice creams and jellies. Chlorella, a unicellular alga rich in proteins, is used as a food supplement even by space travellers. The old NCERT example of Spirulina is no longer in the text; Spirulina is a cyanobacterium, so stick to Chlorella.

Members of Chlorophyceae are commonly called green algae. The plant body may be unicellular, colonial or filamentous. They are grass green because of the dominant pigments chlorophyll a and b, which are localised in definite chloroplasts. The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species; cup-shaped in Chlamydomonas and spiral (ribbon-shaped) in Spirogyra. Most members have one or more storage bodies called pyrenoids located in the chloroplasts. Pyrenoids contain protein besides starch. Some algae may store food as oil droplets. The cell wall has an inner layer of cellulose and an outer layer of pectose.

Vegetative reproduction usually takes place by fragmentation or by the formation of different types of spores. Asexual reproduction is by flagellated zoospores produced in zoosporangia. Sexual reproduction shows considerable variation in the type and formation of sex cells and may be isogamous, anisogamous or oogamous. The flagella, when present, are 2 to 8, equal and apical (Table 3.1). Habitat: fresh water, brackish water and salt water.

Commonly found green algae you must be able to quote: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara. NCERT Figure 3.1 shows Chlamydomonas (unicellular, biflagellate, cup-shaped chloroplast with a pyrenoid, an eye spot and two contractile vacuoles), Volvox (a hollow spherical colony with daughter colonies inside) and Chara (a multicellular form with a stem-like axis and whorls of branches bearing the sex organs). A question that asks 'which alga has a cup-shaped chloroplast' or 'which is a colonial green alga' is testing this figure.

Brown algae are found primarily in marine habitats. They show great variation in size and form: from simple branched filamentous forms (Ectocarpus) to profusely branched forms represented by kelps, which may reach a height of 100 metres. They possess chlorophyll a, chlorophyll c, carotenoids and xanthophylls. Their colour varies from olive green to various shades of brown depending on the amount of the xanthophyll pigment fucoxanthin. Food is stored as complex carbohydrates, laminarin or mannitol (the exemplar asks for mannitol as the reserve food of brown algae). The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin. The protoplast contains plastids, a centrally located vacuole and a 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. This holdfast-stipe-frond organisation is a standard MCQ (answer: Phaeophyceae) and a standard diagram (NCERT Figure 3.2, Laminaria, Fucus, Dictyota).

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. Habitat: fresh water (rare), brackish water and salt water. Common forms: Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus. Note two life-cycle facts used later: Fucus is diplontic and oogamous, while Ectocarpus and kelps are haplo-diplontic.

Members of Rhodophyceae are commonly called red algae because of the predominance of the red pigment r-phycoerythrin in their body. They possess chlorophyll a and d along with it (Table 3.1). Majority of red algae are marine, with a greater concentration 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. Phycoerythrin absorbs the blue-green light that reaches deep water, which is the reasoning behind an Assertion–Reason item on red algae at depth. The red thalli of most red algae are multicellular, and some of them have complex body organisation.

The food is stored as floridean starch, which is very similar to amylopectin and glycogen in structure. The cell wall is of cellulose. Red algae usually reproduce vegetatively by fragmentation. They reproduce asexually by non-motile spores and sexually by non-motile gametes. Sexual reproduction is oogamous and is accompanied by complex post-fertilisation developments. Flagella are absent altogether, which is why the spores and gametes are non-motile. Habitat: fresh water (some), brackish water and salt water (most).

Common members: Polysiphonia, Porphyra, Gracilaria and Gelidium. Tie the names to uses: Porphyra is eaten, Gelidium and Gracilaria give agar, and carrageen is a red-algal hydrocolloid. Polysiphonia is the NCERT example of a haplo-diplontic alga along with Ectocarpus and kelps.

Table 3.1, which you should be able to reproduce from memory, reads: Chlorophyceae: chlorophyll a and b; starch; cellulose; flagella 2–8, equal, apical; fresh, brackish and salt water. Phaeophyceae: chlorophyll a and c, fucoxanthin; mannitol, laminarin; cellulose and algin; flagella 2, unequal, lateral; fresh water (rare), brackish, salt water. Rhodophyceae: chlorophyll a and d, phycoerythrin; floridean starch; cellulose; flagella absent; fresh water (some), brackish, salt water (most).

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 antherozoids must swim to the archegonium. This single sentence is the most frequently asked 1-mark and 2-mark item in the chapter. They usually occur in damp, humid and shaded localities and play an important role in plant succession on bare rocks and soil.

The plant body is more differentiated than that of algae. It is thallus-like and prostrate or erect, attached to the substratum by unicellular or multicellular rhizoids. Bryophytes lack true roots, stem or leaves; they may possess root-like, leaf-like or stem-like structures. The main plant body is haploid. It produces gametes, hence is called a gametophyte. The sex organs are multicellular (a contrast with algae). The male sex organ, the antheridium, produces biflagellate antherozoids. The female sex organ, the archegonium, is flask-shaped and produces a single egg. The antherozoids are released into water where they come in contact with the archegonium. An antherozoid fuses with the egg to produce the zygote. The zygote does not undergo reduction division immediately; it produces 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, which germinate to produce the gametophyte. So in a bryophyte meiosis happens in the capsule of the sporophyte, at spore formation.

Economic importance: bryophytes are of little economic importance in general, but some mosses provide food for herbaceous mammals, birds and other animals. Species of Sphagnum, a moss, provide peat, which has long been used as fuel, and because of its capacity to hold water it is used as packing material for trans-shipment of living material. Mosses along with lichens are the first organisms to colonise rocks; 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.

Liverworts grow usually in moist, shady habitats such as banks of streams, marshy ground, damp soil, bark of trees and deep in the woods. The plant body of a liverwort is thalloid, for example Marchantia. The thallus is dorsiventral (an upper and a lower surface that differ) and closely appressed to the substrate. The leafy members have tiny leaf-like appendages 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. Because a gemma is a piece of the gametophyte, a gemma cell is haploid (n), which is one of the ploidy items in NCERT exercise 4.

Sexual reproduction: the male and female sex organs are produced either on the same or on different thalli. In Marchantia they are carried on upright stalked structures, the antheridiophore and archegoniophore, which NCERT Figure 3.3 shows and which are useful labels in a diagram. The sporophyte is differentiated into a foot, seta and capsule. After meiosis, spores are produced within the capsule. These spores germinate to form free-living gametophytes. The examiner wants you to use the exact NCERT triplet foot, seta, capsule for the liverwort sporophyte, the same three parts as in 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: it develops directly from a spore and is a creeping, green, branched and frequently filamentous stage. Its ploidy is n; the exemplar MCQ 'protonema is haploid and found in mosses' tests exactly this, and the exercise asks for the ploidy of a protonemal cell (n). 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. The leafy stage is attached to the soil through multicellular and branched rhizoids and bears the sex organs. A leaf cell of a moss is therefore also haploid.

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. 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 capsule contains spores, which are formed after meiosis. Mosses have an elaborate mechanism of spore dispersal (the peristome teeth of the capsule, though NCERT does not name them). Common examples are Funaria, Polytrichum and Sphagnum. NCERT Figure 3.3 shows Funaria as a leafy gametophyte with an attached sporophyte, plus Sphagnum and Polytrichum.

Liverworts versus mosses is a standard 2-mark or 3-mark question (NCERT exercise 9). Points: the liverwort gametophyte is thalloid and dorsiventral, the moss gametophyte has a protonema stage and a leafy stage with spirally arranged leaves; liverwort rhizoids are unicellular, moss rhizoids are multicellular and branched; liverworts reproduce asexually by gemmae, mosses by fragmentation and budding of the secondary protonema; the moss sporophyte is more elaborate than the liverwort sporophyte and has an elaborate spore-dispersal mechanism. Examples: Marchantia versus Funaria, Polytrichum, Sphagnum.

Pteridophytes include horsetails and ferns. They are used for medicinal purposes and as soil-binders, and are frequently grown as ornamentals. Evolutionarily they are the first terrestrial plants to possess vascular tissues, xylem and phloem. They are found in cool, damp, shady places, though some flourish in sandy-soil conditions.

In pteridophytes the main plant body is a sporophyte, differentiated into true root, stem and leaves, with well-differentiated vascular tissues. The leaves may be small (microphylls) as in Selaginella or large (macrophylls) as in ferns. The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. In some cases the sporophylls form distinct compact structures called strobili or cones (Selaginella, Equisetum). The sporangia produce spores by meiosis in spore mother cells; this is the site of reduction division in a fern. The spores germinate to give rise to inconspicuous, small, multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus. These gametophytes require cool, damp, shady places to grow, and 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, antheridia and archegonia. Water is required for the transfer of antherozoids, the male gametes released from the antheridia, to the mouth of the archegonium. Fusion of the male gamete with the egg produces a zygote, which develops into a multicellular well-differentiated sporophyte, the dominant phase. The prothallus is haploid, free-living and short-lived; the zygote of a fern is diploid.

Homospory and heterospory. In the majority of pteridophytes all the spores are of similar kind; such plants are homosporous (ferns such as Dryopteris, Pteris, Adiantum, and Equisetum). Genera like Selaginella and Salvinia produce two kinds of spores, macro (large) and micro (small), and are heterosporous. The megaspores and microspores germinate and give rise to female and male gametophytes respectively. In Selaginella and Salvinia the female gametophytes are retained on the parent sporophyte for variable periods, and the development of the zygote into a young embryo takes place within the female gametophyte. This event is a precursor to the seed habit, considered an important step in evolution. This is the significance of heterospory that the exercise question wants: two kinds of spores, two kinds of gametophyte, retention of the female gametophyte and embryo on the parent, hence the beginning of the seed habit. The exemplar asks why Selaginella still does not form a true seed: the female gametophyte is retained only for a while and there is no integument around the megasporangium and no protective seed coat, so the embryo is not released as a dormant seed.

Classes. Pteridophytes are further classified into four classes: Psilopsida (Psilotum); Lycopsida (Selaginella, Lycopodium); Sphenopsida (Equisetum); and Pteropsida (Dryopteris, Pteris, Adiantum). NCERT Figure 3.4 shows Selaginella, Equisetum and a fern; the fern sporophyte with a rhizome, fronds and sori on the underside of the leaflets is the diagram most often asked.

The name comes from gymnos, naked, and sperma, seed. In gymnosperms the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds that develop after fertilisation are not covered, that is, they are naked. This is the whole answer to exercise 6 (why gymnosperms and angiosperms are classified separately though both bear seeds): in angiosperms the ovules are enclosed in the ovary and the seeds are enclosed in fruits.

Vegetative features. 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 (the exemplar MCQ on the giant redwood expects gymnosperm). 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 N₂-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; their thick cuticle and sunken stomata help to reduce water loss. Any 'adaptations of gymnosperm leaves' question wants these three phrases.

Reproduction. Gymnosperms are heterosporous: they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia 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 which is highly reduced and confined to only a limited number of cells; this reduced gametophyte is called a pollen grain, and its development takes place within the microsporangia. The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili. The male and female cones may be borne on the same tree (Pinus) or on different trees (Cycas).

The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes, 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. The multicellular female gametophyte is also retained within the megasporangium. Unlike bryophytes and pteridophytes, in gymnosperms the male and 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, carried in air currents, 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 ovules into seeds; these seeds are not covered.

Gymnosperms are the third group (after bryophytes and pteridophytes) that bears archegonia; that is the answer to exercise 3. Economic importance (exercise 5): timber and soft wood from conifers, paper pulp from Pinus and other conifers, resin and turpentine from Pinus, edible seeds such as pine nuts (chilgoza) and the sago-like starch from the stem of Cycas, ephedrine from Ephedra, and ornamentals such as Cycas, Araucaria and Thuja.

In the rationalised NCERT this section is short: it defines the group and leaves the detail of the flower, gametophytes and double fertilisation to Class 12 (Sexual Reproduction in Flowering Plants). Unlike the gymnosperms where the ovules are naked, in angiosperms 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 a wide range of habitats. They range in size from the tiny, almost microscopic Wolffia to the tall trees of Eucalyptus (over 100 metres). They provide us with food, fodder, fuel, medicines and several other commercially important products.

They are divided into two classes: the dicotyledons and the monocotyledons. The dicotyledons are characterised by having two cotyledons in their seeds while the monocotyledons have only one. NCERT exercise 10 asks how you would distinguish monocots from dicots, and the school answer draws on what you already know from Class 9 and 10: dicots have two cotyledons, reticulate leaf venation, a tap-root system, tetramerous or pentamerous flowers and vascular bundles in a ring (open, with cambium); monocots have one cotyledon, parallel venation, fibrous roots, trimerous flowers and scattered closed vascular bundles.

Two facts from the old, now-deleted text still appear in school papers and in the NCERT exemplar, so keep them ready in one line each. The angiosperm embryo sac (female gametophyte) has 7 cells and 8 nuclei: an egg apparatus of one egg and two synergids at the micropylar end, three antipodals at the chalazal end, and a large central cell with two polar nuclei. Double fertilisation is unique to angiosperms: one male gamete fuses with the egg (syngamy, giving the diploid zygote) and the other fuses with the two polar nuclei (triple fusion, giving the triploid primary endosperm nucleus). If a plant has 2n = 36, its gametes have 18 and its endosperm has 3 × 18 = 54 chromosomes; that is the exemplar calculation. The primary endosperm nucleus in a dicot is 3n and the meristem cell of a monocot is 2n, the two ploidy items in exercise 4 that come from angiosperms.

In plants, both haploid and diploid cells can divide by mitosis. This property 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. The three patterns differ only in which phase is dominant and free-living (NCERT Figure 3.7).

(a) Haplontic. The sporophytic generation is represented only by the one-celled zygote. There are no free-living sporophytes. Meiosis in the zygote results in the formation of haploid spores. The haploid spores divide mitotically and form the gametophyte. The dominant, photosynthetic phase is the free-living gametophyte. Examples: many algae such as Volvox, Spirogyra and some species of Chlamydomonas.

(b) Diplontic. The diploid sporophyte is the dominant, photosynthetic, independent phase. The gametophytic phase is represented by a single to few-celled haploid gametophyte. All seed-bearing plants, gymnosperms and angiosperms, follow this pattern with some variations, wherein the gametophytic phase is few to multi-celled. Among algae, Fucus is diplontic. Here meiosis happens at spore (gamete) formation in the sporophyte.

(c) Haplo-diplontic. This is the intermediate condition. Both phases are multicellular and often free-living; however, they differ in their dominant phases. In bryophytes a dominant, independent, photosynthetic, thalloid or erect gametophyte alternates with a short-lived multicellular sporophyte that is totally or partially dependent on the gametophyte for anchorage and nutrition. In pteridophytes the diploid sporophyte is the dominant, independent, photosynthetic, vascular plant body; it alternates with a multicellular, saprophytic or autotrophic, independent but short-lived haploid gametophyte. Interestingly, while most algal genera are haplontic, some such as Ectocarpus, Polysiphonia and kelps are haplo-diplontic.

Exercise 2 asks when and where reduction division takes place in five plants; the answer is one line each. Liverwort: in the capsule of the sporophyte, at spore formation. Moss: in the capsule of the sporophyte, at spore formation. Fern: in the spore mother cells inside the sporangia on the sporophylls (sori), at spore formation. Gymnosperm: in the microspore mother cells in the microsporangia (male cone) and in the megaspore mother cell in the nucellus of the ovule (female cone). Angiosperm: in the microspore mother cells of the anther and in the megaspore mother cell of the ovule inside the ovary. In every case meiosis happens at spore formation, never at gamete formation, because plant gametes are made by mitosis.