Plant Kingdom | CBSE Class 11 Biology Notes

This note covers the plant kingdom: what Kingdom Plantae includes today, how systems of plant classification have changed, the algae and their three classes, the bryophytes (liverworts and mosses), the pteridophytes, the gymnosperms, the angiosperms, and how the life cycles of these groups compare.

What does the plant kingdom include?

In the Five Kingdom classification proposed by Whittaker (1969), living organisms are placed in five kingdoms: Monera, Protista, Fungi, Animalia and Plantae. Kingdom Plantae is popularly known as the plant kingdom.

Our understanding of the plant kingdom has changed over time. Earlier classifications placed fungi, and the members of Monera and Protista that have cell walls, in the same kingdom as plants. These have now been excluded from Plantae.

So the cyanobacteria, which are also referred to as blue green algae, are not 'algae' any more. Kingdom Plantae is now described under five groups: algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

How have systems of plant classification changed?

Classification within the angiosperms shows the concerns that shaped classification systems. Three broad kinds of system have been used: artificial, natural and phylogenetic.

Artificial systems

The earliest systems used only gross superficial morphological characters such as habit, colour, and the number and shape of leaves. They were based mainly on vegetative characters or on the structure of the androecium; the system given by Linnaeus was of this kind.

Such systems were artificial. Because they were based on a few characteristics, they separated closely related species.

They also gave equal weightage to vegetative and sexual characteristics. This is not acceptable, because vegetative characters are often more easily affected by the environment.

Natural systems

Natural classification systems were based on natural affinities among organisms. They considered not only external features but also internal features, such as ultrastructure, anatomy, embryology and phytochemistry.

Such a classification for flowering plants was given by George Bentham and Joseph Dalton Hooker.

Phylogenetic systems

At present, phylogenetic classification systems, based on evolutionary relationships between organisms, are acceptable. They assume that organisms belonging to the same taxa have a common ancestor.

Information from many other sources is now used to resolve difficulties in classification. These sources become more important when there is no supporting fossil evidence.

Newer sources of information

What are algae, and how do they reproduce?

Definition: Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms, found in both fresh water and marine habitats.

Algae also occur in a variety of other habitats: moist stones, soils and wood. Some occur in association with fungi, as in a lichen, and some with animals, for example on the sloth bear.

Form and size

The form and size of algae is highly variable. It ranges from colonial forms like Volvox to filamentous forms like Ulothrix and Spirogyra. A few marine forms, such as the kelps, form massive plant bodies.

Three methods of reproduction

Algae reproduce by vegetative, asexual and sexual methods.

• 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 are zoospores, which are flagellated (motile) and on germination give rise to new plants.
• Sexual reproduction takes place through the fusion of two gametes.

Types of sexual reproduction

Gametes differ in size and motility, giving three types of sexual reproduction.

Why are algae useful?

Algae are useful to man in a variety of ways.

Role in nature

• At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.
• Being photosynthetic, algae increase the level of dissolved oxygen in their immediate environment.
• They are of paramount importance as primary producers of energy-rich compounds, which form the basis of the food cycles of all aquatic animals.

Uses as food and commercial products

• Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food.
• Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), which are used commercially. Examples are algin from brown algae and carrageen from red algae.
• Agar is a commercial product obtained from Gelidium and Gracilaria. It is used to grow microbes and in preparations of ice-creams and jellies.
• Chlorella, a unicellular alga rich in proteins, is used as a food supplement, even by space travellers.

How do green, brown and red algae differ?

The algae are divided into three main classes: Chlorophyceae, Phaeophyceae and Rhodophyceae. They are classified by the type of pigment they possess and the type of stored food.

Chlorophyceae (green algae)

The members of Chlorophyceae are commonly called green algae. The plant body may be unicellular, colonial or filamentous. They are usually grass green because of the dominance of the pigments chlorophyll a and b.

The pigments are localised in definite chloroplasts. In different species the chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped.

Most 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.

Vegetative reproduction usually takes place by fragmentation. Asexual reproduction is by flagellated zoospores produced in zoosporangia. Sexual reproduction shows considerable variation in the type and formation of sex cells; it may be isogamous, anisogamous or oogamous.

Some commonly found green algae are Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.

Phaeophyceae (brown algae)

The members of Phaeophyceae, or 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 kelps, which may reach a height of 100 metres.

They possess chlorophyll a, c, carotenoids and xanthophylls. Their colour varies from olive green to various shades of brown, depending on the amount of the xanthophyll pigment fucoxanthin present.

Food is stored as complex carbohydrates, 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. It 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. The union of gametes may take place in water or within the oogonium (in oogamous species). The gametes are pyriform (pear-shaped) and bear two laterally attached flagella.

The common forms are Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.

Rhodophyceae (red algae)

The members of Rhodophyceae are commonly called red algae because of the predominance of the red pigment r-phycoerythrin in their body.

The majority of red algae are marine, with greater concentrations in the warmer areas. They occur both in well-lighted regions close to the surface of water and at great depths in oceans, where relatively little light penetrates.

The red thalli of most red algae are multicellular, and some have a complex body organisation. Food is stored as floridean starch, which is very similar to amylopectin and glycogen in structure.

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. The common members are Polysiphonia, Porphyra, Gracilaria and Gelidium.

The three classes side by side

Green algae: (a-i) Volvox, a round colony of many small cells, labelled parent colony, with daughter colonies inside it; (a-ii) Ulothrix, an unbranched filament of cells in a single row.

Brown algae: (b-i) Laminaria, labelled frond, stipe and holdfast; (b-ii) Fucus, a branched body labelled air bladder, frond, midrib and holdfast; (b-iii) Dictyota, a repeatedly branched body labelled frond and stipe.

Red algae: (c-i) Porphyra, a broad, folded, red thallus labelled frond; (c-ii) Polysiphonia, a finely branched body labelled main axis and branches.

See Fig. 3.1 in your NCERT textbook

What are bryophytes, and why are they important?

Bryophytes include the various mosses and liverworts that are commonly found growing in moist, shaded areas in the hills. They usually occur in damp, humid and shaded localities.

Bryophytes are called the amphibians of the plant kingdom because they can live in soil but are dependent on water for sexual reproduction. 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 is attached to the substratum by unicellular or multicellular rhizoids.

Bryophytes lack true roots, stem or leaves, though they may possess root-like, leaf-like or stem-like structures.

The main plant body of a bryophyte is haploid. It produces gametes, and so it is called a gametophyte.

Sexual reproduction and the life cycle

The sex organs of bryophytes are multicellular. The male sex organ is the antheridium, which produces biflagellate antherozoids. The female sex organ, the archegonium, is flask-shaped and produces a single egg.

1. The antherozoids are released into water, where they come in contact with the archegonium.
2. An antherozoid fuses with the egg to produce the zygote.
3. The zygote does not undergo reduction division immediately. It produces a multicellular body called a sporophyte.
4. The sporophyte is not free-living. It is attached to the photosynthetic gametophyte and derives nourishment from it.
5. Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores.
6. The spores germinate to produce the gametophyte.

Economic and ecological 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. Peat has long been used as fuel, and as packing material for the trans-shipment of living material because of its capacity to hold water.

Mosses, along with lichens, are the first organisms to colonise rocks, and so 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 two groups: liverworts and mosses.

How do liverworts and mosses differ?

Liverworts

Liverworts usually grow in moist, shady habitats such as the banks of streams, marshy ground, damp soil, the bark of trees and deep in the woods.

The plant body of a liverwort is thalloid, for example Marchantia. The thallus is dorsiventral and closely appressed to the substrate. The leafy members have tiny leaf-like appendages in two rows on the stem-like structures.

Asexual reproduction takes place by fragmentation of the thalli, or by specialised structures called gemmae (singular, gemma).

1. Gemmae, which are green, multicellular, asexual buds, develop in small receptacles called gemma cups.
2. The gemma cups are located on the thalli.
3. The gemmae become detached from the parent body.
4. They germinate to form new individuals.

During sexual reproduction, male and female sex organs are produced either on the same thallus or on different thalli. The sporophyte is differentiated into a foot, seta and capsule. After meiosis, spores are produced within the capsule, and they germinate to form free-living gametophytes.

Mosses

The predominant stage of the life cycle of a moss is the gametophyte, which consists of two stages.

• The protonema stage develops directly from a spore. It is a creeping, green, branched and frequently filamentous stage.
• The leafy stage develops from the secondary protonema as a lateral bud. It consists of upright, slender axes bearing spirally arranged leaves, attached to the soil by 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 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. Spores are formed in the capsule after meiosis, and mosses have an elaborate mechanism of spore dispersal.

Common examples of mosses are Funaria, Polytrichum and Sphagnum.

(a) Female thallus of the liverwort Marchantia: a flat, lobed green thallus bearing a stalked archegoniophore, with gemma cups on the thallus and rhizoids below. (b) Male thallus of Marchantia: a similar thallus bearing a stalked antheridiophore, with gemma cups and rhizoids labelled.

(c) The moss Funaria: the lower gametophyte, labelled leaves, main axis and rhizoids, carries the upper sporophyte, a long seta ending in a capsule. (d) Sphagnum gametophyte: a leafy shoot labelled branches, antheridial branch and archegonial branch.

See Fig. 3.2 in your NCERT textbook

What are pteridophytes?

The pteridophytes include horsetails and ferns. They are used for medicinal purposes and as soil-binders, and they are also 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 may flourish well in sandy-soil conditions.

The sporophyte is the main plant body

In bryophytes the dominant phase in the life cycle is the gametophytic plant body. In pteridophytes, however, the main plant body is a sporophyte, which is differentiated into true root, stem and leaves. These organs possess well-differentiated vascular tissues.

The leaves in pteridophytes may be small, called microphylls, as in Selaginella, or large, called macrophylls, as in ferns.

Sporangia, sporophylls and strobili

The sporophytes bear sporangia, which are subtended by leaf-like appendages called sporophylls. In some cases the sporophylls form distinct, compact structures called strobili or cones, as in Selaginella and Equisetum.

The sporangia produce spores by meiosis in spore mother cells.

Classes of pteridophytes

The pteridophytes are further classified into four classes.

(a) Selaginella: a spreading, branched plant with small leaves along the stem and roots hanging down, labelled leaves, stem and roots.

(b) Equisetum: an upright, jointed stem with whorls of slender branches at the nodes and a strobilus at the tip, rising from a horizontal rhizome; labels read strobilus, node, internode, branch and rhizome. (c) A photograph of a fern growing in a pot, with many long, divided leaves. (d) Salvinia: a plant with oval leaves along a horizontal axis and long, hair-like structures hanging below.

See Fig. 3.3 in your NCERT textbook

How do pteridophytes reproduce, and what is heterospory?

The life cycle of a pteridophyte has a dominant sporophyte and a small, free-living gametophyte. The steps run as follows.

1. The sporangia on the sporophyte produce spores by meiosis in spore mother cells.
2. The spores germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes, called prothallus.
3. The gametophytes bear male and female sex organs, called antheridia and archegonia respectively.
4. Water is required to transfer the antherozoids, the male gametes released from the antheridia, to the mouth of the archegonium.
5. The fusion of the male gamete with the egg present in the archegonium results in the formation of a zygote.
6. The zygote produces a multicellular, well-differentiated sporophyte, which is the dominant phase of the pteridophytes.

Why pteridophytes are restricted in spread

The 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.

Homospory and heterospory

In the majority of pteridophytes all the spores are of a similar kind; such plants are called homosporous.

Genera like Selaginella and Salvinia produce two kinds of spores, macro (large) and micro (small) spores, and are known as heterosporous. The megaspores and microspores germinate to give rise to female and male gametophytes respectively.

In these plants the female gametophytes are retained on the parent sporophytes for variable periods. The development of the zygotes into young embryos takes place within the female gametophytes.

Note: The retention of the female gametophyte on the parent sporophyte, with the zygote developing into a young embryo inside it, is a precursor to the seed habit. This is considered an important step in evolution.

What are gymnosperms?

Definition: Gymnosperms (gymnos: naked, sperma: seeds) 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 after fertilisation are not covered, that is, they are naked.

Gymnosperms include medium-sized trees, tall trees and shrubs. One of the gymnosperms, the giant redwood tree Sequoia, is one of the tallest tree species.

Roots

The roots are generally tap roots. Roots in some genera have a fungal association in the form of mycorrhiza, as in Pinus.

In some others, such as Cycas, small specialised roots called coralloid roots are associated with N₂-fixing cyanobacteria.

Stems and leaves

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 of 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 also help to reduce water loss.

(a) A photograph of Cycas, with a crown of long, pinnate leaves. (b) A photograph of a Pinus tree with drooping clusters of needle-like leaves.

(c) A drawing of a Ginkgo branch with fan-shaped leaves, labelled dwarf shoot, long shoot and seeds.

See Fig. 3.4 in your NCERT textbook

How do gymnosperms reproduce?

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.

Male strobili and the pollen grain

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 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.

Female strobili and the ovule

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, as in Pinus. In Cycas, however, the male cones and megasporophylls are borne on different trees.

The female gametophyte forms inside the ovule in the following order.

1. The megaspore mother cell is differentiated from one of the cells of the nucellus.
2. The nucellus is protected by envelopes, and this composite structure is called an ovule. The ovules are borne on megasporophylls, which may be clustered to form the female cones.
3. The megaspore mother cell divides meiotically to form four megaspores.
4. One of the megaspores, enclosed within the megasporangium, develops into a multicellular female gametophyte that bears two or more archegonia, the female sex organs.
5. The multicellular female gametophyte is also retained within the megasporangium.

Pollination and fertilisation

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.

1. The pollen grain is released from the microsporangium.
2. Pollen grains are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls.
3. The pollen tube, carrying the male gametes, grows towards the archegonia in the ovules and discharges its contents near the mouth of the archegonia.
4. The male gamete fuses with the egg cell in the archegonia.
5. Following fertilisation, the zygote develops into an embryo and the ovules into seeds. These seeds are not covered.

What are angiosperms, and how do they differ from gymnosperms?

In the angiosperms, or flowering plants, the pollen grains and ovules develop in specialised structures called flowers, and the seeds are enclosed in fruits. This is unlike the gymnosperms, where the ovules are naked.

The angiosperms are an exceptionally large group of plants, occurring in a wide range of habitats. They range in size from the smallest, Wolffia, to tall trees of Eucalyptus, which are over 100 metres tall.

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.

(a) A photograph of a dicotyledon: a potted plant with broad leaves and yellow flowers on stalks. (b) A photograph of a monocotyledon: a clump of long, narrow, strap-like leaves with a few white flowers.

See Fig. 3.5 in your NCERT textbook

Why gymnosperms and angiosperms are classified separately

Both groups bear seeds, but they differ in how the ovules and seeds are covered.

How do the life cycles of the plant groups compare?

Every group with archegonia, namely bryophytes, pteridophytes and gymnosperms, has a gamete-producing gametophyte and a spore-producing sporophyte. The groups differ in which of the two is dominant and in how independent each one is.

Ploidy of common cells

The gametophyte is haploid, and it produces gametes. Meiosis in the sporophyte produces haploid spores. The zygote, formed by the fusion of two haploid gametes, is diploid, and so is the sporophyte that grows from it.

Glossary

• Isogamy — Sexual reproduction by fusion of two gametes similar in size, which may be flagellated, as in Ulothrix, or non-flagellated, as in Spirogyra.
• Anisogamy — Sexual reproduction by fusion of two gametes that are dissimilar in size, as in species of Eudorina.
• Oogamy — Fusion of one large, non-motile female gamete with a smaller, motile male gamete, as in Volvox and Fucus.
• Fucoxanthin — The xanthophyll pigment of brown algae; its amount decides whether a brown alga looks olive green or a shade of brown.
• Gametophyte — The gamete-producing plant body; in bryophytes it is the haploid main plant body, and in pteridophytes it is the prothallus.
• Protonema — The first stage of the moss gametophyte; it develops directly from a spore and is creeping, green, branched and frequently filamentous.
• Gemma — A green, multicellular, asexual bud of liverworts that develops in a gemma cup on the thallus and grows into a new individual.
• Antheridium — The male sex organ of bryophytes and pteridophytes; in bryophytes it produces biflagellate antherozoids.
• Archegonium — The female sex organ; in bryophytes it is flask-shaped and produces a single egg. Pteridophytes and gymnosperms also bear archegonia.
• Sporophyll — A leaf-like appendage that subtends or bears sporangia; sporophylls may form compact strobili, as in Selaginella and Equisetum.
• Heterospory — The production of two kinds of spores, large megaspores and small microspores, as in Selaginella, Salvinia and the gymnosperms.

Common errors and misconceptions

• Misconception: Blue green algae are true algae in the plant kingdom. Correct: Blue green algae are cyanobacteria. Members of Monera with cell walls have been excluded from Plantae, so cyanobacteria are not 'algae' any more.
• Misconception: Linnaeus gave a natural system of classification. Correct: Linnaeus gave an artificial system based on androecium structure. The natural system for flowering plants was given by George Bentham and Joseph Dalton Hooker.
• Misconception: Isogamy means the gametes are always flagellated. Correct: Isogamous gametes are similar in size. They are flagellated in Ulothrix but non-flagellated in Spirogyra.
• Misconception: Red algae live only near the surface of the sea. Correct: Red algae occur both in well-lighted regions near the surface and at great depths where relatively little light penetrates.
• Misconception: The main plant body of a moss is the sporophyte. Correct: The main plant body of a bryophyte is the haploid gametophyte. The sporophyte is attached to it and derives nourishment from it.
• Misconception: The zygote of a bryophyte undergoes meiosis straight away. Correct: The zygote does not undergo reduction division immediately. It forms a multicellular sporophyte, some of whose cells later undergo meiosis to form spores.
• Misconception: All pteridophytes produce two kinds of spores. Correct: Most pteridophytes are homosporous. Genera like Selaginella and Salvinia are heterosporous.
• Misconception: Male and female cones of every gymnosperm are borne on different trees. Correct: In Pinus they may be borne on the same tree. In Cycas the male cones and megasporophylls are borne on different trees.

Exam-style questions with model answers

Q1. Why are red algae red? Name the pigment responsible. [1 mark]

1. Red algae (Rhodophyceae) are red because of the predominance of the red pigment r-phycoerythrin in their body.

Q2. Why are bryophytes called the amphibians of the plant kingdom? [2 marks]

1. Bryophytes can live in soil.
2. However, they are dependent on water for sexual reproduction, because the biflagellate antherozoids are released into water to reach the archegonium. Living on land but needing water to reproduce is why they are called amphibians of the plant kingdom.

Q3. What is the basis of classification of algae? Name the three classes. [3 marks]

1. Algae are classified mainly on the type of pigment they possess and the type of stored food.
2. Chlorophyceae (green algae) have chlorophyll a and b and store starch.
3. Phaeophyceae (brown algae) have chlorophyll a, c and fucoxanthin and store mannitol and laminarin.
4. Rhodophyceae (red algae) have chlorophyll a, d and phycoerythrin and store floridean starch.
5. The classes also differ in cell wall, flagella and habitat.

Q4. Distinguish between isogamy, anisogamy and oogamy, giving an example of each. [3 marks]

1. Isogamy is the fusion of two gametes similar in size. The gametes may be flagellated, as in Ulothrix, or non-flagellated (non-motile), as in Spirogyra.
2. Anisogamy is the fusion of two gametes dissimilar in size, as in species of Eudorina.
3. Oogamy is the fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete, as in Volvox and Fucus.

Q5. What is heterospory? Briefly comment on its significance and give two examples. [3 marks]

1. Heterospory is the production of two kinds of spores, macro (large) spores and micro (small) spores. The megaspores give rise to female gametophytes and the microspores to male gametophytes.
2. Examples: Selaginella and Salvinia.
3. Significance: the female gametophytes are retained on the parent sporophyte for variable periods, and the zygotes develop into young embryos within them. This is a precursor to the seed habit, considered an important step in evolution.

Q6. Both gymnosperms and angiosperms bear seeds. Why are they classified separately? [3 marks]

1. In gymnosperms the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation.
2. The seeds that develop after fertilisation are therefore not covered; gymnosperms are naked-seeded plants.
3. In angiosperms the pollen grains and ovules develop in specialised structures called flowers, and the seeds are enclosed in fruits. This difference in the covering of ovules and seeds is why they are placed in separate groups.

Q7. Differentiate between liverworts and mosses. [4 marks]

1. Plant body: in liverworts it is thalloid, dorsiventral and closely appressed to the substrate, as in Marchantia; mosses have upright, slender axes bearing spirally arranged leaves.
2. Gametophyte: in liverworts it is a thallus, as in Marchantia; the moss gametophyte has two stages, the protonema, which develops directly from a spore, and the leafy stage, which bears the sex organs.
3. Asexual reproduction: liverworts use fragmentation or gemmae formed in gemma cups; mosses use fragmentation and budding in the secondary protonema.
4. Sporophyte: both have a foot, seta and capsule, but the moss sporophyte is more elaborate, with an elaborate mechanism of spore dispersal. Examples of mosses are Funaria, Polytrichum and Sphagnum.

Q8. Name three groups of plants that bear archegonia. Briefly describe the life cycle of a pteridophyte. [5 marks]

1. Bryophytes, pteridophytes and gymnosperms bear archegonia.
2. The main plant body of a pteridophyte is a sporophyte, differentiated into true root, stem and leaves with vascular tissues. It bears sporangia subtended by sporophylls, which in some cases form strobili, as in Selaginella and Equisetum.
3. Spore mother cells in the sporangia undergo meiosis to produce spores.
4. The spores germinate into small, multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus, which need cool, damp, shady places.
5. The prothallus bears antheridia and archegonia. Water carries the antherozoids from the antheridia to the mouth of the archegonium.
6. The male gamete fuses with the egg in the archegonium to form a zygote, which produces a multicellular, well-differentiated sporophyte, the dominant phase.

Q9. Describe the important characteristics of gymnosperms. [5 marks]

1. The ovules are not enclosed by any ovary wall and remain exposed before and after fertilisation, so the seeds are naked. Gymnosperms are trees and shrubs; Sequoia is one of the tallest tree species.
2. The roots are generally tap roots. Pinus has mycorrhiza, while Cycas has coralloid roots associated with N₂-fixing cyanobacteria.
3. Stems are unbranched (Cycas) or branched (Pinus, Cedrus). Leaves are simple or compound; in conifers the needle-like leaves, thick cuticle and sunken stomata reduce water loss.
4. They are heterosporous. Microspores and megaspores form in sporangia on sporophylls arranged spirally into male and female strobili or cones. The male gametophyte is reduced to a pollen grain; one megaspore forms a female gametophyte with two or more archegonia.
5. The gametophytes are not free-living. Air currents carry pollen to the ovule; the pollen tube takes the male gametes to the archegonia. The zygote becomes an embryo and the ovule a seed.

Q10. Write a note on the economic importance of algae. [5 marks]

1. At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis, and they increase the level of dissolved oxygen in their immediate environment.
2. As primary producers of energy-rich compounds, they form the basis of the food cycles of all aquatic animals.
3. Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food.
4. Certain marine brown and red algae produce hydrocolloids used commercially: algin (brown algae) and carrageen (red algae).
5. Agar, obtained from Gelidium and Gracilaria, is used to grow microbes and in preparing ice-creams and jellies.
6. Chlorella, a unicellular alga rich in proteins, is used as a food supplement, even by space travellers.

Key takeaways

• The plant kingdom now includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms; fungi and walled members of Monera and Protista have been excluded.
• Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic, and reproduce by fragmentation, spores such as zoospores, and isogamy, anisogamy or oogamy.
• Algae are classified into Chlorophyceae, Phaeophyceae and Rhodophyceae by their pigments and stored food, which are starch, mannitol or laminarin, and floridean starch respectively.
• Bryophytes are the amphibians of the plant kingdom; the haploid gametophyte is the main body and carries the dependent sporophyte.
• Pteridophytes are the first terrestrial plants with xylem and phloem; the sporophyte is dominant and the gametophyte is a small free-living prothallus.
• Heterospory in Selaginella and Salvinia, with embryos developing inside retained female gametophytes, is a precursor to the seed habit.
• Gymnosperms have naked ovules and seeds, are heterosporous, and their reduced gametophytes are not free-living; the male gametophyte is the pollen grain.
• Angiosperms bear pollen grains and ovules in flowers and enclose their seeds in fruits; they are divided into dicotyledons and monocotyledons.

Test yourself

Why are cyanobacteria no longer called algae?

Members of Monera and Protista that have cell walls have been excluded from Plantae. Cyanobacteria, also called blue green algae, are therefore not 'algae' any more.

What do cytotaxonomy and chemotaxonomy use to classify plants?

Cytotaxonomy uses cytological information such as chromosome number, structure and behaviour. Chemotaxonomy uses the chemical constituents of the plant to resolve confusions.

Name the three parts of the plant body of a brown alga.

A holdfast, which attaches it to the substratum; a stalk called the stipe; and a leaf-like photosynthetic organ called the frond.

How does Sphagnum help people?

Species of Sphagnum provide peat, which has long been used as fuel and, because it holds water, as packing material for the trans-shipment of living material.

What are the two stages of the moss gametophyte?

The protonema stage, which develops directly from a spore, and the leafy stage, which develops from the secondary protonema as a lateral bud and bears the sex organs.

Why is the spread of living pteridophytes limited?

Their gametophytes need cool, damp, shady places to grow, and water is needed for fertilisation. So they are restricted to narrow geographical regions.

What is a pollen grain in a gymnosperm?

It is the highly reduced male gametophyte, confined to a limited number of cells, which develops from a microspore within the microsporangium.