Plant Kingdom | ISC Class 11 Biology Notes

This note covers the major plant groups, algal structure and reproduction, green, brown and red algae, liverworts and mosses, the life cycles of Funaria, a typical fern and Pinus, spore types, alternation of generations, and the economic importance of algae, bryophytes, pteridophytes and gymnosperms.

What features help us distinguish the major plant groups?

Plant Kingdom, or Plantae, includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms. A cell wall alone does not place an organism in this kingdom. Fungi and the organisms called blue-green algae, or cyanobacteria, are excluded from Plantae in the five-kingdom classification.

Which structural features matter?

A thallus is a plant body not differentiated into true roots, stems and leaves. Algae have a thalloid body. Bryophytes may have root-like, stem-like and leaf-like structures, whereas pteridophytes possess true roots, stems and leaves with vascular, or conducting, tissues.

Xylem is vascular tissue that conducts water and minerals; phloem conducts food materials. Pteridophytes possess these tissues. Gymnosperms produce exposed seeds. Angiosperms, the flowering plants, produce seeds enclosed within fruits.

Which reproductive terms are needed?

A gamete is a reproductive cell that fuses with another gamete during fertilisation. Their fusion produces a zygote, the cell formed by fertilisation. A spore can germinate to begin a new plant generation without fusing with another cell.

Haploid means having one chromosome set, written as n; diploid means having two sets, written as 2n. Chromosomes are cellular structures carrying hereditary information. These symbols refer to chromosome sets, not the number of cells in a plant.

A gametophyte is the haploid, gamete-producing generation. A sporophyte is the diploid generation that produces haploid spores through meiosis, the division that reduces chromosome sets from two to one. Alternation of generations is the succession of these two phases in a life cycle.

Note: A dominant generation is the main, conspicuous plant phase. Bryophytes have a dominant gametophyte; pteridophytes have a dominant sporophyte. The leafy appearance of a plant does not by itself establish which generation it represents.

What are algae, and how do they reproduce?

Algae are simple, thalloid organisms bearing chlorophyll, a light-absorbing pigment. They are autotrophic, meaning that they make their own food, and largely aquatic. Their habitats include fresh water and the sea, as well as moist stones, soil and wood.

Algal bodies vary greatly. Volvox is colonial, with cells grouped into a colony. Ulothrix and Spirogyra are filamentous, with cells arranged in threads. A few marine forms, such as kelps, develop massive bodies. Algae are therefore not all single-celled or microscopic.

What distinguishes the three methods of reproduction?

Vegetative reproduction produces new individuals from parts of an existing plant body. In algal fragmentation, the body breaks into pieces, each of which develops into a new thallus. Asexual reproduction involves no fusion of gametes and can occur through spores.

The most common algal spores are zoospores, motile spores bearing flagella, the slender structures used for movement. Motile means capable of movement. Zoospores germinate into new plants. Sexual reproduction involves fusion of two gametes and is classified by their relative sizes and movement.

The female gamete in oogamy is called an egg. Size and motility must be considered separately: similar-sized gametes need not be motile, and the male gamete in red algae is non-motile even though their sexual reproduction is oogamous.

These reproductive methods should also be distinguished from the division into green, brown and red algae. Those classes differ in their major pigments, stored food, cell walls and flagellar features; a sexual method alone does not identify the algal class.

How do green, brown and red algae differ?

What characterises green algae?

Chlorophyceae, the green algae, may be unicellular, colonial or filamentous. They are usually grass green because chlorophyll a and b dominate. Here, the letters identify different forms of chlorophyll. The pigments occur within chloroplasts, the cell structures in which photosynthesis, the production of organic food using light energy, takes place.

Chloroplasts may be disc-shaped, plate-like, net-like, cup-shaped, spiral or ribbon-shaped. Most members contain one or more pyrenoids, storage bodies in the chloroplasts containing protein besides starch. Some algae may also store food as oil droplets.

Green algae usually have a rigid wall with an inner cellulose layer and an outer pectose layer. Cellulose and pectose are cell-wall materials. Vegetative reproduction usually occurs by fragmentation. Zoospores develop inside zoosporangia, structures that produce these motile spores.

Sexual reproduction may be isogamous, anisogamous or oogamous. Examples include Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara. The range of body forms and reproductive methods is important: no single body shape describes all green algae.

What characterises brown and red algae?

Phaeophyceae, the brown algae, occur primarily in marine habitats. They range from simple branched filaments, such as Ectocarpus, to profusely branched kelps that may reach 100 metres. Their pigments include chlorophyll a and c, together with accessory carotenoid pigments such as xanthophylls.

Fucoxanthin is the xanthophyll pigment whose amount influences colour, from olive green to shades of brown. Food reserves include laminarin and mannitol. Their cellulosic walls are usually covered externally by a gelatinous coating of algin, a water-holding substance.

Rhodophyceae, the red algae, owe their colour to the predominance of r-phycoerythrin, a red pigment. The majority are marine, with greater concentrations in warmer areas. They occur near well-lit water surfaces and at great ocean depths where relatively little light penetrates.

Most red algal thalli are multicellular; some have complex organisation. Their reserve is floridean starch, a stored carbohydrate structurally similar to amylopectin and glycogen, two storage carbohydrates. Examples include Polysiphonia, Porphyra, Gracilaria and Gelidium.

Which features should be compared together?

In the comparison below, apical means at the tip and lateral means at the side. Brackish water is water with a salt content between fresh water and seawater. Flagellar entries describe the characteristic flagellated cells, not every cell or every genus in a class.

Pectin and poly sulphate esters are constituents of red algal cell walls.

The table records the main wall materials; the more detailed green algal description also distinguishes the inner cellulose and outer pectose layers.

How are algal body parts and reproductive cells related to their functions?

A brown algal body is usually attached to its substratum, the surface on which it grows, by a holdfast. The stalk is the stipe, and the leaf-like photosynthetic part is the frond. These terms describe a thallus rather than true root, stem and leaf differentiation.

In most brown algae, asexual reproduction involves pear-shaped zoospores with two unequal flagella attached laterally. Their sexual reproduction may be isogamous, anisogamous or oogamous. Gamete fusion can occur in water or within the oogonium, the female reproductive structure in oogamous forms.

Why must flagellar descriptions be used carefully?

The brown algal description of two laterally attached flagella applies to motile reproductive cells. It does not make the large female gamete in oogamy motile. Fucus, for example, illustrates fusion of a large, non-motile female gamete with a smaller, motile male gamete.

Red algae usually reproduce vegetatively by fragmentation. Their asexual spores and sexual gametes are non-motile. Sexual reproduction is oogamous and is followed by complex developments after fertilisation. Thus, the absence of flagella does not mean the absence of sexual reproduction.

An air bladder is a gas-containing swelling, and a midrib is the central rib of the frond. These labels belong to the illustrated Fucus body. They should not be added indiscriminately to drawings of every brown alga.

Volvox is drawn as a rounded colony with daughter colonies inside; Ulothrix is a cellular filament. Laminaria has a labelled frond, stipe and holdfast. Fucus has labelled air bladders, frond, midrib and holdfast. Porphyra is broad and sheet-like; Polysiphonia is finely branched.

See Fig. 3.1 in your NCERT textbook

Body form and microscopic characters provide complementary information. A broad frond does not by itself distinguish a brown alga from a red alga. Pigments, reserve food, wall composition and reproductive cells provide additional features for identification.

What is the economic and ecological importance of algae?

Algae contribute to both natural food systems and useful products. Through photosynthesis, the production of organic food using light energy, they carry out at least a half of the total carbon dioxide fixation on Earth. Fixation means incorporating carbon dioxide into organic compounds.

Photosynthetic algae increase dissolved oxygen in their immediate environment. As primary producers, organisms that make the energy-rich compounds supporting food chains, they form the basis of aquatic animal food cycles. These ecological roles are distinct from their direct commercial uses.

Which five uses can be described with examples?

1. Food: Many species of Porphyra, Laminaria and Sargassum are among the marine algae eaten as food. The edible examples include red and brown algae.
2. Algin production: Certain brown algae yield algin, a commercially used hydrocolloid. A hydrocolloid is a water-holding substance.
3. Carrageen production: Certain red algae yield carrageen, another commercially used hydrocolloid. Its algal source differs from that of algin.
4. Agar production: Gelidium and Gracilaria yield agar, a product used to grow microbes and to prepare ice-creams and jellies. Microbes are microscopic organisms.
5. Protein supplement: Chlorella is a protein-rich unicellular alga used as a food supplement, including by space travellers.

Keep each product paired with its source. Agar should be linked with Gelidium and Gracilaria; algin with brown algae; and carrageen with red algae. Chlorella illustrates a protein supplement, while Porphyra, Laminaria and Sargassum illustrate marine food algae.

The water-holding property explains why algin and carrageen belong to the hydrocolloid category. Their commercial use and the role of algae in oxygen production are different kinds of importance, so they should be expressed as separate ideas rather than merged into one vague statement.

What distinguishes bryophytes, liverworts and mosses?

Bryophytes include liverworts and mosses. They usually grow in damp, humid and shaded places. They are called the amphibians of the plant kingdom because they can live on soil but depend on water for sexual reproduction.

The main body is a haploid gametophyte. It lacks true roots, stems and leaves and attaches to the substratum through rhizoids, root-like anchoring structures that may be unicellular or multicellular. The body may lie along the surface or grow upright.

How do their sex organs function?

The multicellular male sex organ, the antheridium, produces antherozoids, male gametes bearing two flagella. The multicellular female sex organ, the archegonium, is flask-shaped and produces one egg. Water allows the antherozoids to reach an archegonium.

Fertilisation produces a zygote. It develops into a multicellular sporophyte rather than immediately undergoing meiosis. The sporophyte remains attached to the photosynthetic gametophyte and obtains nourishment from it. Meiosis within the sporophyte produces haploid spores that germinate into gametophytes.

How do liverworts and mosses compare?

In the liverwort Marchantia, the thallus is dorsiventral, with distinct upper and lower surfaces, and lies close to the substratum. Gemmae are green, multicellular asexual buds produced in cup-like receptacles called gemma cups. Detached gemmae germinate into new individuals.

In mosses, the protonema is the creeping, green, branched and frequently filamentous gametophyte stage that develops from a spore. A leafy stage arises as a lateral bud from the secondary protonema, the later protonemal growth.

The foot is the basal part attaching the sporophyte to the gametophyte; the seta is its stalk; the capsule is the spore-containing part. These structures belong to the sporophyte, not to the leafy gametophyte.

Why are bryophytes useful?

Bryophytes in general have little economic importance, but some mosses provide food for animals. Sphagnum provides peat, accumulated plant material used as fuel, and serves as packing material for transporting living material because of its capacity to hold water.

Mosses and lichens, associations of fungi with algae, are the first organisms to colonise rocks. They help break down rocks and make the substratum suitable for higher plants. Dense moss mats reduce the impact of falling rain and prevent soil erosion.

How does Funaria show alternation of generations?

Funaria is a moss whose dominant generation is the gametophyte. Both the protonema and the leafy stage belong to this haploid generation. The leafy shoots bear the sex organs, while the attached sporophyte produces spores in its capsule.

What is the sequence of the life cycle?

1. Spore germination: A haploid spore develops into a green, creeping, branched protonema. This begins the gametophyte generation.
2. Leafy shoot formation: Lateral buds from the secondary protonema develop into upright leafy shoots attached by multicellular, branched rhizoids.
3. Gamete production: Antheridia and archegonia form at the tips of leafy shoots. They produce antherozoids and eggs respectively.
4. Fertilisation: Antherozoids travel through water to the archegonium. Fusion with the egg forms a diploid zygote.
5. Sporophyte development: The zygote develops into the attached sporophyte, differentiated into a foot, seta and capsule. It receives nourishment from the gametophyte.
6. Spore production: Meiosis in the capsule produces haploid spores. Their dispersal and germination begin the gametophyte generation again.

The drawing labels the leafy lower portion as the gametophyte, with leaves, main axis and rhizoids. The long seta and terminal capsule form the visible upper sporophyte, which remains attached to the gametophyte.

See Fig. 3.2c in your NCERT textbook

Join these stages with arrows: spore (n) → protonema (n) → leafy gametophyte (n) → gametes (n) → fertilisation → zygote (2n) → attached sporophyte (2n) → meiosis in capsule → spores (n). Here n means one chromosome set and 2n means two sets.

Where does the chromosome-set number change?

Fertilisation changes the condition from haploid gametes to a diploid zygote. Meiosis restores the haploid condition when spores form. The protonema is not a sporophyte merely because it grows from a spore; it is part of the gametophyte generation.

Alternation therefore involves two multicellular generations with different reproductive roles. The gametophyte produces gametes, while the sporophyte produces spores. In Funaria, the conspicuous green leafy plant is the gametophyte, and the sporophyte is nutritionally dependent on it.

What characterises pteridophytes and their four classes?

Pteridophytes include horsetails and ferns. They are the first terrestrial plants, in evolutionary terms, to possess vascular tissues. Their main body is a sporophyte differentiated into true roots, stems and leaves containing well-developed xylem and phloem.

They occur in cool, damp and shady places, though some may flourish in sandy soil. Leaves may be small, called microphylls, as in Selaginella, or large, called macrophylls, as in ferns. These terms describe contrasting leaf forms.

Where are spores formed?

A sporangium is a spore-producing structure. Pteridophyte sporangia are subtended by leaf-like appendages called sporophylls. In some plants, sporophylls form compact groups called strobili, or cones; examples include Selaginella and Equisetum. The singular of strobili is strobilus.

Spore mother cells are diploid cells that undergo meiosis to produce spores. A germinating spore develops into a small, inconspicuous, multicellular gametophyte called a prothallus. This gametophyte is free-living, thalloid and mostly photosynthetic.

A node is a point on a stem where leaves arise; an internode is the stretch between successive nodes. A rhizome is an underground stem. These terms identify the labelled parts in the Equisetum drawing.

Selaginella is drawn with labelled leaves, stem and roots. Equisetum has a labelled terminal strobilus, node, internode, branch and rhizome. The fern panel is a photograph of leafy plants; the Salvinia panel is a drawing.

See Fig. 3.3 in your NCERT textbook

What is their economic importance?

Pteridophytes are used for medicinal purposes and as soil-binders, plants that help hold soil in place. They are also frequently grown as ornamentals, meaning plants cultivated for decoration. These are distinct uses of the group.

How do the fern life cycle and heterospory explain reproductive change?

In a typical fern, the conspicuous leafy plant is the diploid sporophyte. The gametophyte is a much smaller prothallus. This reverses the dominance seen in mosses, although both groups require water for the male gametes to reach the female sex organ.

What is the graphic outline of a typical fern life cycle?

1. Spore formation: Spore mother cells within the sporangia of the sporophyte undergo meiosis, producing haploid spores.
2. Gametophyte formation: A spore germinates into a small, multicellular, free-living prothallus. Pteridophyte gametophytes are mostly photosynthetic.
3. Sex organ formation: The prothallus bears antheridia and archegonia, which produce male gametes and eggs respectively.
4. Water-dependent fertilisation: Antherozoids move through water to the archegonium. A male gamete fuses with the egg to form a diploid zygote.
5. New sporophyte formation: The zygote develops into a well-differentiated multicellular sporophyte with true roots, stem and leaves.

Draw the sequence sporophyte (2n) → sporangia and meiosis → spores (n) → prothallus (n). Branch to antheridia and archegonia, then join their gametes at fertilisation → zygote (2n) → new sporophyte (2n). Label the male-gamete transfer as requiring water.

The gametophytes require cool, damp and shaded conditions. This requirement, together with the need for water during fertilisation, restricts the distribution of living pteridophytes. Possession of vascular tissue does not remove that reproductive dependence on water.

What is the significance of two spore types?

Homospory means producing one kind of spore. The majority of pteridophytes are homosporous; Dryopteris is a homosporous fern. Heterospory means producing two kinds: large megaspores, also called macrospores, and small microspores. Selaginella and Salvinia are heterosporous.

Megaspores develop into female gametophytes, while microspores develop into male gametophytes. The female gametophytes are retained on the parent sporophyte for variable periods. Within them, zygotes develop into young embryos, the early developing sporophytes.

This retention and internal embryo development are a precursor to the seed habit, an earlier condition leading towards reproduction by seeds. It does not mean that Selaginella or Salvinia produces seeds. Spore type, gametophyte retention and seed formation remain separate features.

What makes gymnosperms distinctive, and why are they useful?

Gymnosperms are plants whose ovules are not enclosed by an ovary wall. An ovule is the structure that develops into a seed after fertilisation; an ovary is the enclosing floral structure present around ovules in flowering plants. Gymnosperm ovules remain exposed before and after fertilisation.

The resulting seeds are therefore described as naked, meaning not enclosed within fruits. Gymnosperms include medium-sized or tall trees and shrubs. The giant redwood Sequoia is one of the tallest tree species. Cycas, Pinus and Cedrus are other examples.

How are roots, stems and leaves organised?

The roots are generally tap roots, with a main root and branches. Some genera, such as Pinus, have roots associated with fungi in mycorrhiza. Cycas has specialised coralloid roots associated with nitrogen-fixing cyanobacteria. Nitrogen fixation converts atmospheric nitrogen into forms usable in biological systems.

Stems may be unbranched, as in Cycas, or branched, as in Pinus and Cedrus. Leaves may be simple, with a blade not divided into separate leaflets, or compound, with separate leaflets. Cycas has pinnate leaves, with leaflets arranged along a common axis, which persist for a few years.

Gymnosperm leaves are adapted to extremes of temperature, humidity and wind. In conifers, the group containing Pinus, needle-like leaves reduce surface area. A thick cuticle, the protective outer covering, and sunken stomata, gas-exchange pores below the surface, help reduce water loss.

Which economic uses are important?

Pinus wood is used as timber and for paper pulp, the fibrous raw material used in paper-making. Pine resin, a sticky plant secretion, is a source of turpentine and rosin, commercially useful products. Cycas is cultivated as an ornamental plant.

These examples link gymnosperms with construction materials, paper manufacture, resin products and decorative planting. Keep the plant and product associated: timber and resin here refer to Pinus, while the ornamental example is Cycas. A shared naked-seed condition does not imply identical uses for every gymnosperm.

How does the Pinus life cycle produce naked seeds?

Pinus is a gymnosperm with male and female cones on the same tree. Gymnosperms are heterosporous, producing microspores and megaspores. Their gametophytes are reduced and do not have an independent, free-living existence like the typical fern prothallus.

What do the two cones contain?

Male cones bear microsporophylls, sporophylls carrying microsporangia, the structures in which microspores are produced. A microspore develops into a highly reduced male gametophyte called a pollen grain. Pollen development takes place inside the microsporangium.

Female cones bear megasporophylls, sporophylls carrying ovules. The nucellus is the tissue of the megasporangium, the large-spore-producing structure within an ovule. Protective envelopes surround it. A cell of the nucellus differentiates into the megaspore mother cell.

The megaspore mother cell undergoes meiosis to produce four megaspores. One develops into a multicellular female gametophyte that remains within the megasporangium. It bears two or more archegonia. The female gametophyte is retained even though the ovule is not enclosed by an ovary.

What is the sequence from spores to seeds?

1. Spore production: The diploid tree produces cones. Meiosis produces haploid microspores in microsporangia and megaspores within ovules.
2. Gametophyte development: Microspores develop into pollen grains. One megaspore in an ovule develops into the retained female gametophyte bearing archegonia.
3. Pollination: Pollen grains are released and carried by air currents to the openings of ovules. Pollination is pollen transfer; it is not gamete fusion.
4. Pollen-tube growth: A pollen tube, a tubular outgrowth of the pollen grain, grows towards the archegonia and carries the male gametes.
5. Fertilisation: The tube discharges its contents near the archegonia. Fusion of a male gamete with an egg produces a diploid zygote.
6. Seed development: The zygote develops into an embryo and the ovule develops into a seed. The seed remains uncovered by a fruit. On germination, its embryo grows into a new sporophyte.

Begin with the Pinus sporophyte (2n). Draw a male branch through meiosis, microspores (n), pollen grains and male gametes; draw a female branch through meiosis, a megaspore (n), female gametophyte and egg. Join at fertilisation, then show zygote (2n), embryo and naked seed. Add germination and an arrow returning to the sporophyte. Label wind transfer of pollen and pollen-tube growth.

The two branches show alternation of generations despite the small size and dependence of the gametophytes. Pollination brings pollen to an ovule; the pollen tube then carries the male gametes towards the egg. Meiosis and fertilisation retain their distinct roles in changing chromosome-set number.

Glossary

• Thallus — A plant body that is not differentiated into true roots, stems and leaves.
• Isogamy — Sexual reproduction involving fusion of two gametes that are similar in size.
• Anisogamy — Sexual reproduction involving fusion of two gametes that differ in size.
• Oogamy — Fusion involving a large, non-motile female gamete and a smaller male gamete.
• Gametophyte — The haploid generation of a plant life cycle that produces gametes.
• Sporophyte — The diploid generation of a plant life cycle that produces haploid spores through meiosis.
• Rhizoid — A root-like structure that attaches a bryophyte to the surface on which it grows.
• Protonema — The creeping, green, branched and frequently filamentous moss gametophyte stage developing from a spore.
• Archegonium — The flask-shaped female sex organ in bryophytes that produces a single egg.
• Prothallus — A small, multicellular, free-living, mostly photosynthetic thalloid gametophyte found in pteridophytes.
• Homospory — Production of one kind of spore, the condition in the majority of pteridophytes.
• Heterospory — Production of two kinds of spores: large megaspores and small microspores.
• Microspore — The smaller spore type that develops into a male gametophyte in heterosporous plants.
• Megaspore — The larger spore type that develops into a female gametophyte in heterosporous plants.
• Pollination — In Pinus, the transfer of pollen grains by air currents to the openings of ovules.

Common errors and misconceptions

• Misconception: Every organism with a cell wall belongs to Plantae. Correct: Fungi and cyanobacteria are excluded from Plantae in the five-kingdom system, despite possessing cell walls.
• Misconception: Isogamy requires swimming gametes. Correct: Isogamy concerns similar gamete size. Spirogyra has non-flagellated gametes, whereas Ulothrix has flagellated gametes.
• Misconception: All algae are green and aquatic. Correct: Algae include green, brown and red groups and are largely aquatic; some occur on moist stones, soil and wood.
• Misconception: The leafy Funaria plant is the sporophyte. Correct: The leafy plant is the gametophyte. The attached foot, seta and capsule constitute the sporophyte.
• Misconception: Vascular tissue makes fern fertilisation independent of water. Correct: Fern antherozoids still require water to reach the archegonium, despite the sporophyte having vascular tissues.
• Misconception: Heterosporous pteridophytes produce seeds. Correct: Selaginella and Salvinia produce two spore types. Retention of the female gametophyte and embryo development are precursors to the seed habit.
• Misconception: Naked gymnosperm ovules lack protective envelopes. Correct: The nucellus has protective envelopes; naked means that the ovule is not enclosed by an ovary wall.
• Misconception: Pollination and fertilisation are the same event. Correct: Pollination transfers pollen. Fertilisation fuses a male gamete with the egg and forms a zygote.

Exam-style questions with model answers

Q1. In Ulothrix, the fusing gametes are flagellated and similar in size. In Spirogyra, they are non-flagellated and similar in size. Identify the sexual method in each and justify it. [2 marks]

1. Ulothrix shows isogamy because its two fusing gametes are similar in size, although they are flagellated.
2. Spirogyra also shows isogamy because its gametes are similar in size; the absence of flagella does not change that classification.

Q2. Compare green, brown and red algae using three headings: major pigments, stored food, and flagellar number with position of insertion where flagella occur. [3 marks]

1. Pigments: Green algae have chlorophyll a and b; brown algae have chlorophyll a and c with fucoxanthin; red algae have chlorophyll a and d with phycoerythrin.
2. Stored food: Green algae store starch, brown algae store mannitol and laminarin, and red algae store floridean starch.
3. Flagella: The characteristic flagellated cells have 2-8 equal, apical flagella in green algae and two unequal, lateral flagella in brown algae. Red algae lack flagella.

Q3. State four differences between liverworts such as Marchantia and mosses such as Funaria, covering body form, vegetative reproduction, position of sex organs and sporophyte complexity. [4 marks]

1. Body: Marchantia has a dorsiventral thallus, whereas the moss gametophyte includes a protonema and an upright leafy stage.
2. Vegetative reproduction: Liverworts reproduce by thallus fragmentation or gemmae; mosses reproduce by fragmentation and budding in the secondary protonema.
3. Sex organs: Liverwort sex organs occur on the same or different thalli; moss sex organs occur at the tips of leafy shoots.
4. Sporophyte: Both have a foot, seta and capsule, but the moss sporophyte is more elaborate than the liverwort sporophyte.

Q4. Outline the Funaria life cycle in five stages, beginning with a haploid spore. Include protonema, leafy gametophyte, fertilisation, attached sporophyte and meiosis, identifying the chromosome-set changes. [5 marks]

1. A haploid spore germinates into a green, creeping, branched protonema. This is the first stage of the gametophyte generation.
2. Lateral buds on the secondary protonema produce leafy shoots. Their tips bear antheridia and archegonia, which produce male gametes and eggs.
3. Water enables the male gametes to reach an archegonium. Fusion of a haploid male gamete with a haploid egg produces a diploid zygote.
4. The zygote develops into a sporophyte with a foot, seta and capsule. It remains attached to the gametophyte and receives nourishment from it.
5. Meiosis in the capsule produces haploid spores, reducing the chromosome sets from two to one. The spores disperse and germinate to restart the cycle.

Q5. Describe a typical fern life cycle in six points, from the mature sporophyte to the new sporophyte. Include spore formation, the prothallus, sex organs and the role of water. [6 marks]

1. The mature fern is the dominant diploid sporophyte. Its body is differentiated into true roots, a stem and leaves with vascular tissues.
2. Within its sporangia, diploid spore mother cells undergo meiosis. This produces haploid spores and begins the transition towards the gametophyte generation.
3. A spore germinates into a small, free-living, multicellular prothallus. Pteridophyte gametophytes are mostly photosynthetic and require cool, damp, shaded conditions.
4. The prothallus bears antheridia and archegonia. The antheridia produce male gametes, while the archegonia contain the eggs needed for fertilisation.
5. Water enables male gametes to reach an archegonium. Fusion of a male gamete with the egg forms a diploid zygote.
6. The zygote develops into a multicellular, well-differentiated sporophyte. This becomes the dominant plant generation and can again produce spores in sporangia.

Q6. Selaginella and Salvinia produce large megaspores and small microspores. Their female gametophytes remain on parent sporophytes for variable periods, and embryos develop within them. Name this spore condition, state each spore's fate, and explain its significance without claiming that these plants form seeds. [4 marks]

1. The condition is heterospory, meaning the production of two different kinds of spores rather than one kind.
2. The larger megaspores germinate and develop into female gametophytes, which are retained on the parent sporophyte for variable periods.
3. The smaller microspores germinate and develop into male gametophytes. Thus, the two spore types give rise to gametophytes of different sexes.
4. Development of embryos within retained female gametophytes is a precursor to the seed habit. It does not mean that Selaginella or Salvinia produces seeds.

Q7. Explain five stages of reproduction in Pinus: formation of the male gametophyte, formation of the female gametophyte, pollination, pollen-tube action with fertilisation, and embryo with seed development. [5 marks]

1. Microspores produced in microsporangia develop into pollen grains. Each pollen grain is a highly reduced male gametophyte, rather than an independent, free-living plant.
2. Within an ovule, a megaspore mother cell undergoes meiosis to form four megaspores. One develops into a retained female gametophyte bearing archegonia.
3. Air currents carry released pollen grains to the openings of ovules. This transfer is pollination and is separate from the later fusion of gametes.
4. The pollen tube grows towards the archegonia and carries the male gametes. Fusion of a male gamete with the egg produces a diploid zygote.
5. The zygote develops into an embryo and the ovule becomes a seed. The seed is naked because it is not enclosed within a fruit.

Q8. State five economic uses of algae, giving the relevant algal examples or groups for food, algin, carrageen, agar and protein supplementation. [5 marks]

1. Food: Many species of the marine algae Porphyra, Laminaria and Sargassum are used as food. These examples include red and brown algae.
2. Algin: Certain brown algae produce algin, a commercially used water-holding substance. It is an example of a hydrocolloid obtained from algae.
3. Carrageen: Certain red algae produce carrageen, another commercially useful hydrocolloid. The term hydrocolloid refers to a substance with water-holding properties.
4. Agar: Gelidium and Gracilaria provide agar. It is used for growing microbes and in the preparation of ice-creams and jellies.
5. Protein supplement: Chlorella is a protein-rich unicellular alga. It is used as a food supplement, including by space travellers.

Key takeaways

• Algae are largely aquatic, chlorophyll-bearing organisms whose pigments, reserve food, cell walls and reproductive features distinguish three major classes.
• Isogamy concerns similar gamete size, anisogamy concerns unequal size, and oogamy involves a large, non-motile female gamete.
• Green algae store starch, brown algae store mannitol and laminarin, and red algae store floridean starch.
• Bryophytes have a dominant gametophyte, depend on water for sexual reproduction, and nourish an attached sporophyte.
• Funaria alternates between a haploid gametophyte, including protonema and leafy shoots, and a diploid sporophyte with foot, seta and capsule.
• Pteridophytes possess vascular tissues and a dominant sporophyte, but their male gametes still need water to reach archegonia.
• Heterospory produces microspores and megaspores; retention of female gametophytes and embryo development are precursors to the seed habit.
• Pinus produces pollen and retained female gametophytes; after fertilisation, the zygote forms an embryo and the ovule forms a naked seed.

Test yourself

Which characteristic, rather than motility alone, defines isogamy?

Isogamy is defined by fusion of gametes similar in size. Those gametes may be flagellated, as in Ulothrix, or non-flagellated, as in Spirogyra.

What are the major reserve foods of brown and red algae?

Brown algae store mannitol and laminarin. Red algae store floridean starch, which resembles amylopectin and glycogen in structure.

Why are bryophytes called amphibians of the plant kingdom?

They can live on soil but depend on water for sexual reproduction, because their male gametes must reach the archegonia.

Which Funaria stages belong to the gametophyte generation?

Both the protonema and the leafy shoots are haploid gametophyte stages. The leafy shoots bear antheridia and archegonia.

Which two divisions or events change chromosome-set number in these life cycles?

Meiosis reduces the diploid condition to haploid spores. Fertilisation fuses haploid gametes and restores the diploid condition in the zygote.

Name the four pteridophyte classes with one example of each.

Psilopsida includes Psilotum; Lycopsida includes Selaginella; Sphenopsida includes Equisetum; Pteropsida includes Dryopteris. Other listed examples include Lycopodium, Pteris and Adiantum.

What develops from a microspore, and what develops from a megaspore?

A microspore develops into a male gametophyte, whereas a megaspore develops into a female gametophyte in heterosporous plants.

Why are Pinus seeds called naked even though ovules have protective envelopes?

Naked refers to the absence of an enclosing ovary and fruit. It does not mean that the ovule lacks protective envelopes around its nucellus.