NCERT Solutions Class 11 Biology Chapter 3 - Plant Kingdom

Q1: What is the basis of classification of Algae?
Ans: Algae are classified into three main classes namely Chlorophyceae, Phaeophyceae, and Rhodophyceae. These divisions are based on a combination of structural and physiological characters that reflect differences in their organisation and adaptation. The important criteria used are:

• Major photosynthetic pigments present
• Form of stored food
• Cell wall composition
• Number of flagella and position of insertion

(i) Chlorophyceae

• Common name: Green algae
• Major pigments: Chlorophylls a and b
• Stored food: Starch
• Cell wall composition: Cellulose
• Flagella number and position: 2-8; equal and apical

(ii) Phaeophyceae

• Common name: Brown algae
• Major pigments: Chlorophylls a and c, and fucoxanthin (gives brown colour)
• Stored food: Mannitol and laminarin
• Cell wall composition: Cellulose and algin (alginates)
• Flagella number and position: 2; unequal and lateral

(iii) Rhodophyceae

• Common name: Red algae
• Major pigments: Chlorophylls a and d, and phycoerythrin (gives red colour)
• Stored food: Floridean starch
• Cell wall: Cellulose, pectin, and sulphated polysaccharides
• Flagella number: Absent

These distinguishing characters (pigments, reserve food, cell wall chemistry and flagellar features) help group algae into broad divisions that reflect their evolutionary relationships and ecological roles.

Q2: When and where does reduction division take place in the life cycle of a liverwort, a moss, a fern, a gymnosperm and an angiosperm?
Ans: 

(i) Liverwort

• Reduction division (meiosis) occurs in the capsule of the sporophyte.
• In liverworts the dominant plant-body is haploid (gametophytic), which bears sex organs that produce gametes. After fertilisation the zygote develops into a sporophyte bearing a capsule where sporogenous cells undergo meiosis to produce haploid spores.

(ii) Moss

• Reduction division takes place inside the capsule (sporangium) of the sporophyte.
• Mosses have a dominant gametophyte (protonema → gametophyte). The gametophyte bears sex organs that form gametes; the zygote develops into a sporophyte whose capsule produces spores by meiosis.

(iii) Fern

• In ferns, meiosis occurs in the sporangia borne on the sporophylls (leaves). These sporangia produce haploid spores by reduction division.
• The dominant plant is the sporophyte; spores germinate to form the gametophyte (prothallus).

(iv) Gymnosperm

• Reduction division takes place in the microsporangia (on microsporophylls) to produce microspores (pollen) and in the megasporangia (on megasporophylls) to produce megaspores.
• Gymnosperms have a dominant sporophyte; pollen grains are the male gametophytes and the female gametophyte develops within the ovule from the megaspore.

(v) Angiosperm

• Meiosis occurs in the anthers (microsporangia) to produce microspores (pollen) and in the ovule (within the ovary) to produce megaspores.
• Angiosperms have a dominant sporophyte (flowering plant); male gametophyte is pollen and the female gametophyte (embryo sac) develops inside the ovule.

Q3: Name three groups of plants that bear archegonia. Briefly describe the life cycle of any one of them.
Ans: Archegonium is the female sex organ that produces the egg. It occurs in the life cycles of bryophytes, pteridophytes, and gymnosperms.

Life cycle of a fern (Dryopteris)

• Dryopteris is a common fern with pinnately compound leaves. The dominant, conspicuous plant is the sporophyte. Many sporangia are borne on the lower surface of mature fronds (sporophylls), often clustered in sori.
  
• Each sporangium contains spore mother cells that undergo meiosis to produce haploid spores. On maturity the spores are released and, on finding a suitable substratum, they germinate to form a heart-shaped gametophyte called the prothallus.
• The prothallus bears the male sex organs (antheridia) and the female sex organs (archegonia). Antheridia produce motile sperm that swim through a film of water to reach the archegonia and fertilise the egg.
• Fertilisation produces a zygote which develops into an embryo and then into a young sporophyte. The young sporophyte grows out of the archegonium and becomes the independent fern plant that will form sporangia, completing the cycle.

Q4: Mention the ploidy of the following: 
protonemal cell of a moss; primary endosperm nucleus in dicot, leaf cell of a moss; prothallus cell of a ferm; gemma cell in Marchantia; meristem cell of monocot, ovum of a liverwort, and zygote of a fern.
Ans:
Q5: Write a note on economic importance of algae and gymnosperms.
Ans: 

(i) Economic Importance of Algae
Algae have many direct and indirect uses and are important in aquatic ecosystems as primary producers. They contribute substantially to global carbon fixation and form the base of many aquatic food chains.

• Food source: Several marine algae such as Porphyra, Sargassum, and Laminaria are edible. Microalgae like Chlorella and Spirulina are rich in protein and used as dietary supplements.
• Commercial products: Agar (from Gelidium and Gracilaria) is used in food (jellies, ice-cream) and in microbiological media. Carrageenan (from red algae) is an emulsifier used in foods, cosmetics and toothpastes.
• Medicines and other uses: Some algae yield substances used in pharmaceuticals and traditional remedies; certain species are used for treating parasitic worm infections. Algal extracts also find uses in biotechnology and as biofertilisers.

(ii) Economic importance of Gymnosperms

• Timber and construction: Conifers such as pine and cedar provide softwood used in construction, furniture and packing.
• Medicinal products: Taxus yields taxol, an anticancer compound. Species of Ephedra produce ephedrine, used in respiratory medicines.
• Edible seeds and food: Seeds of Pinus gerardiana (chilgoza) are edible and valued as food.
• Resins and industrial products: Resins from pines are used to make varnishes, sealing wax and turpentine, which have various industrial applications.

Q6: Both gymnosperms and angiosperms bear seeds, then why are they classified separately?
Ans:

• Although both are seed-bearing and have a dominant diploid sporophyte (diplontic life cycle), they differ fundamentally in reproductive structures and seed development.
• In gymnosperms the megasporophylls bear exposed ovules and the seeds are said to be naked because there is no enclosing fruit. The nutritive tissue for the embryo is the haploid female gametophyte (derived from the megaspore).
• In angiosperms the megasporophylls are folded to form carpels, which enclose the ovules inside an ovary. Flowers are the reproductive structures, and seeds develop within fruits.
• Fertilisation also differs: angiosperms show double fertilisation, where one sperm fuses with the egg to form the zygote and the second fuses with the two polar nuclei to form a triploid (3n) endosperm. Gymnosperms lack true double fertilisation and their nutritive tissue in seeds is haploid (n).
• These differences in ovule enclosure, gametophyte structure and fertilisation justify the separate classification of gymnosperms and angiosperms.

Q7: What is heterospory? Briefly comment on its significance. Give two examples.
Ans: 

• Heterospory is the production of two distinct types of spores by a single plant: a smaller microspore and a larger megaspore.
• The microspore develops into the male gametophyte (which produces male gametes) and the megaspore develops into the female gametophyte (which bears the egg).
• Significance: Retention and germination of the megaspore within the megasporangium provide protection and nourishment to the developing female gametophyte and the zygote. This condition is an important step towards the evolution of the seed habit because it leads to closer association of the sporophyte and developing sporophyte embryo, improving survival.
• Examples: Heterospory is seen in some pteridophytes such as Selaginella and Salvinia.

Q8: Explain briefly the following terms with suitable examples:
(i) protonema
(ii) antheridium
(iii) archegonium
(iv) diplontic
(v) sporophyll
(vi) isogamy
Ans.: 

(i) Protonema: It is the first stage in the life cycle of a moss, developing directly from a spore. It is a creeping, green, branched filamentous structure from which the leafy gametophyte develops.

(ii) Antheridium: It is the male sex organ present in bryophytes and pteridophytes. It is usually surrounded by a jacket of sterile cells and contains sperm mother cells that give rise to motile sperm.

(iii) Archegonium: It is the female sex organ present in bryophytes, pteridophytes and gymnosperms. It generally has a swollen venter and a tubular neck and contains a single egg (female gamete).

(iv) Diplontic: This term describes life cycles in which the diploid sporophyte is the dominant, free-living phase and the gametophyte is reduced to a few cells or a microscopic structure. Seed plants (gymnosperms and angiosperms) show diplontic life cycles.

(v) Sporophyll: These are modified leaves that bear sporangia. In pteridophytes sporophylls bear sporangia on their surfaces; in seed plants they occur as microsporophylls (bearing microsporangia) and megasporophylls (bearing megasporangia).

(vi) Isogamy: A form of sexual reproduction where the gametes are morphologically similar (same size and shape) but functionally different. An example is Spirogyra, where the fusing gametes are similar.

Q9: Differentiate between the following:
(i) red algae and brown algae
(ii) liverworts and moss
(iii) homosporous and heterosporous pteridophyte
Ans:

(i) Red algae and Brown algae

(ii) Liverworts and Moss

(iii) Homosporous and Heterosporous Pteridophyte

Q10: Match the followings (column I with column II)

Ans:

Q11: Describe the important characteristics of gymnosperms.
Ans: Important features of gymnosperms are:

• The term gymnosperm means plants with naked seeds (seeds are not enclosed within a fruit).
• Plant habit ranges from shrubs to large trees; some, like Sequoia, are among the tallest trees.
• Root system is typically a tap root. Some roots (coralloid roots in Cycas) host nitrogen-fixing cyanobacteria.
• Stems may be branched (e.g., Pinus, Cedrus) or unbranched (e.g., Cycas).
• Leaves vary from needle-like to pinnate; they often have a thick cuticle and sunken stomata to reduce water loss.
• Gymnosperms are heterosporous, producing microspores and megaspores.
• Reproductive structures: They lack true flowers; microsporophylls and megasporophylls are organised into male and female cones (strobili).
• Pollination is mostly by wind; pollen grains reach the ovule and enter via the micropyle into the pollen chamber.
• The male and female gametophytes are reduced and dependent on the sporophyte; the nutritive tissue of the seed (female gametophyte) is haploid.
• Seeds remain exposed (not enclosed in fruits) and development of seeds completes the life cycle.