Class 11-science NCERT Solutions Biology Chapter 3 - Plant Kingdom
Plant Kingdom Exercise 35
The main basis of classification of algae is the presence or absence of pigments, which impart the algae with colour. Example: Chlorophyceae contains chlorophyll a and b, which gives it the green colour. Phaeophyceae contains chlorophyll a and c and fucoxanthin, which imparts the brown colour. Rhodophyceae contains chlorophyll a and d and phycoerythrin, which imparts the distinct red colour.
Algae are now classified on the basis of flagellation, cellular organisation, storage products and chemistry of the cell wall.
i. In liverworts like Marchantia, meiosis occurs in spore mother cells of the capsule in sporogonium leading to the formation of haploid spores.
ii. Moss: In moss like Funaria, meiosis takes place in spore mother cells of spore sacs in the capsule of sporogonium.
iii. Fern: Fern plants like Dryopteris bear fertile leaves called sporophylls on which sporangia are borne. Meiosis takes place in spore mother cells of sporangium to form haploid spores, which on germination give rise to prothalli on which gametes are borne.
iv. Gymnosperm: In gymnosperms like Pinus, meiosis takes place in microsporangia present on microsporophylls. Meiosis occurs in microspore mother cells to form haploid pollen grains which give rise to the male gametophyte.
v. Angiosperm: Pollen grains are formed in microspore mother cells of pollen sacs in the anther of stamen. Pollen grains lead to the formation of the male gametophyte. Ovules are borne inside the ovary. In the nucellus of the ovule, the megaspore mother cell is present which undergoes meiosis to form haploid megaspores. The megaspore forms the female gametophyte.
Bryophytes, Pteridophytes and Gymnosperms bear distinct archegonia.
Life Cycle of Fern (Dryopteris):
i. The leaflets of Dryopteris bear sporangia and are known as sporophylls.
ii. The sporangium bears a large number of homosporous spores. The aggregate is called a sorus.
iii. Sori are green in the beginning but turn brown at maturity.
iv. Each sporangium consists of a stalk and capsule.
v. The spores are minute, dark brown, uninucleated and have stored food inside them.
vi. The prothallus is green and heart shaped. On its under surface, male sex organs (called antheridia) and female sex organs (called archegonia) are borne.
vii. The antheridium is small and spherical or oval body and produces antherozoids.
viii. Archegonium is a small flask-shaped structure and contains an egg and a venter canal cell.
ix. Antherozoids swim in water by cilia. The venter canal cell and neck cell disintegrate in the archegonium to produce mucilage and malic acid.
x. Many sperms pass down into the venter, and one of them fuses with the egg to form a zygote.
xi. The zygote develops into an embryo which in turn develops into a new sporophyte.
Protonemal cell of moss
Primary endosperm nucleus
Leaf cell of moss
Prothallus of fern
Gemma cells in Marchantia
Meristem cell of monocot
Ovum of liverwort
Zygote of fern
Plant Kingdom Exercise 36
Economic importance of algae:
i. At least half of the total carbon dioxide fixation on the Earth is carried out by algae through photosynthesis.
ii. They are primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.
iii. Certain marine brown and red algae produce large amounts of hydrocolloids (water-holding substances), e.g. algin (brown algae) and carrageen (red algae), which are used commercially.
iv. Agar, one of the commercial products obtained from Gelidium and Gracilaria, is used to grow microbes and in preparations of ice-creams and jellies.
v. Chlorella and Spirulina are unicellular algae which are rich in proteins and are used as food supplements even by space travellers.
Economic importance of gymnosperms:
i. Gymnospermous plants are widely used as ornamentals. Conifers are often featured in formal gardens and are used for bonsai.
ii. Fibres of conifers make up paper pulp and may occasionally be used for creating artificial silk or other textiles.
iii. Turpentine and resin are derived from conifer resins. A hardened form of resin from a kauri (Agathis australis), called copal, is used in the manufacture of paints and varnishes.
iv. Some useful oils are derived from conifers such as junipers, pines, hemlock, fir, spruces and arborvitae. These oils serve as air fresheners, disinfectants and scents in soaps and cosmetics.
v. The seeds of some gymnosperms serve as food sources. Pine seeds may be eaten plain or used as a garnish on bakery products.
Gymnosperms and angiosperms are classified separately because of the following reasons:
i. The ovules of gymnosperms are naked; however, in angiosperms, they are enclosed inside the ovary.
ii. In gymnosperms, the wood is non-porous, i.e. the vessels are absent. In angiosperms, the wood is porous, i.e. the vessels are present.
iii. In gymnosperms, the endosperm is haploid and is formed before fertilisation; double fertilisation is absent. In angiosperms, the endosperm is triploid and formed after double fertilisation.
Certain pteridophytes produce two kinds of spores in the same plant. This phenomenon is called heterospory.
i. Heterospory has led to the development of seeds in gymnosperms and angiosperms.
ii. It has helped in the differentiation of male and female gametophytes.
Examples: Selaginella, Salvinia
(i) Protonema: A protonema is a thread-like chain of cells which forms the haploid phase of a bryophyte life cycle. When a moss or liverwort first grows from the spore, it grows as a protonema which develops into a leafy gametophore.
Example: Some mosses and all liverworts.
(ii) Antheridium: An antheridium is a haploid structure which produces male gametes. It is present in bryophytes and pteridophytes. Many algae and some fungi (e.g. ascomycetes and water moulds) also have antheridia during their reproductive stages.
(iii) Archegonium: It is a multicellular, jacketed, flask-shaped female sex organ in bryophytes, pteridophytes and gymnosperms. It has a neck and swollen venter and produces a single female gamete called egg or ovum.
(iv) Diplontic: It is a type of life cycle in which the dominant free living phase is diploid (2n). It produces haploid gametes. Examples: Fucus, Sargassum
(v) Sporophyll: Sporophyll is a leaf which bears sporangia. Sporophyll may be microsporophyll or megasporophyll. Sporophylls aggregate to form cones or strobili. Example: Pinus
(vi) Isogamy: Isogamy refers to a form of sexual reproduction involving gametes of similar morphology and function. Examples: Ulothrix, Ectocarpus
(i) Red algae and brown algae:
i. Chlorophyll a and d present.
ii. Phycobilins are present.
iii. Reserve food material is floridean starch.
iv. Motile and flagellated bodies are absent.
i. Chlorophyll a and c present.
ii. Phycobilins are absent.
iii. Reserve food material is laminarin.
iv. Motile and flagellated bodies are present.
(ii) Liverworts and moss:
i. In liverworts, there is no protonema stage.
ii. The gametophytic plant body is dorsoventral, leaf-like and has the appearance of a liver.
i. In moss, the life cycle begins with a protonema stage.
ii. The gametophytic plant body of moss is differentiated into a stem-like axis and leaf-like lobes.
(iii) Homosporous and heterosporous pteridophytes: Homosporous pteridophytes possess only one type of spores, whereas heterosporous pteridophytes possess two morphological distinct types of spores in separate sporangia
(a)-(iii); (b)-(iv); (c)-(ii); (d)-(i)
Characteristics of gymnosperms:
i. The 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 which develop after fertilisation are not covered, i.e. they are naked.
ii. The plant body is a sporophyte and is differentiated into root, stem and leaves.
iii. The roots are generally tap roots. Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas), the small specialised roots called coralloid roots are associated with N2-fixing cyanobacteria.
iv. The stems are unbranched (Cycas) or branched (Pinus, Cedrus).
v. 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 also help to reduce water loss.
vi. They are heterosporous, i.e. they produce two different kinds of spores.
vii. Fertilisation occurs by siphonogamy, i.e. the male gametes are carried to the archegonia through the pollen tube. Thus, fertilisation takes place in the absence of external water.