RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Green plants range from a pond scum you can barely see to a hundred-metre conifer, and this chapter walks that whole span in order of increasing complexity. Each group is defined by a few decisive innovations — vascular tissue, seeds, an ovary — and by the way its life cycle alternates between a haploid gametophyte and a diploid sporophyte. Keep an eye on which generation dominates as you move from mosses to flowering plants; the shift explains a great deal about why plants conquered dry land.
Which pigment is chiefly responsible for the characteristic colour of the brown algae (Phaeophyceae)?
Answer
Fucoxanthin is correct — this brown xanthophyll masks the chlorophylls in Phaeophyceae, while r-phycoerythrin gives red algae their colour and phycocyanin is found in cyanobacteria.
In bryophytes such as Funaria, the dominant, photosynthetic, free-living stage of the life cycle is the:
Answer
Haploid gametophyte is correct — in bryophytes the leafy or thalloid gametophyte is the main plant body, and the sporophyte stays attached to it and draws nourishment from it.
Gymnosperms are so named because their:
Answer
Ovules are not enclosed by an ovary wall is correct — since there is no ovary, no fruit is formed and the seeds remain exposed on the megasporophyll, which is exactly what the term 'naked seeded' means.
The triploid primary endosperm nucleus of an angiosperm is formed by the fusion of:
Answer
One male gamete and two polar nuclei is correct — this triple fusion produces the 3n primary endosperm nucleus, while the other male gamete fuses with the egg in syngamy to give the diploid zygote.
Assertion (A): Bryophytes cannot complete their life cycle in a completely dry habitat. Reason (R): The male gametes of bryophytes are flagellated and must swim through a film of water to reach the egg in the archegonium.
Answer
Both A and R are true and R is the correct explanation of A — because fertilisation depends on motile antherozoids swimming in water, bryophytes are restricted to damp, shady places and are called the amphibians of the plant kingdom.
What is heterospory, and why is it considered an important evolutionary step?
Answer
Heterospory is the production of two different kinds of spores by the same plant — large megaspores that give rise to the female gametophyte and small microspores that give rise to the male gametophyte — as seen in Selaginella and Salvinia. It is important because the megaspore is retained within the megasporangium on the parent sporophyte, where the female gametophyte develops and the young embryo is protected and nourished. This retention and protection of the developing embryo is precisely the condition that led to the origin of the seed habit in gymnosperms and angiosperms.
List three features that distinguish a monocotyledonous plant from a dicotyledonous plant.
Answer
First, the seed of a monocotyledon has a single cotyledon while that of a dicotyledon has two. Second, monocot leaves show parallel venation in which the veins run side by side along the length of the leaf, whereas dicot leaves show reticulate venation with veins forming a branching network. Third, monocots typically develop a fibrous root system with many similar roots arising from the stem base, while dicots develop a tap root system with one dominant primary root bearing lateral branches. In addition, monocot flowers are usually trimerous with parts in threes, whereas dicot flowers are usually tetramerous or pentamerous.
Compare bryophytes, pteridophytes and angiosperms with respect to vascular tissue, water requirement for fertilisation, and the relative dominance of the gametophyte and sporophyte.
Answer
Bryophytes have no true vascular tissue; conduction of water and food occurs by simple diffusion and through undifferentiated cells, which is one reason they remain small and grow flat against damp ground. Their male gametes are flagellated and must swim through a film of external water to reach the egg, so fertilisation is impossible in dry conditions. In their life cycle the haploid gametophyte is the dominant, green, independent plant body, and the diploid sporophyte is a small structure that remains permanently attached to the gametophyte and depends on it for nutrition. Pteridophytes mark a major advance: they are the first group with true xylem and phloem organised into vascular bundles, and they possess a proper root, stem and leaf, which allows them to grow considerably larger. However, their antherozoids are still flagellated and still need water for fertilisation, so pteridophytes too are largely confined to moist, shady habitats. Here the diploid sporophyte is the dominant plant body that bears sporangia on sporophylls, while the gametophyte is a small, short-lived but independent green prothallus. Angiosperms carry vascular tissue to its most advanced condition, with vessels in the xylem and sieve tubes with companion cells in the phloem, giving efficient long-distance transport. They have eliminated the water requirement altogether: pollen grains carrying the male gametes are transferred by wind, water or animals to the stigma, and a pollen tube then delivers the non-motile male gametes directly to the ovule. Their sporophyte is overwhelmingly dominant and forms the entire visible plant, while the gametophytes are reduced to a few cells — a three-celled pollen grain and a seven-celled embryo sac — that remain enclosed within and nourished by the sporophyte. The sequence therefore shows a steady increase in vascular efficiency, a steady loss of dependence on external water, and a steady shift of dominance from gametophyte to sporophyte.
Describe the classification of algae into its three main classes on the basis of pigments, stored food and cell wall, and state the economic importance of algae.
Answer
Algae are grouped into three classes. Chlorophyceae, the green algae, contain chlorophyll a and b in definite chloroplasts that may be cup-shaped, spiral, ribbon-like or discoid, and most carry one or more pyrenoids storing protein and starch. Their stored food is starch and their cell wall has an inner cellulose layer with an outer pectose layer; examples include Chlamydomonas, Volvox, Ulothrix and Spirogyra. Phaeophyceae, the brown algae, are mainly marine and owe their olive-green to dark brown colour to the xanthophyll fucoxanthin present along with chlorophyll a and c. Food is stored as the complex carbohydrate laminarin or as mannitol, and the wall is made of cellulose covered on the outside by a gelatinous algin coating; the plant body is often differentiated into a holdfast, a stalk called the stipe and a leaf-like frond, as in Laminaria, Fucus, Sargassum and Ectocarpus. Rhodophyceae, the red algae, occur largely in warm marine waters, often at considerable depth, and are red because of the abundant pigment r-phycoerythrin. Their stored food is floridean starch and the wall is of cellulose, pectin and polysulphate esters; examples are Polysiphonia, Porphyra, Gracilaria and Gelidium. Economically, algae are of great value. Being photosynthetic, they fix an enormous share of the carbon dioxide dissolved in water and form the base of almost every aquatic food chain. Many are eaten directly — Porphyra, Laminaria and Sargassum are used as food in coastal countries — and Chlorella is cultivated as a protein-rich supplement. Hydrocolloids obtained from algae are industrially important: algin comes from brown algae and carrageenan from red algae, while agar, obtained from Gelidium and Gracilaria, is used to grow microbes in the laboratory and to prepare ice creams and jellies.
On a monsoon field trip to a hill slope, a student collects four plants from four different spots: (i) a flat green thallus growing on wet rock, bearing a small brown capsule on a stalk, (ii) a plant with coiled young leaves and brown patches on the underside of mature leaves, (iii) a tree bearing woody cones, whose seeds sit exposed on the cone scales, and (iv) a herb with parallel-veined leaves, fibrous roots and flowers with three petals. (a) Identify the group to which plant (i) belongs and name the generation that the brown capsule represents. (b) What are the brown patches on plant (ii), and to which group does it belong? (c) State the single feature of plant (iii) that places it in its group. (d) Is plant (iv) a monocot or a dicot? Give two reasons from the description.
Answer
(a) Plant (i) is a bryophyte, specifically a liverwort or moss-like thalloid form. The flat green thallus is the gametophyte, and the stalked brown capsule is the diploid sporophyte, which stays attached to the gametophyte and draws its nutrition from it. (b) The brown patches are sori, clusters of sporangia borne on the underside of the sporophyll. The coiled young leaves showing circinate vernation together with the sori identify plant (ii) as a pteridophyte, a fern. (c) Plant (iii) is a gymnosperm because its ovules and the seeds developing from them are naked, that is, not enclosed within an ovary, so no fruit is formed and the seeds lie exposed on the surface of the cone scale. (d) Plant (iv) is a monocotyledon. The leaves show parallel venation rather than a reticulate network, and the root system is fibrous rather than a tap root; the trimerous flower with parts in threes confirms this.
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