RowQ
The Vault
RowQ
The Vault
CBSE Class 12 Biology · 11 questions · 26 marks
A flower is not decoration — it is a precisely engineered reproductive organ, and this chapter takes it apart layer by layer. You will follow a pollen grain from the anther wall through pollination, double fertilisation, and finally into a seed carrying an embryo and its food store. Once you can label a mature embryo sac and explain why endosperm is triploid, most board questions from this chapter become straightforward.
Which anther wall layer supplies nourishment to the developing microspores and degenerates as the pollen matures?
Answer
Tapetum is correct — it is the innermost layer of the anther wall, is rich in cytoplasm with often multiple nuclei, and secretes nutrients, enzymes, and sporopollenin precursors for the developing pollen. The epidermis is protective, the endothecium has fibrous thickenings that help the anther dehisce, and the middle layers are ephemeral.
A researcher counts the chromosome number in the endosperm cells of a flowering plant and finds 36. What is the chromosome number of the leaf cells of the same plant?
Answer
24 is correct — the endosperm is triploid (3n), so 3n = 36 gives n = 12 and the diploid somatic leaf cells are 2n = 24. The value 12 is the gametic number, 18 would wrongly assume the endosperm is 2n or the leaf 4n, and 36 is the endosperm value itself.
In a typical mature angiosperm embryo sac, how many nuclei and how many cells are present respectively?
Answer
8 nuclei and 7 cells is correct — three successive free-nuclear mitotic divisions of the functional megaspore give 8 nuclei, but the two polar nuclei remain together inside a single large central cell, so the sac is organised into only 7 cells: 3 antipodals, 2 synergids, 1 egg, and 1 central cell.
The exceptionally tough, decay-resistant material forming the outer wall of a pollen grain is:
Answer
Sporopollenin is correct — it resists high temperature, strong acids and alkalis, and known enzymes, which is why pollen walls persist as fossils. Cellulose and pectin make up the inner intine, and suberin is a waterproofing material of cork cells, not pollen exine.
Assertion (A): A pollen grain that lands on the stigma of a different species usually fails to produce a pollen tube that reaches the ovule. Reason (R): The pistil recognises compatible pollen through chemical dialogue at the stigma surface and rejects incompatible pollen by blocking germination or tube growth.
Answer
Both A and R are true and R is the correct explanation of A — pollen-pistil interaction is an active recognition event, not a passive one. Proteins on the pollen exine interact with the stigma; if the signals are accepted, the pistil supports germination and tube growth through the style, and if they are rejected, germination is prevented or the tube is arrested before it reaches the ovary.
A gardener notices that in a certain flower the anthers shed their pollen several days before the stigma of the same flower becomes receptive. Name this condition and state the advantage it gives the plant.
Answer
This condition is protandry, a form of dichogamy in which the male part matures before the female part of the same flower. Its advantage is that it acts as an outbreeding device — because pollen is no longer available when the stigma turns receptive, self-pollination is prevented and the flower must receive pollen from another plant. This promotes cross-pollination, maintains genetic variation in the population, and avoids the loss of vigour that comes with continued inbreeding.
Explain double fertilisation and name the two products formed, giving the ploidy of each.
Answer
Double fertilisation is the fusion of both male gametes released by a single pollen tube inside the embryo sac, and it occurs only in angiosperms. After the pollen tube discharges its contents into one synergid, one male gamete fuses with the egg cell — this is syngamy — and the other fuses with the two polar nuclei of the central cell, which is called triple fusion. The two products are the zygote and the primary endosperm nucleus. The zygote is diploid (2n) and develops into the embryo. The primary endosperm nucleus is triploid (3n) because it is formed from one haploid male gamete plus two haploid polar nuclei, and it develops into the endosperm, the nutritive tissue of the seed.
Why is apomixis considered useful to a seed company that sells hybrid seed?
Answer
In apomixis, seeds are formed without fertilisation, so the embryo is genetically identical to the mother plant. Hybrid plants show high vigour and yield, but if a farmer sows seed collected from a hybrid crop, meiosis and recombination cause the hybrid characters to segregate and the yield drops in the next generation, forcing farmers to buy fresh seed each season. If hybrid varieties could be made apomictic, the hybrid genotype would be transmitted unchanged through seed year after year, so farmers could keep and re-sow their own seed without losing hybrid vigour, greatly reducing seed cost.
Describe the development of a male gametophyte in a flowering plant, from the microspore mother cell up to the discharge of male gametes.
Answer
Development begins in the microsporangium of the anther, where the sporogenous tissue gives rise to diploid microspore mother cells. Each of these undergoes meiosis, producing a tetrad of four haploid microspores. As the anther matures, the tetrad separates and each microspore develops a two-layered wall — the hard outer exine of sporopollenin, interrupted at germ pores, and the thin inner intine of cellulose and pectin. The microspore nucleus then divides mitotically and unequally, producing a large vegetative cell rich in food reserves and a small generative cell floating in its cytoplasm. In most species the pollen grain is shed at this 2-celled stage; in about 40 percent of species the generative cell divides once more before shedding, so the grain is released in the 3-celled condition. On reaching a compatible stigma, the pollen grain germinates. The intine grows out through a germ pore as the pollen tube, and the contents of the pollen grain move into it, with the vegetative nucleus at the tip guiding growth. If the generative cell has not already divided, it does so within the tube, forming two non-motile male gametes. The pollen tube grows through the tissue of the style, enters the ovule through the micropyle, and is guided by the filiform apparatus of a synergid. It penetrates that synergid and bursts, discharging the two male gametes into the embryo sac, where double fertilisation follows.
With reference to a mature angiosperm ovule, describe the structure of the embryo sac and state the fate of each of its components after fertilisation.
Answer
The ovule consists of a stalk (funicle) attached to the placenta, one or two protective integuments enclosing the nucellus, and an opening called the micropyle at one end; the region opposite the micropyle is the chalaza. Embedded in the nucellus is the embryo sac, the female gametophyte. The typical embryo sac is monosporic, 8-nucleate, and 7-celled. At the micropylar end lies the egg apparatus — one egg cell flanked by two synergids that bear a specialised thickening called the filiform apparatus. At the chalazal end lie three antipodal cells. Occupying the large central region is the central cell, containing two polar nuclei. After double fertilisation the fates are as follows. The egg cell, fertilised by one male gamete, becomes the diploid zygote and develops into the embryo with its radicle, plumule, and cotyledons. The central cell, fertilised by the second male gamete, gives the triploid primary endosperm nucleus, which divides to form the endosperm that nourishes the growing embryo. The synergids and antipodal cells have no further role and degenerate soon after fertilisation. Outside the sac, the integuments harden into the seed coat, the nucellus may persist as perisperm, the ovule as a whole becomes the seed, and the surrounding ovary wall becomes the pericarp of the fruit.
A horticulturist in Coorg maintains an orchard of a mandarin orange variety. When she cuts open ripe fruits and sows the seeds, she is surprised to find that several seeds each produce more than one seedling, and that most of these seedlings look exactly like the parent tree rather than showing the variation she expected. She also notices that one of her table-grape vines produces plump seedless berries every year without any change in yield. (a) Name the phenomenon of more than one embryo developing in a single seed. (b) Explain why most of the mandarin seedlings resemble the parent tree. (c) Name the condition responsible for the seedless grape berries. (d) State one practical advantage the horticulturist gains from the mandarin seedlings being uniform.
Answer
(a) The phenomenon is polyembryony — the presence of more than one embryo in a single seed, which is common in citrus fruits such as mandarin orange. (b) In citrus, most of these extra embryos are nucellar embryos: they arise from diploid cells of the nucellus surrounding the embryo sac rather than from a fertilised egg. Because they are formed without meiosis and without fusion of gametes, they are genetically identical to the mother plant, so the seedlings from them are clones and show no variation. Only the one zygotic embryo in the seed is a product of fertilisation and can differ from the parent. (c) The seedless grape berries result from parthenocarpy — development of the fruit from the ovary without fertilisation, so no seeds are formed. (d) Because the nucellar seedlings are genetically identical to the mother tree, the horticulturist can raise true-to-type rootstock and new trees from seed at low cost, with the certainty that fruit quality, size, and flavour will match the selected parent variety.
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