31 Years NEET Previous Year Questions: Sexual Reproduction in Flowering Plants - 2 - NEET MCQ

Somatic embryogenesis is a process where a plant or embryo is derived from a single somatic cell or group of somatic cells. Somatic embryos are formed from plant cells that are not normally involved in the development of embryos, i.e., ordinary plant tissue. No endosperm or seed coat is formed around a somatic embryo.

Endosperm is the nutritive tissue which provides nourishment to the embryo in seed plant. Albuminous seeds retain a part of endosperm as it is not completely used up during embryo development (e.g., wheat, maize, barley, castor, sunflower).

Topic in NCERT: Albuminous seeds

Line in NCERT: "Albuminous seeds retain a part of endosperm as it is not completely used up during embryo development (e.g., wheat, maize, barley, castor)."

Pollen grains are generally spherical and a prominent two-layered wall. The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali.

Topic in NCERT: Sporopollenin properties

Line in NCERT: "It can withstand high temperatures and strong acids and alkali."

There are different types of vectors involved in pollination. For example, Maize, mulberry → pollination by wind. Vallisneria → pollination through water (hydrophily). Cucumber → Bees are brought for the commercial plantings of cucumber.

Single Megaspore Mothe Cell (MMC) with dense cytoplasm aond a prominent nucleus gets differentiated from nucellus near the micropylar region. This Megaspore Mother Cell (MMC) undergoes meiosis to form ‘4’ haploid cells called megaspores and the process of formation of known as megasporogenesis.

Topic in NCERT: Megasporogenesis

Line in NCERT: "The MMC undergoes meiotic division."

Cleistogamous flowers do not expose their reproductive parts. Anthers and stigma lie close to each other. Pure autogamy occurs since there is no chance of crosspollination. Cleistogamy is the most efficient floral adaptation for promoting self-pollination. E.g., Viola mirabilis and Oxalis autosella.

Topic in NCERT: Cleistogamous flowers

Line in NCERT: "Cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma."

Dehydrogenase enzymes present in living tissue reduce the tetrazolium chloride to formazan, a reddish, water insoluble compound. This reaction occurs in or near living cells which are releasing hydrogen in respiration processes. Viable tissues produce a normal red-colour, weak living tissue produce an abnormal colour. Dead tissues do not stain, remaining usually white.

Sporogenous tissue is always diploid, endothecium is second layer of anther wall and perform the function of protection and help in dehiscence of anther to release the pollen. Hard outer layer of pollen is called exine but tapetum always nourishes the developing pollen.
Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus (polypoid).

Topic in NCERT: Tapetum

Line in NCERT: "The innermost wall layer is the tapetum. It nourishes the developing pollen grains."

Sexual reproduction leads to formation of new combination  and appearance of variations. Genetic recombination, interaction etc. during sexual reproduction provides vigour and vitality to the offsprings. They better adapt themselves to changing  environmental conditions and also plays an important role in evolution.

Topic in NCERT: Sexual reproduction in flowering plants

Line in NCERT: "Being products of sexual reproduction, they generate new genetic combinations leading to variations."

Cleistogamy favours no dependence on pollinator because flowers never open. In such flowers, the anthers and stigma lie close to each other. When anthers dehisce in flower buds pollen grains come in contact with the stigma to effect pollination.

Topic in NCERT: Cleistogamous flowers

Line in NCERT: "Cleistogamous flowers produce assured seed-set even in the absence of pollinators."

The seed coat develops from integuments originally surrounding the ovule. It is thick and hard in coconut which protect the embryo from mechanical injury and from drying out.

Ovule is also called integumented megasporangium. It develops into seed after fertilisation in spermatophytes. It occurs singly or in a cluster inside ovary with parenchymatous cushions called placenta.

Topic in NCERT: Megasporogenesis

Line in NCERT: "Only the functional megaspore develops into the female gametophyte (embryo sac)."

Perisperm is remnants of nucellus which is diploid (2n) but endosperm is triploid (3n). Perisperm occurs in the seeds of Black pepper, coffee, castor, cardamum, Nymphaea. Endosperm is the food  laden tissue which is meant for nourishing the embryo in seed plants. In angiosperms the endosperm is formed as a result of vegetative fertilization, triple fusion or fusion of a male gamete with diploid secondary nucleus of the central cell.

Topic in NCERT: Endosperm

Line in NCERT: "Occasionally, in some seeds such as black pepper and beet, remnants of nucellus are also persistent. This residual, persistent nucellus is the perisperm."

Wind pollinated flowers have generally single ovule in each ovary.

Topic in NCERT: Agents of pollination

Line in NCERT: "Wind-pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence; a familiar example is the corn cob - the tassels you see are nothing but the stigma and style which wave in the wind to trap pollen grains."

In over 60 per cent of angiosperms, pollen grains are shed at cell 2-celled stage. In the remaining species the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage.)

Topic in NCERT: Pollen grains and fertilization

Line in NCERT: "The generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage)."

The germ pores are apertures in the exine layer of the pollen grain where the sporopollenin is absent. The germ pore helps in the formation of the germ tube or pollen tube and which makes its exit on germination. 

Topic in NCERT: Germ pore

Line in NCERT: "As mentioned earlier, following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores (Figure 1.12a)."

Sporopollenin is a fatty substance present in the exine of pollen grains. It is resistant to microbial and chemical decomposition and can withstand the extreme environmental conditions. 

Topic in NCERT: Sporopollenin

Line in NCERT: "The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known."

Commelina bears aerial, chasmogamous (stigma and anthers exposed to pollinating agents), insect pollinated flowers and underground cleistogamous flowers. Cleistogamous flowers are bisexual flowers which never open. In such flowers, the anther and stigma lie very close to each other. When anther dehisce in the flower buds, pollen grains come in contact with the stigma of the same flower i.e, autogamy occurs. So, these flowers produce an assured seed set, even in the absence of pollinators.

Autogamy involves pollination within the same flower, while geitonogamy involves transfer of pollen grains from the anther of one flower to the stigma of another flower of the same plant. Both the process are prevented in papaya because it is dioecious plant(i.e, male and female sex organs are borne on separate plants) and it always needs cross-pollination. 

Topic in NCERT: Preventing self-pollination

Line in NCERT: "In several species such as papaya, male and female flowers are present on different plants, that is each plant is either male or female (dioecy). This condition prevents both autogamy and geitonogamy."

Some fleshy fruits such as mango, plum etc. usually have a single hard stone that encloses a seed, called drupe.

Wind pollination is common in grasses. Grasses produce large amount of pollen which by the help of wind reach to opposite sex for reproduction.

Topic in NCERT: Wind pollination

Line in NCERT: "Wind-pollination is quite common in grasses."

Cleistogamy is autogamous pollination. When pollination and fertilization occur in unopened flower bud, it is known as cleistogamy. It ensures self-pollination and prevents cross-pollination.

Topic in NCERT: Cleistogamous flowers

Line in NCERT: "Cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma."

Nucellar polyembryony is reported in a Citrus species.

Topic in NCERT: Apomixis and polyembryony

Line in NCERT: "More often, as in many Citrus and Mango varieties some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos."

A synergid cell wall forms a highly thickened structure called the filiform apparatus at the micropylar end consisting of numerous finger like projections into synergid cytoplasm. These synergid cells are necessary for pollen tube guidance in ovule.

Topic in NCERT: Egg apparatus

Line in NCERT: "The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid."

(b) Pollination by wind is called anemophily. Wind pollinated flowers are small in size, producing large number of dry pollen grains. Pollens are small, dry and light in weight. Grasses are anemophilous plants.

Topic in NCERT: Wind-pollinated flowers

Line in NCERT: "Wind pollination also requires that the pollen grains are light and non-sticky so that they can be transported in wind currents."