NCERT Solutions for Class 12 Biology Chapter 1 Sexual Reproduction in Flowering Plants

Short Note:

• The flower is the reproductive unit of angiosperms; the androecium (stamens) is the male reproductive part and the gynoecium (pistil/carpel) is the female reproductive part.

• Each anther is bilobed with four microsporangia that develop into pollen sacs, where microsporogenesis produces haploid microspores (pollen grains) through meiosis of microspore mother cells.

• The pollen grain has a hard outer exine (made of sporopollenin, one of the most resistant organic materials known) and a soft inner intine; at shedding it may be 2-celled (vegetative + generative cell) or 3-celled.

Short Note:

• The ovule consists of the nucellus, one or two integuments, and a micropylar opening; inside it, a megaspore mother cell undergoes meiosis to form four megaspores, of which usually only one survives.

• The functional megaspore develops into the embryo sac (female gametophyte) through three rounds of mitotic division, forming the typical 7-celled, 8-nucleate structure with an egg apparatus (1 egg cell + 2 synergids), 3 antipodal cells, and a central cell with two polar nuclei.

Short Note:

• Pollination is the transfer of pollen grains from the anther to the stigma; it may be autogamy (self), geitonogamy (between flowers of the same plant), or xenogamy (between genetically different plants), and is aided by agents such as wind, water, insects and animals.

• After reaching the stigma, the pollen grain germinates to form a pollen tube that grows through the style, carrying the two male gametes, and enters the ovule usually through the micropyle (porogamy).

• Double fertilisation, unique to angiosperms, involves one male gamete fusing with the egg to form the diploid zygote (syngamy), and the other fusing with the two polar nuclei to form the triploid primary endosperm nucleus (triple fusion).

Short Note:

• The primary endosperm nucleus develops into the endosperm, which nourishes the developing embryo; in many dicots, the endosperm is completely consumed by the embryo, and food is stored in the cotyledons instead.

• The zygote develops into the embryo, passing through globular, heart-shaped and mature stages, while the ovule as a whole develops into the seed, and the ovary develops into the fruit.

• In some species, fruits and seeds can develop without fertilisation, a phenomenon known as apomixis, which is of interest to plant breeders as it produces seeds genetically identical to the parent, useful in hybrid seed production.