Sexual Reproduction in Flowering Plants Test - 1

In decussate arrangement, the microspores are arranged in two pairs. One pair is situated at right angles to the other. This type of arrangement of microspore tetrads is found in Magnolia.

The cells of the innermost wall layer are radially elongated and rich in protoplasmic contents. This layer is called tapetum. The tapetum forms the nutritive tissue nourishing the developing microspores and provides nourishment to young microspore mother cells.

There are 2 polar nuclei which may remain separate until the discharge of male gametes or fuse to form diploid secondary nucleus of central cell. During fertilisation, one male gamete fuses with the secondary nucleus to form triploid nucleus. After fertilisation, the central cell develops endosperm which is triploid primary endosperm nucleus (3 n).

Flowers possess some mechanical barrier on their stigmatic surface. The Calotropis is characterised by the presence of gynostegium and pollinia, where gynostegium is formed by the fusion of stigma and androecium and pollens are aggregated in pollinia. Self pollination becomes difficult and hence, cross-pollination is achieved with the help of bees.

In some plants, such as Urtica, the anthers burst suddenly to throw the pollen grains into the air. This is termed as gunpowder mechanism. The anthers are often versatile, swinging freely in the air and the pollens are light and smooth-walled. The wind-pollinated flower, Urtica dioica shows an explosive mechanism by which the anthers burst and surcharge the air with pollen.

Carrion flies and flesh flies are important for some species of plants whose flowers exude a fetid odour. Hence, carrion flies are attracted by the unpleasant smell (smell of rottening meat) of flowers of Rafflesia.

Bat pollination is called chiropterophily. The flowers they visit are large and have a strong scent. Flowers usually produce abundant pollen grains and secrete more nector.
Flowers of Kigelia pinnata are large bell-shaped and their scent is most notable at night indicating that they are adapted to pollination by bats. Also, in Anthocephalus cadamba, the pollination takes place by bats which visit several flowers to collect nectar.

Ornithophily is the pollination of flowering plants by birds. These plants typically have brightly coloured (often red to yellow), large conspicuous, odourless flowers. The flowers have long tubular funnel shaped corollas holding ample nectar.

The outer integument becomes hard and leathery testa or outer seed coat which ensures survival of seeds. The inner integument, if persists, forms the tegmen.

On opening of seed coat, the kernel is obtained in both pea and gram. The kernel in dicotyledonous seed is the embryo. In it the two fleshy cotyledons are very conspicuous. These are fleshy as all the nutrients required by the growing seedlings are kept stored here. Seeds of both pea and gram is characterised by the absence of endosperm.