NEET Practice Questions (MCQs) with Answers & Solutions

Exine is outer layer of pollen grain which is acetolysis resistant layer. It is also resistant to physical and biological degradation. Because of this property of exine, pollen grains are found well preserved for long periods in fossil deposits.

After entering the wall of the embryo sac the pollen tube during its entry into the embryo sac passes through the nucellar cells and the synergids, e.g. Fagopyrum. The pollen tubes passes between the egg and one synergid. In Cardiospermum, it passes between embryo sac wall and synegid. In Oxalis, it passes directly through a synergid. In Viola, it enters a synergid and makes its way thorough the base of the synergid. In normal cases only one synergid destroys by the impact of the pollen tube and the other remains intact until sometimes afterwards.

The propagation through vegetative multiplication is used to maintain the genetic traits of a given plant. It gives rise to genetically uniform population or clone. in case of plants propagated through seeds, variations creep in due to chance segregation of genes during meiosis and their chance combination during fertilizaton.

(d)  During the post-pollination development, germination of pollen grain takes place. The pollen grain absorbs water and nutrients on the stigma from the stigmatic secretion through its germ pores. The tube or vegetative cell enlarges and comes out of the pollen grain through one of the germ pores or germinal furrows to form a pollen tube. Generative cell does not produces pollen tube instead it passes into the pollen tube and divides into 2 male gametes. Thus even after killing the generative cell with a laser beam, the pollen grain produces normal pollen tube because the vegetative cell has not been damaged.

Cleistogamous flowers do not open at all. In such flowers, the anhters and stigma lie close to each other. Anthers when dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination Thus, cleistogamous flowers are autogamous asn there is no chance of cross-pollination. So, no pollination agent is required in these flowers. 

The outer layer of pollen grain is called exine. It is thick and smooth and culticularised. The cutin is called sporopollenin. It is not degraded by any enzyme. It is not affected by high temperature, strong acid or strong alkali. Thus it is resistant to chemical and biological decomposition. Because of sporopollenin, pollen grains are well preserved as microfossils. 

Pollen viability is the period for which pollen grians retian the ability to germinate. Pollen viability is little in flowers which are pollinated in bud condition. It is 30 minutes in rice and wheat. It depends upon environmental conditions of temperature and humidity.

In angiosperms, the body of the ovule consists of a mass of parenchymatous cells called nucellus. In the hypodermal region of nucellus towards the micropylar end develops a primary archesporial cell, which by further divisions forms a diploid megaspore mother cell. The latter undergoes meiosis to form megaspore tetrad. Usually the chalazal megaspore remians functional while the other three degenerate. The functional megaspore is the first cell of female gametophyte (embryo sac).

During the event of double fertilisation in angiosperms, the second male gamete fuses with the two haploid polar nuclei or diploid secondary nucleus of the central cell to form a triploid primary endosperm nucleuus (PEN). This second fertilisation is called vegetative fertilisation.

Non-endospermic seeds or exalbuminous seeds have no residual endosperm as it is completely consumed during embryo development. Gram, pea, bean and groundnut have non-endospermic seeds.