31 Years NEET Previous Year Questions: Sexual Reproduction in Flowering Plants - 1

Xenogamy – Transfer of pollen grains from anther to the stigma of a different plant. This is the only type of pollination which during pollination brings genetically different types of pollen grains to the stigma.

Thus, hilum represents the junction between ovule and funicle. Each ovule has one or two protective envelopes called integuments. Integuments encircle the nucellus except at the tip where a small opening called the micropyle is organised. Opposite the micropylar end, is the chalaza, representing the basal part of the ovule.

In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes of their release, and in some members of Rosaceae, Leguminoseae and Solanaceae, they maintain viability for months.

• Plum/peach belongs to the family Rosaceae and it shows half-inferior ovary. So the flowers are perigynous. 
• Sunflower have inferior ovary.
• Brinjal (Solanaceae) has a superior ovary. 
• Mustard have superior ovary.

In a majority of aquatic plants such as water hyacinth and water lily, the flowers emerge above the level of water and are pollinated by insects or wind as in most of the land plants. Their stem part which is above the thalamus is not in the water. The pollen grains are in the upper part of thalamus so pollination can't done by water. That's why it is done by insects and wind.

• Vallisneria is an aquatic plant. In case of Vallisneria water is the agent for cross pollination. 
• In female Vallisneria, the flower reaches to the surface of water by the long stalk and the male flower or pollen grains are released into the surface of water. 
• Female flowers or stigma carried them by water current.

Autogamy and geitonogamy are two forms of self-pollination. In autogamy, pollen falls on stigma of the same flower. While in geitonogamy pollens from a flower fall on the stigma of some other flower on the same plant. Papaya is a dioecious plant thus both autogamy and geitonogamy are prevented in it.

Exine of pollen grain is made up of highly resistant fatty substance called sporopollenin, which is not degraded by any enzyme. It is not affected by high temperature, strong acid or strong alkali. Because of the sporopollenin, pollen grains are well preserved as microfossils.

• Double fertilisation is unique and universal feature of angiosperms. Total number of nuclei involved in double fertilisation is five, i.e., 2 in syngamy and 3 in triple fusion. 
• Syngamy is fusion of one male gamete with egg to form zygote. 
• Triple fusion occurs when the second male gamete with 2 polar nuclei or secondary nucleus to form triploid primary endosperm nucleus.

Each pollen grain of Pinus has two wing-like structures which enable it to float in the air, as an adaptation for dispersal by wind. Pollen grains of mustard, Cycas and mango are not winged.

• Pollen grains of different species can land on the stigma, but pistil has an ability to recognise the compatible pollens and accept it for germination on stigma and growth of pollen tube in the style.
• There occurs a chemical mediated interaction between pollen and pistil, which facilitates the recognition and acceptance of the right type of pollen for further processes and the rejection of the wrong type of pollen grains.

So, the correct option is 'Pollen grains of many species can germinate on the stigma of a flower, but one pollen tube of the same species grow into the style'.

• The tapetum is a specialized layer of nutritive cells found within the sporangium, particularly within the anther, of flowering plants, where it is located between the sporangenous tissue and the anther wall.
• Tapetum is important for the nutrition and development of pollen grains, as well as a source of precursors for the pollen coat, however it does not help in dehiscence of the anther.

So, the correct answer is 'Tapetum helps in the dehiscence of anther.'

The synergid cell wall forms a highly thickened structure called the filiform apparatus at the micropylar end, consisting of numerous finger-like projections into the synergid cytoplasm. It is believed to play a major role in pollen tube guidance and reception.

Nectar and pollen grains are the usual floral rewards which the animal pollinators get. 

Additional Information:

• Flowering plants need to get pollen from one flower to another, either within a plant for self-pollination or between plants of the same species for cross-pollination to occur.
• However, pollen cant move on its own, so animals or the wind (and water in rare cases) move the pollen for plants.Many flowers use colours to attract insects, sometimes helped by coloured guiding marks. Some have ultraviolet marks that can be seen by insects but are invisible to human eyes.
• Flowers are often shaped to provide a landing platform for visiting insects or to force them to brush against anthers and stigmas.
• Some flowers have scent to attract insects. Many of these scents are pleasing to humans too, but not all some flowers attract flies with a smell of rotting meat.
• Colours cant be seen in the dark, so scent is important for flowers that are pollinated by night-flying insects such as moths.
• Most bird-pollinated flowers have lots of nectar, often at the bottom of a tube of petals.
• Birds need to brush against anthers and stigmas when reaching for the sugary reward with their long beaks. Some birds, such as stitchbirds and bellbirds, have special brush-like tips to their tongues to help them soak up the nectar.

• Ovule is an integumented megasporangium found in spermatophytes which develops into seed after fertilization.
• An angiospermic ovule is typically an ovoid and whitish structure. It occurs inside ovary where it is attached to a parenchymatous cushion called placenta either singly or in a cluster.
• The ovule is stalked. The stalk is called funiculus or funicle. The point of attachment of the body of the ovule with the funiculus is known as hilum. It is present as a scar on a mature seed.

Honey is made by nectar and pollens. Bees collect pollen and nectar in the spring when most flowers and plants are in bloom. They collect them in their stomach but an hour, the nector mixes with the proteins and enzymes produced by bees, which convert nectar into honey. 

• Geitonogamy involves transfer of the pollen from one flower of a plant to the stigma of another flower of the same plant.
• As the pollen has to move from one flower to another flower, it requires a pollinating agent. Yet it is genetically similar to autogamy, as both the flowers of the plant, share the same genotype of the plant.
• Xenogamy is pollination between two flowers of two different plants.
• Apogamy: It can be defined as the development of a sporophyte directly from the gametophyte without the intervention of sex organs and gametes. The sporophyte formed usually have the same chromosome number as the gametophyte (i.e. the haploid number for the species). It is common and wide spread phenomenon in ferns.
• Cleistogamy or automatic self-pollination describes the trait of certain plants to propagate by using non-opening, self-pollinating flowers. Especially well known in peanuts, peas, and beans. 

• Lilium is an angiosperm plant, in which male gametophyte is 3 celled and most reduced gametophyte as compared to Funaria (bryophyte). 
• As we proceed from Thallophyta to angiosperms, there is a reduction in the gametophytic generation they become more and more dependent on sporophyte.
• In each of the plant groups that succeeded bryophytes, the gametophyte became less and less prominent. 
• In angiosperms like Lilium, the male gametophyte (pollen - grain) is highly reduced and is only a 3 celled structure.

• Filiform apparatus helps in the entry of pollen tube into a synergid in ovule.
• Filiform apparatus is in form of finger like projection comprising a core of micro fibrils enclosed in a sheath. It resembles transfer cells meant for short distance movement of metabolites.
• It is responsible for the absorption of food from the nucleus.

• The tapetum is a specialised layer of nutritive cells found within the anther of flowering plants, where it is located between the sporangenous tissue and the anther wall.
• Tapetum is important for the nutrition and development of pollen grains, as well as a source of precursors for the pollen coat.