Plant Reproduction Test - 9

• Pollen grain protoplasts are uninucleate at the beginning but at the time of liberation it becomes 2 or 3 celled.
• Pollen grains are the male gametes.
• They are present inside the pollen sacs.

• The ovule bearing parenchymatous tissue is known as the placenta.
• It is also the part of the ovary where the funiculus attaches itself.
• Just like in humans, the placenta provides nutrition to the growing ovules.

The development of the male gametophyte in angiosperms is called as microgametogenesis. Pollen grain is the first cell of a male gametophyte. This cell undergoes only two divisions, with the result of first division two cells are formed - a large vegetative cell and a small generative cell. The second division is concerned with generative cell only. This division may take place either in pollen grain or in the pollen tube and gives rise to two male gametes.

• Pollen allergies are usually caused by plants that produce pollens in a large number.
• Because of this they might float in air as well causing them to enter respiratory tracts.
• Carrot grass, Parthenium are a major contributor to pollen allergies.

In the ovule, the pollen tube is attracted by secretions of synergids. Jensen (1965) suggested that the filiform apparatus may be aiding the synergid in the absorption and transportation of materials into the embryo sac from the nucellus. He held the opinion that the filiform apparatus

Stigma is the terminal receptive part of the pistil which functions as landing platform for the pollen grains. It also determines the compatibility of the pollen grains.

A pollen grain with pollen tube carrying male gametes represents mature male gametophyte. It is 3-celled (one tube cell +2 male gametes) and 3-nucleated (one tube nucleus + two nuclei of each male gamete) structure.

Each pollen mother cell (PMC), by a meiotic division, gives rise to a group of four haploid microspores. Thus, 16 pollen mother cells undergo meiotic divisions to produce 64 pollen grains.

The tapetal layer is of great physiological significance as all the food material entering into the sporogenous tissue diffuses through this layer. Ultimately the cells of tapetal layer disorganise. Thus, tapetum makes a nutritive layer for the developing microspores.

• Pollens are spiny because they make it easy for the pollens to attach themselves to the insects that come to suck the nectar.
• This makes cross pollination possible.

• Gynoecium represents the female part of the flower.
• It consists of a stigma, style and ovaries.
• The gynoecium is non motile.
• It ’s the pollens that are motile.
• They fall on the stigma which starts the fertilization process.

Outer layer (exine) of pollen grain is made of a highly resistant substance called sporopollenin. Sporopollenin is not degraded by any enzyme. It is not affected by high temperature, strong acid or strong alkali. Because of sporopollenin, pollen grains are well preserved as fossils.

• Pistil is the free unit of a gynoecium.
• A pistil has three parts-style, stigma and ovaries.
• Stigma acts as a landing platform for the pollens.
• Style is the long tube-like structure that carries the male gamete to the ovaries.

Wheat is an anemophilous plant. Its flowers are not very showy. They do not have petals or sepals. Each female flower consists of an ovary from which two styles emerge, finished with two feathery sticky stigmas to catch pollen.

Entomophilous flowers produce an odour which may be pleasant (e.g., jasmine) or foul (e.g., Aristolochia, Arum, Rafflesia) Foul smell attracts flies and beetles. Malacophily is the kind of pollination in which snails perform pollination e.g. in Arisaema (cobra plant). Hydrophily refers to water and anemophily refers to wind pollination.