CBSE Class 12 Biology

The flower as the reproductive unit

A flower is the part of an angiosperm that carries out sexual reproduction. Its four whorls, from outside in, are the calyx (sepals), the corolla (petals), the androecium and the gynoecium. The last two are the reproductive whorls.

The androecium is the set of stamens. Each stamen has a long, slender filament and a terminal anther, which is usually bilobed. The gynoecium is made of one or more carpels (together called the pistil), and each carpel has a stigma that receives pollen, a style that connects the stigma to the ovary, and an ovary that contains one or more ovules. When a gynoecium has several carpels they may be free (apocarpous, as in a lotus or a rose) or fused (syncarpous, as in a mustard flower).

The male and female gametes are made by different structures and in different ways, and in both cases the cells involved pass through a sporophytic stage first. A gametophyte is the small haploid body that produces gametes, and in flowering plants it is reduced to a few cells living inside the flower. The pollen grain is the male gametophyte and the embryo sac is the female gametophyte.

The anther and microsporogenesis

A typical anther is bilobed, and each lobe has two chambers, so a cross-section is roughly four-sided with a microsporangium (pollen sac) in each corner. Each lobe is called a theca, so the anther is dithecous and tetrasporangiate. The four microsporangia are the sites where pollen grains develop.

A microsporangium is surrounded by four wall layers, from outside in.

• The epidermis is a protective outer layer.
• The endothecium is a layer whose cells thicken unevenly as the anther matures, which helps the anther open (dehisce) and release the pollen.
• The middle layers, usually one to three, are short-lived.
• The tapetum is the innermost layer. Its cells have dense cytoplasm and often more than one nucleus, and they nourish the developing pollen grains.

In the young anther the centre of each microsporangium is filled with compactly arranged sporogenous tissue. Its cells are diploid, and each one can become a microspore mother cell (also called a pollen mother cell). A microspore mother cell divides by meiosis to give a cluster of four haploid microspores, called a microspore tetrad. This process is microsporogenesis. As the anther matures the microspores separate from one another and each develops into a pollen grain, so one microspore mother cell gives four pollen grains. After this the anther dehisces and releases the pollen.

The pollen grain

A pollen grain is the male gametophyte. It is usually spherical, about 25 to 50 micrometres across, and has a two-layered wall.

• The outer layer, the exine, is made of sporopollenin, one of the most resistant organic materials known. It withstands high temperatures and strong acids and alkalis, and no enzyme that degrades it is known. It is what allows pollen grains to be preserved as fossils.
• The inner layer, the intine, is thin and continuous and is made of cellulose and pectin.

The exine has prominent apertures called germ pores, where sporopollenin is absent, and the pollen tube emerges through one of them. The exine is often ornamented with patterns, which is why pollen grains can be identified under a microscope.

Inside the wall the cytoplasm is surrounded by a plasma membrane. The pollen grain starts with one haploid nucleus, which divides mitotically to form two cells. The vegetative cell is large, has abundant food reserves and a large irregularly shaped nucleus. The generative cell is small, floats in the cytoplasm of the vegetative cell and has a spindle shape with dense cytoplasm. In over sixty per cent of angiosperms pollen is shed at this two-celled stage. In the others the generative cell divides mitotically before shedding to form two male gametes, and the grain is shed three-celled.

Pollen grains are not always short-lived. They stay viable for a period that depends on the species: in rice and wheat the viability is lost within about thirty minutes of release, while in members of the Rosaceae, Leguminosae and Solanaceae pollen can remain viable for months. Pollen can be stored for years in liquid nitrogen at $-196\,^\circ\mathrm{C}$, in pollen banks, which is used in crop breeding in the same way as seed banks.

Pollen grains are rich in nutrients and are sold as food supplements. Pollen of some plants, such as carrot grass (Parthenium), causes severe allergies and respiratory disorders in many people.

The ovule

The ovule is a small structure attached to the placenta inside the ovary by a stalk, the funicle. The point where the body of the ovule fuses with the funicle is the hilum. The body of the ovule is made of a mass of parenchymatous cells, the nucellus, which holds food reserves. It is protected by one or two layers called integuments, which cover all of the nucellus except at the tip, where a small opening, the micropyle, is left. The end of the ovule opposite the micropyle is the chalaza, where the nucellus and integuments meet. Inside the nucellus is the embryo sac, the female gametophyte. A single ovary may contain one ovule (wheat, paddy, mango) or many (papaya, watermelon, orchids).

Megasporogenesis and the embryo sac

In the part of the nucellus near the micropyle, one cell becomes the megaspore mother cell. It is diploid and large, with dense cytoplasm and a prominent nucleus. It undergoes meiosis to form four haploid megaspores, in a process called megasporogenesis. In most flowering plants only one of the four megaspores is functional, and the other three degenerate. This is why a single megaspore develops into the embryo sac (monosporic development).

The nucleus of the functional megaspore divides by mitosis three times in a row, and there is no cell wall formation between divisions, so the first stage has two nuclei, then four, then eight, all in one cell. The nuclei then arrange themselves in a fixed pattern. Four sit at each end of the sac, and then cell walls form around six of the eight nuclei while two are left uncovered.

• Three cells at the micropylar end form the egg apparatus, made of two synergids and one egg cell. The synergids have a special thickening called the filiform apparatus, which guides the pollen tube into the synergid.
• Three cells at the chalazal end are the antipodals.
• The two remaining nuclei, called polar nuclei, lie in the large central cell and are below the egg apparatus. They may fuse before fertilisation.

The mature embryo sac is therefore 8-nucleate and 7-celled. The large central cell has two nuclei and the other six cells have one each. This is the typical form, and it is the female gametophyte.