Microsporogenesis in Angiosperms
The terminal part of the stamen bears the anther. It consists of two lobes. Each lobe has two chambers or loculi, called the pollen sacs or microsporangia. Thus a typical anther is tetrasporangiate. The four pollen sacs appear to lie in the four corners of an anther. The pollen sacs or microsporangia develop hypodermally in anther from strips of archesporial cells.
The transverse section of a mature anther shows the following:
Wall of the anther: The anther wall consists of four layers. These are epidermis, endothecium, middle layers and tapetum.
Epidermis: This is the outermost layer. It is only one cell in thickness.
Endothecium: This layer is situated just below the epidermis. It is a single layer of radially elongated cell with fibrous thickening. Because of the presence of fibrous thickenings, the endothecium is also called fibrous layer. In between these cells, a few cells without thickening are also present. These thinned wall cells collectively form the stomium or line of dehiscence.
Middle layers: The middle layers consist of three to four layers of thinned wall cells. These layers are situated below the endothecium. These layers generally degenerate to provide nourishment to the growing microspore mother cells.
Tapetum: Tapetum is the innermost layer of the wall and is multinucleate. The tapetal cells provide nutrition to the developing microspores. Tapetum secretes both enzymes and hormones. Tapetum is of two types-Secretory or glandular tapetum.and amoeboid or plasmodial tapetum. Pollen chamber: The pollen chamber forms the central cavity of anther lobe. The process of microsporogenesis takes place in the pollen chamber.
Development of anther
The young anther is represented by a group of parenchymatous cells surrounded by epidermis. It soon becomes four lobed. In each of the four lobes, some of the hypodermal cells begin to act as archesporial initials. Each archesporial initial divides into an outer primary parietal cell and an inner primary sporogenous cell. The primary parietal cell divides to form three to five wall layers – endothecium, middle layers and tapetum. The primary sporogenous cells divide to produce a mass of sporogenous cells or microsporocytes. These sporogenous cells undergo a few mitotic divisions and form microspore mother cells.
Microsporogenesis is the process in which each micropore mother cell divides meiotically to form four haploid microspores or pollen grains. The arrangement of pollen grain in a tetrad is affected by cytokinesis during meiosis. It is of following types:
- Simultaneous type: In simultaneous type, the cytokinesis occurs only at the end of meiosis II. This results in the production of four cells by cell wall formation between the four nuclei. The resultant tetrad shows tetrahedral arrangement and is common in dicotyledons.
- Successive type: In this type, cytokinesis occurs twice – once at the end of meiosis I, forming two cells and then again at the end of meiosis II to form four cells. The resultant tetrad shows isobilateral arrangement and is found in monocotyledons.
There are five types of tetrads – tetrahedral, isobilateral, decussate, T-shaped and linear. Among these, tetrahedral tetrad is the most common. In the members of family Cyperaceae, pollen mother cell divides meiotically to form four nuclei. Of these, three degenerate, the remaining nucleus form the only microspore. Thus, in the members of Cyperaceae, only one microspore is formed by one pollen mother cell instead of usual four.
In Drosera, Typha etc., all the four pollen grains do not separate but together form compound pollen grains. In Mimosaceae, 8 to 64 pollen grains remain united together. In Calotropis and some orchids, all the pollen grains of an anther lobe form a typical structure called pollinium.
Microspore or the pollen grains
Microspore or the pollen grains is the first cell of male gametophyte. It is unicellular and haploid and its shape varies from oval to polyhedral. Inside each anther, there are several thousands of pollen grains.
The wall or covering of pollen grain is called sporoderm. It is made of two layers. The outer layer is called exine. It is thick and ornamented. The exine is made of fatty substance called sporopollenin. It is biologically the most resistant substance and can withstand high temperature and strong acids and alkali.
In insect pollinated flowers, the exine of the pollen grain is covered with a yellowish viscous and sticky substance called pollenkitt. This is a protective envelop which sticks to the body of the insect and helps in pollination
The inner layer, called intine, is thin and uniform. It is made of pecto-cellulose. At the time of pollen germination, intine comes out of the germ pores in the form of a pollen tube.
The cytoplasm of pollen grain is rich in nutrients. The size and form of pollen grain and ornamentation of exine are characteristic of a plant. Pollen grains of many species cause severe allergies and respiratory disorders such as asthma, bronchitis etc.
It is the process of formation of male gametophyte from a pollen grain or microspore. The nucleus of a pollen grain divides mitotically to form a vegetative nucleus (tube nucleus) and a generative nucleus. The generative nucleus gets surrounded by cytoplasm to become generative cell. At this stage, pollen grain is two celled – a large vegetative cell and a small lenticular generative cell. The pollen grain may be discharged from the anther at this stage (two-celled stage). However, in some plants, generative cell divides further to give rise to two male gametes before the pollen grains are shed. These pollen grains are thus, three-celled at the time of shedding.
In most of the plants, three-celled condition is formed after pollen grain has been shed. The liberated pollen grain germinates on the stigma and produces a pollen tube. The pollen tube is covered by intine. It secretes pectinases and other hydrolytic enzymes to create a passage for it in the style. The pollen tube absorbs nourishment from the cells of the style for its growth. The tube nucleus descends to the tip of the pollen tube. The generative cell also passes into it and divides inside the pollen tube to form two male gametes.