In the life history of Angiosperms, there is a highly developed sporophyte and highly reduced gametophyte. Which alternate with each other (Alternation of generations). These two generations are clearly distinguishable in their morphological as well as cytological characters and are formed from one another i.e. gametophytic generation gives rise to sporophytic generation and vice versa. The sporophytic generation is reported by roots, stems, leaves and flowers (which bears calyx, corolla, androecium and gynoecium).
Each cell bears 2n number of chromosomes i.e., the sporophyte is diploid. The gametophytic generation is represented by pollen grains (male gametophyte) and embryo sac (female gametophyte) and each cell bears n number of chromosomes i.e., the gametophyte is haploid.
The fertile portion of stamens is called anther. Each anther is usually made up of two lobes (antherlobes) connected by a connective. In turn, each anther lobe contains two pollen chambers, placed longitudinally. Each pollen chamber represents a microsporangium and is filled with a large number of pollen grains or microspores.
Thus a typical anther consists of four microsporangia (pollen chambers). This type of anther is known as dithechous. In Malvaceae monothecous condition is found i.e. anthers bears only one lobe (theca with 2 microporangia).
Each lobe of the mature anther is an elongated cavity or pollen sac or microsporangium which consists of pollen grains. The outermost layer of the wall of microsporangium is epidermis.
The next layer made up of large columnar cells with fibrous thickening arising from inner side is called endothecium. The cells of the endothecium opposite the partition between two microsporangia are thin- walled and constitute the stomium through which pollen grains are discharged from the anther lobes.
Beneath the endothecium are present two to three layers of cells called middle layers. Below the middle layers and outer to the pollen sac the innermost layer of cells known as tapetum is present which surrounds the microsporogenous tissue.
Its cells divide freely and form a layer of enlarged cubical cells which elongate centripetally as the microsporogenous tissue develops into pollen mother cells. The layer called tapetum disappears when the pollen grains become mature and before dehiscence of anthers.
A young anther in a T.S. appears quadrilobed. The development of microsporangia is eusporangiate type. Certain cells of hypodermal layer in each lobe begin to enlarge, and protoplasm becomes denser than the neighbouring cells. These are archesporial initials. These cells divide periclinally to form primary parietal layer towards outside and primary microsporogenous cells towards the inner side. The cells of the foremen usually undergo further periclinal and anticlinal divisions to form a series of 3 to 5 concentric layers making the wall of the anther. The primary microsporogenous cells separate from one another and give rise to microspore mother cells.
The epidermal cells of the wall of anther become stretched and flattened as the anther enlarges. The parietal layer lying below the epidermis form endothecium. The endothecium is followed by 2-3 middle layers. The cells between middle layer and microsporogenous cells develop into tapetum.
Later the tapetal cells becomes loose and nuclei may degenerate. Tapetum plays an important role in pollen development (specially microspore walls). Generally, all the microsporogenous cells are capable of producing the microspores but some of them distintegrate and are absorbed by remaining cells. Microspore mother cells divide to form microspores by reduction divisions. The division may be successive type or simultaneous type.
In the former, each of the two nuclear divisions are followed by the formation of wall. In successive type the cell plate is formed in the center and then extends centrifugally on both sides dividing the cell into similar parts. In simultaneous type, a division occurs centripetally dividing the cells into four parts with the formation of furrows.
Generally successive type occurs in monocotyledons and simultaneous type in dicotyledons. The tetrad produced by successive type is called isobilateral tetrad and that produced by simultaneous type is called tetrahedral tetrad. Other tetrads are decussate, T-shaped and linear.
Most commonly the microspores soon separate from one another but sometimes they adhere in tetrads to form compound pollen grains e.g. Drimys, Anona, Drosera, Elodea, Typha. In Asclepiadaceae family all microspores of a sporangium cohere in a single mass called pollinium. Development of microspores (pollen grains) from archesporial cells is called microsporogenesis.