Write about the structure of microsporangium with suitable diagram

Explanation

Microsporangium is cylindrical structure that runs on either side of each anther lobe. It appears circular in transverse section. Microsporangium consists of two parts, microsporangial wall and sporogenous tissue.
1. Microsporangial Wall. Including the common epidermis of anther, each.microsporangium has four types of wall layers-epidermis, endothecium, middle layers and tapetum. The outer three layers are mainly protective in nature while the fourth layer of tapetum provides nourishment to sporogenous tissue.
(i) Epidermis. It is common covering layer of the anther. The cells are small and thin walled. In the mature anther, the epidermal cells get stretched and shrivel.
(ii) Endothecium. It is the hypodermal wall layer of microsporangium or pollen so also called as fibrous layer sac. The cells are large-sized. They often develop fibrous thickenings of cellulose on inner and radial walls. The mature cells become dead. In such anthers, the hypodermal (as well as epidermal) cells present in the region of shallow groove between the microsporangia of an anther lobe remain thin-walled to function as stomium or line of dehiscene.
(iii) Middle Layers. They are 1-3 layers of thin-walled cells present in the wall of each microsporangium below the layer of endothecium. The cells of middle layers are ephemeral. However, they may contain starch grains and even develop some fibrous thickenings.
(iv) Tapetum. It forms the inner most layer of microsporangial wall. The tapetal cells enlarge radially and become filled with dense protoplasmic contents as well as nutrients. They either become multinucleate or their nucleus becomes polyploid due to endoploidy. Tapetum is of two typesamoeboid (=invasive) and secretory (= glandular). In amoeboid type the tapetal cells fuse to form a plasmodium or periplasmodium because it passes in between the sporogenous cells to nourish them. The cells of secretary tapetum pass out substances over the sporogenous cells for their growth and differentiation. Ultimately both the types of tapetum degenerate.
Tapetum has a number of functions : (i) Nourishment of the developing microspore mother cells and pollen grains, (ii) It produces lipid rich Ubisch granules containing sporopollenin for exine formation, pollenkitt (oily, sticky covering of lipids and carotenoids) in case of entomophilous pollen grains, special proteins for the pollen grains to recognise compatibility and hormone IAA.
(iii) It secretes enzymes like callase responsible for the degradation callose wall arround pollen tetrad.
2. Sporogenous tissue fills the whole interior of a microsporangium. Its cells divide with the growth of anther and increase their number. Ulitimately they are transformed into microspore or pollen mother cells which are diploid. The microspore mother cells or microsporocytes develop an internal layer of callose (B-1, 3 glucan) which breaks the plasmodesmal connections among themselves. The separated mother cells round off and undergo meiosis to produce tetrads.