Why are angiosperm anthers called dithecous? Describe the structure of its microsporangium

A typical angiosperm anther is bilobed with each lobe having two theca, thus they are called dithecous.

Microsporangium is nearly circular in outline. It is surrounded by four wall layers that is the epidermis, endothecium, middle layers and the tapetum. The outer three wall layers perform the function of protection and help in dehiscence of anther to release the pollen. The innermost wall layer is the tapetum which nourishes the developing pollen grains. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus. When the anther is young, a group of compactly arranged homogenous cells called the sporogenous tissue occupies the centre of each microsporangium.