Why are angiosperm anthers called dithecous?

The anthers of androecium of angiospermic flowers are bilobed and each lobe is divided into two parts or theca. Thus, the anther of angiosperms are dithecous. For structure of microsporangium.
A typical microsporangium is surrounded by four wall layers, i.e. the epidermis, endothecium, middle layers and the tapetum. The outer three wall layers are protective in function and help in dehiscence of anther to release pollen grains. Tapetum (innermost layer) nourishes the developing microspores or pollen grains and the cells of 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.