What's the name of the part??

The given diagram represents microsporangium. The label marked by you is endothecium layer.
 

The microsporangium is surrounded by four wall layers (epidermis, endothecium, middle layers, and tapetum). The outer three layers are protective and help in dehiscence of anther to release the pollen grains. The tapetum provides nourishment to the developing pollen grains. In the young anther, the sporogenous tissue forms the centre of each microsporangium.