What is the thick walled layer of anther  helpful in its dehiscence on its dehydration and subsequent release of pollen ?

Structure of microsporangium: In a transverse section, a typical microsporangium appears near circular in outline. It is generally surrounded by four wall layers – the epidermis, endothecium, the middle layer and the tapetum. The outer three wall layers perform the function of protection and help in the dehiscence of anther to release the pollen. The innermost wall layer is the tapetum. It nourishes the developing pollen grains.