The tapetum is the only layer of the anther wall that is physiologically active in nourishing the pollen; the outer three layers (epidermis, endothecium, and middle layers) are primarily protective.
Step 1: The tapetum is the innermost layer of the microsporangium (anther wall) and consists of cells with dense cytoplasm and generally more than one nucleus. Step 2: Its primary function is to provide nutrition to the developing microspores (pollen grains). Step 3: It also secretes various enzymes, hormones, and the fatty substance sporopollenin, which forms the tough outer layer (exine) of the pollen grain.