Which one of the following layers of microsporangium nourishes the developing pollen grains?

Tapetum

The correct answer is Option (2) → Tapetum

Atypical stamen has two parts – the long and slender stalk called the filament, and the terminal generally bilobed structure called the anther. The anther is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe.
The microsporangia develop further and become pollen sacs. They extend longitudinally all through the length of an anther and are packed with pollen grains. In a transverse section, a typical microsporangium appears near circular in outline. It is generally surrounded by four wall layers– 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. It nourishes the developing pollen grains.

The function of the tapetum in the microsporangium is the nourishment of pollen grains. The tapetum is the innermost layer of cells in the wall of the microsporangium. Its primary function is to provide nourishment and support to the developing pollen grains. The tapetum cells are rich in cytoplasm and contain various nutrients, including carbohydrates, proteins, and lipids.

During microsporogenesis, the tapetum supplies essential nutrients and metabolites to the developing pollen grains, promoting their growth and maturation. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.