Final answer:
The tapetum is the innermost layer of the pollen sac wall, providing essential support and materials for pollen grain development.
Step-by-step explanation:
The innermost layer of the pollen sac wall is known as the tapetum. The tapetum is critical for the development of pollen grains, as it supplies nutrients and also contributes essential components to the pollen wall, including sporopollenin, which is an important component of the pollen grain's outer layer responsible for durability and protection.
Pollen grains are the male gametophytes in seed plants and have two main layers: the exine (outer layer) and intine (inner layer). The exine contains sporopollenin, a complex and highly durable material that allows pollen grains to withstand various environmental conditions and contributes to the dispersal of pollen by wind, water, or organisms.