Final answer:
The tapetum is the layer responsible for secreting sporopollenin and pollenkitt, crucial substances involved in the formation and protection of pollen grain walls.
Step-by-step explanation:
The layer which secretes sporopollenin and pollenkitt is the tapetum, an inner layer of cells within the microsporangium of seed plants. The tapetum plays a crucial role in the development of pollen grains by providing nutrition and contributing to the formation of the pollen wall, which includes both sporopollenin and pollenkitt.
Sporopollenin is a tough polymer that is resistant to chemical and biological degradation, aiding in the durability of pollen grains. This substance facilitates the survival of pollen under diverse environmental conditions and contributes to the yellow color of most pollen. Pollenkitt, on the other hand, is a sticky substance that helps in the adhesion of pollen grains to pollinators.
During the maturation of pollen grains, the tapetum's cells degrade, releasing their contents, which include sporopollenin, to form the exine layer of the pollen grain's outer wall. This process ensures that pollen grains are well-equipped for dispersal and can successfully contribute to fertilization.