Final answer:
Exines in the context of pollen grains are the protective outer layers composed of sporopollenin, allowing pollen to be carried without damage and reach the female organs for fertilization.
Step-by-step explanation:
In the context of pollen grains, exines refer to the thicker, outer layer that encapsulates the pollen. This layer is composed of sporopollenin, a complex substance that ensures the pollen's durability and resistance to environmental stress, thereby enabling it to survive under unfavorable conditions. The importance of sporopollenin within the exine layer is significant as it allows for the protection and transportation of pollen by various means such as wind, water, and biological agents like insects without the risk of damage.
The exine layer ensures that the male gametophytes, which the pollen grains represent, can effectively reach the female organs of plants for successful fertilization. This is crucial because it helps the plant species to reproduce and spread their genetic material. The sporopollenin within the exine is supplied by tapetal cells, which are part of the internal structure supporting pollen development.