Explanation

Pollen wall comprises of two principal layers the inner intine and outer exine. The intine is pectocettulosic in nature. A special feature of intine is the presence of beads, ribbons or plates of enzymatic proteins particularly in vicinity of germ pores. The exine is composed of sporopollenin which is derived from carotenoids by oxidative polymerization. It is resistant to physical and biological decomposition. Due to this, pollen walls are often preserved for long periods in fossil deposits.