Question

why the exine should be hard

Answer 1

so that the viability of seeds can be maintained even in harsh conditions

Answer 2

$\tt\pink{\underline{Exine}}$

→ It age outer layer of pollen grain and very hard.

→ It age made up of sporopollenin in which is most resistance organic material present in nature .

→Sporopollenin is Resistance to high temperatures stronger acids and alkaline and also to enzyme .