Question

why do you think the exine should be hard

Answer 1

As it is outermost layer of pollens so it protect pollen from external harsh environment like winter and summer also from acid and alkali etc so it is made of hard substance called sporopollenin.

Answer 2

Explanation:

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

→ 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 .