Question

why phylloclade of opuntia is thick and fleshy

Answer 1

Because it is structure modified to store water in xerophytic condition.

Answer 2

Opuntia is a xerophytic plant, that is, it grows in hot and arid deserts.

In normal condition, it is the root of plants that absorb water and minerals from the soil with the help of the root hairs for carrying out photosynthesis.

Since Opuntia grows in deserts where there is scarcity of wter, the root is unable to absorb water.

Hence the phylloclade is miodified into thick and flesh structure to store water and keeping the plant alive in xerophytic conditions.