Question

Why underground swollen part of potato is a stem and hanging brown fibrous part of banyan trees are roots?

Answer 1

because this both are the modifications. ...which is modified for taking food. ...

Answer 2

It is a modification that the stem of potato is underground. It contains all the nutrients required for growth. This swollen part of potato gives rise to many roots and eyes of potatoes.

The hanging part of banyan trees is called adventitious roots. It has an aerial prop roots that mature into thick woody trunks. It is another modification.