Question

how Pipal sapling on the sidewall

Answer 1

a  peepal  sapling   grows  on  the  walls  on  when 

-the  wall  has  a  lot  of  moisture 
- and  enough  sunlight 
and  as  we  know  dispersal  of  seeds  takes  place  and  the  scattered   seed  comes  and  it  starts  to  grow

Answer 2

Pipal tree grows on the walls because peepal tree produces a large number of seeds which is carried by winds, animals, insects and birds etc. to different places including old walls. Pipal trees have very strong root system which absorbs moisture and nutrients from walls. They grow mainly in rainy season.