Question

why does transpiration occur in the lower side of the leaf

Answer 1

the rate of tanspiration is very closely linked with rate of evaporation.
increased air movements across the surface of leaf increases da rate of evaporation of h2o frm da leaf surface.
this happens becoz h20 saturated air moved away.

Answer 2

Transpiration takes place in the lowerpart of the leaf because the plants need to breath ,and need to take in carbon dioxide ,and to do this they need to open their stomata,when this opens the excess water comes out .As stomata is situated in the lower side of the leaf ,Transpiration occurs lower side of the leaf.