Stomata are present in greater number on the lower surface because if more number of stomata will be present on the upper surface, it would lead to great amount of water loss through transpiration.
Thus, to avoid the excessive transpiration, stomata are present in greater number on lower surface of the leaf.

On reducing or removing the fifty per cent of leaves from plant, the rate of transpiration by the remaining leaves will decreases because the rate of transpiration is purely dependent upon the surface area of the leaves. Thus, plant with larger leaf surface area have higher rate of transpiration as compared to the plant with the areas of smaller leaf surface 

Girdling is the removal of a ring of outer up to the depth of the plant of the phloem layer in a woody trunk.Phloem is responsible for downward movement of food, which is stopped bt gridling. The roots will not be able to get food and die first.

The plants survive under flooded condition by losing excess water through transpiration (increased (increased transpiration rate.) It is seen that in prolonged flooded conditions, roots die because of lack oxygen and plants eventually dies.

Various ions from the soil are actively absorbed and accumulated in the root tissues.This accumulation of ions causes a decrease in water potential.So water is absorbed passively through osmosis by the root, which results in hydrostatic pressure.Since, ions are absorbed actively, there is a need for energy to develop root pressure.