The presence of stomata makes the leaves suitable for exchange of gases which goes on during photosynthesis and respiration, and this makes loss of water inevitable. Stomata are tiny pores in the epidermis of leaves, each pore being surrounded by a pair of special cells called guard-cells which control the size of the stomatal pores. The behaviour of the stomata and the rate of transpiration are determined by environmental factors. Transpiration exerts a force against that of gravity, and this is known as transpiration pull, enables water to be absorbed.