TRANSPIRATION 195 



chemical nature of the substances present in the solu- 

 tions absorbed by the plant. 



(i) At night and in darkened rooms plants transpire very little ; 

 in diffuse daylight an increase is noticed, but when exposed to 

 bright sunlight the amount of water given off is vastly augmented. 

 In one of Wiesner's experiments 100 sq. cm. of leaf-surface of 

 a well-grown maize plant gave off in the dark 97 milligrams of 

 water per hour, while in diffuse daylight 114 milligrams were 

 lost and in bright sunlight 785 milligrams. 



Usually under the influence of light the turgidity of the 

 guard-cells is increased, the stomatal pore therefore opens and 

 water-vapour is thus allowed to escape freely from the leaf. 

 The action of light upon transpiration is independent of the 

 effect of heat which usually accompanies it ; it is not, however, 

 simply connected with the increased opening of the stomata 

 under its influence, for a similar increase of transpiration is 

 noticed when fungi which possess no stomata are exposed to 

 light of increasing intensity. Light appears to act as a direct 

 stimulus upon the protoplasm, and under this stimulation the 

 latter becomes more permeable to the water of the cell-sap. 



It must also be remarked that light indirectly influences 

 transpiration by modifying the structure of the tissues and the 

 composition of the cell-walls of the leaves. Plants grown in 

 well-exposed situations with full access to light have a greater 

 development of cuticle and smaller intercellular spaces within 

 the leaves than those grown in shaded situations; from the 

 former less water is transpired than from the latter. 



(ii) When the air is saturated, as on a dull day or in a close 

 damp greenhouse, transpiration is almost entirely checked ; on the 

 other hand a dry atmosphere, even if cold, leads to considerable 

 loss of water, and the injury which occurs to delicate leaves and 

 other recently expanded parts of plants at low temperatures in 

 spring is perhaps caused as much by the dryness of the air at 

 such times as by its coldness. 



