124 TRANSPIRATION IN SUNSHINE, ETC. [CH. 



leaves, and in succulent plants with pronounced cuticles. 

 Some of these specific differences are correlated with such 

 peculiarities of structure as the extent of the leaf-surface, 

 the presence of wax and cuticular layers, and especially 

 the number of stomata per unit of area, and other struc- 

 tural features ; but there can be little doubt that more 

 profound adaptations of the species to its natural environ- 

 ment also complicate the matter. 



In the case of completely submerged leaves, transpira- 

 tion, in the ordinary sense of the word, is of course out of 

 the question, and it is but small in leaves which float on 

 the surface of water. But marsh plants with their leaves 

 in the air, even though moist, transpire large quantities 

 of water in bright sunshine. 



Enough has been said to show that transpiration is 

 not a mere evaporation of water from the surface of a 

 leaf; and it remains to point out that, in the typical leaf, 

 the greater part of the aqueous vapour escapes through the 

 stomata on the lower surface of the lamina, passing out of 

 the intercellular spaces into which it had escaped through 

 the thin cell- walls of the mesophyll. This being the case, 

 the enhanced transpiration in brilliant sunlight has been 

 attributed to the widening of the apertures of the stomata 

 under the influence of light, and the consequent easing 

 of the passage of the aqueous vapour to the exterior. It 

 is true the stomata of many plants experimented upon do 

 open wider as the light increases in intensity, and more 

 vapour escapes from the stomatal area than from the 

 upper surface of the leaf; but it is also to be noted that, 

 according to Wiesner and Van Tieghem, the chlorophyll- 

 corpuscles actually exude more water when active in 

 the sunlight, and that most of the increased production 

 of aqueous vapour in the intercellular passages is to be 

 put down to this process, which the latter author terms 



