CHAPTER II. SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 697 



would be evaporated in a given time by equal areas of water- 

 surface, and of living plant-surface. But this is not the case. 

 All observations show that the amount of water transpired from a 

 given area of living plant-surface in a given time, is only a small 

 fraction of that evaporated in the same time from an equal surface 

 of water. On the other hand, the evaporation from dead plant- 

 surface is as active, or even more so, than from a free surface of 

 water. Transpiration, whilst ultimately depending upon the 

 purely physical process of evaporation, is essentially evaporation 

 modified by the living substance, protoplasm, from and through 

 which it takes place, and is therefore a vital function. 



The activity of transpiration is intimately connected with the 

 external conditions. It is dependent upon the hygrometric state 

 of the atmosphere, upon the temperature of the air, and especially 

 upon exposure to light. Naturally, the drier the atmosphere the 

 more active the transpiration; and, similarly, a rise of temperature 

 promotes transpiration. The effect of exposure to light is very 

 striking. On removing a plant from darkness to even feeble 

 light, its transpiration is markedly increased; but when it is re- 

 moved into bright sunlight it is increased severalfold. This effect 

 of light is especially marked in the case of plants, or parts of 

 plants, which contain chlorophyll. It is clear that light so acts 

 upon the protoplasm of the transpiring tissue as to facilitate the 

 evaporation of the water contained in the cells. 



The activity of the transpiration from the surface of a plant 

 is inversely proportional to the development of the tegumentary 

 tissue. Thus, transpiration is rapid in the case of stems and 

 leaves in which cuticularisation or suberisation of the superficial 

 cell- walls has taken place to but a small extent or not at all, 

 whereas the transpiration of organs which, like the trunks of 

 many trees, are covered externally by layers of cork, or which, 

 like the stems and leaves of succulent plants, such as Cacti, Aloes, 

 Crassulaceae, etc., have a thick cuticle, is comparatively slight. 



However, inasmuch as most aerial leaves and stems have a 

 more or less well-developed and cuticularised tegumentary tissue, 

 the transpiration from the external surface is insignificant. In 

 such cases the transpiration takes place mainly through the thin 

 uiicuticularised walls of the cells of the ground-tissue into the 

 intercellular spaces, and the watery vapour escapes from the 

 intercellular spaces into the external air by means of the stomata 

 and the lenticels. The stomata, especially, are organs for the 



