43] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 179 



of the sub-aerial parts are of great importance in connexion with 

 this process. Submerged water-plants absarb, in solution, the 

 gases dissolved in the water. 



The absorbed gases remain in solution in t-he cell-sap, so that 

 living cells do not contain bubbles of gases. Moreover, gases 

 travel in the plant mainly by diffusion from cell to cell, though 

 their distribution may also be effected by means of the intercellular 

 spaces. 



43. Transpiration. Every part of a plant which is exposed 

 to the air, except such as are covered by a thick layer of cork, is 

 continually exhaling watery vapour. This may be demonstrated 

 by placing a leafy branch under a cold bell-glass, when it will 

 shortly be observed that the internal surface becomes covered with 

 drops of water, the watery vapour exhaled by the branch having 

 condensed upon the cold glass. Again, the drooping of cut flowers 

 or herbaceous branches is due to the loss of water by transpira- 

 tion. 



It must be clearly understood that transpiration is not simply 

 evaporation. If it were so, then clearly equal amounts of water 

 should 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- j 

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

 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. 



Inasmuch as most aerial leaves and stems have a more or less 

 well-developed and cuticularised tegumentary tissue, the transpira- 

 tion from the external surface is insignificant. In such cases the 

 transpiration takes place mainly through the thin uncuticularised 

 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 regulation of transpiration. 

 As already mentioned (p. 163), the stomata open and close, their 

 opening and closing being dependent upon variations in the tur- 

 gidity (p. 159) of the guard-cells. When the guard-cells are highly 



