58 



required to convert it into vapour is derived from the skin, so 

 evaporation takes heat from the parts in order to enable it to 

 take place. The process of evaporation consists in the removal 

 of pure water; hence, although water containing salts in solution 

 is absorbed by the plant, only pure water is given off, and an 

 accumulation of salts takes place in the plant. The loss of 

 water caused by transpiration very materially affects the 

 amount of water in the plant at any moment, and so indirectly 

 affects the growth of the plant. For the growth of each in- 

 dividual cell is due to a condition of turgidity, and if this con- 

 dition ceases to obtain, growth ceases also. If the transpiration 

 is then excessive, and the plant loses more water than it can 

 take up, the cells of the plant lose their turgid condition, and so 

 the growth of the plant is considerably retarded. Transpiration, 

 doubtless, exercises an important influence by causing what is 

 known as the negative pressure in the wood, the reason of which 

 I hope to show presently. 



Now as to the Function of the Stomata. It was formerly 

 thought that the stomata, through which transpiration for the 

 most part takes place, acted as protective regulators of the 

 amount of fluid exhaled : thus it was said that when the plant 

 was transpiring very actively, the stomata remained closed, so 

 as to shield the plant and prevent it from losing too much 

 moisture : when, on the other hand, the atmosphere was nearly 

 saturated with moisture, so that there was little evaporation, and 

 the light was dull, the stomata opened, because then no harm 

 could come of a free passage into the leaves being left open, for 

 moisture to come out. I said that this would be the case if 

 the stomata were isolated; but that since they are placed 

 among the cells of the epidermis, owing to the action of these 

 cells overcoming that of the guard cells of the stomata, the total 

 result was, that when the temperature, was highest and the sun- 

 light brightest, and as a natural consequence the amount of 

 transpiration at a maximum, the stomata were always most 

 widely opened — precisely the reverse of what ought to occur 

 according to the old theory ; so that it is apparent that there is 

 no direct provision in the stomata for checking the amount of 



