52 



proportion of fifty; also in the Dahlia, where the proportion of 

 stomata on the upper surface of the leaf is twenty-two, on the 

 lower surface thirty-three, or as 2:3, while the degree of tran- 

 spiration on the upper surface is 50, and on the lower surface 

 100, or as 1:2. In the Deadly Nightshade the proportion of 

 stomata on the upper surface is 5-5 to 10 on the lower surface, 

 and the relative quantity of water transpired is as 48:60. 

 Recent results have shown further that the quantity of water 

 exhaled is greater in the same proportion as the number of 

 stomata is greater. Those points which particularly affect the 

 degree of transpiration in plants may be grouped under two 

 main heads. 



I. External influences. Of these the first is the presence 

 of light, which, however, is so usually associated with a 

 rise of temperature, that it is a matter of difficulty to 

 isolate them. In the dark, the stomata of most plants 

 are almost or quite closed, hence very little watery vapour 

 can be then exhaled, and if a low degree of temperature be 

 present at the same time, the process of transpiration may 

 become slow, or even cease altogether. The immediate bearing 

 of this statement will become evident when we come to con- 

 sider root-pressure The stomata of most plants are, on the 

 contrary, most widely open in the light, especially if it be bright, 

 even if the temperature be low. Hence, according to the care- 

 ful experiments of MacNab, increase of light, apart altogether 

 from, and independently of, the elevation of temperature, has a 

 distinct influence on the amount of transpiration which it 

 increases. Increase in the amount of humidity of the air also 

 affects it in two ways ; 1st, it acts by simply checking the pro- 

 cess of evaporation, and 2ndly, it acts on the stomata, causing 

 the guard and epidermal cells to swell up and become turgid, 

 and thus diminishing the size of the stomata, that is to say, 

 causing their closure. 



We may consider, together, the effect produced by varia- 

 tions in the moisture of the air surrounding the transpiring 

 organ, and that produced by variations in the temperature of 

 the air ; for each of these conditions is so dependent on the 

 other that it is impossible altogether to eliminate its influence 



