54 



transpired during the day, i.e. in the light, is very large, the 

 amount transpired during the night, i.e. in the dark, is very 

 small indeed. In this case the influence of light on transpira- 

 tion is mainly due to the widely open stomata, though in part 

 to the warmth of the air. 



But light has probably a direct influence upon transpiration, 

 altogether apart from its effect on the stomata ; at least we 

 have some observations on record which seem to show that this 

 is the case, although it is extremely difficult to get the action of 

 light as light solely, and not accompanied by any variations of 

 temperature induced by it. These experiments were carefully 

 performed on plants kept in a moist and nearly saturated at- 

 mosphere, i.e., under conditions which prevented any change in 

 the temperature or humidity of the air around the plant. It- 

 was then found that under such conditions the stomata remained 

 open both in the dark and in the light, so that the effect of light 

 upon these structures was thereby eliminated ; and further, it was 

 observed that exposure to light increased very much the amount 

 of transpiration of the plant as compared with the amount of 

 transpiration of the same plant for an equal time in the dark. 



The nature of the substances present in the water taken up by 

 the roots, and so introduced into the plant, materially affects 

 the amount of transpiration going on in the leaves. 



Movements of the air produce an influence in a twofold 

 fashion — first, by promoting evaporation, and, second, by shak- 

 ing the parts of the plant, e. g. the leaves and branches, there- 

 by promoting transpiration. Thus, if an ordinary branch be 

 taken and violently shaken for a minute or two, the leaves be- 

 come flaccid and hang down for a considerable time, as if they 

 had been supplied with too little moisture. The reason of this 

 is that the mechanical stimulus produced by shaking the leaves 

 causes excessive transpiration. This is an interesting point in 

 connection with the attractive power of the living protoplasm 

 for water, for by shaking this attraction is diminished, and 

 it allows the passage of water through it to take place in conse- 

 quence more readily than is usually the case. 



II. Internal Causes. Transpiration varies in quantity ac- 



