RUST, SMUT, ETC, 117 



207. These and numerous other experiments establish the 

 fact that exhalation from the stomata is greatly dependent upon 

 the moisture of the atmosphere, and that an atmosphere satura- 

 ted with moisture totally arrests this function in plants. Light 

 exercises a most important influence upon exhalation, for it has 

 heen established that if plants in which the process is being vi- 

 gorously performed be carried into a darkened room, the exhala- 

 tion is immediately stopped, and that the absorption by the roots 

 is checked almost as completely as if the plant had been stripped 

 of its leaves.^ 1 ) 



208. " It would not seem improbable, then, that the effect of 

 light is confined to the opening of the stomata, which it is be- 

 lieved to effect ; and that the large quantity of fluid discharged 

 from them may be due to simple evaporation from the extensive 

 surface of succulent and delicate tissue which is thus brought 

 into relation with the air, and to the constant supply of fluid 

 from within, by which it is maintained in a moist condition." (2) 



209. As is shown in the foregoing paragraphs, evaporation 

 may take place from all parts of the surface of a plant in small 

 quantity when air is not saturated with moisture ; and in the 

 absence or presence of light, it is, in a word, independent of vi- 

 tality. Exhalation, on the contrary, is dependent not only upon 

 the dryness of the atmosphere, but upon the opening of the 

 stomata of the plant under the influence of light, it is therefore 

 so far subordinate to vitality. 



210. The stomata opening under the influence of light, the 

 rise of the sap< 3 ) in plants becomes due to evaporation and the 

 pressure of the atmosphere. " By the evaporation of water at 

 the surface of plants, a vacuum arises within them, in conse- 



(1) Senebier, quoted by Carpenter. (2) Carpenter, Prin. Comp. Physiology. 

 (3) The rise of the sap in spring is probably greatly increased by a species of 

 germination liberating gas in the plant. 



