170 PASSAGE OF VAPOUR THROUGH STOMATA. 



account for the large quantity of vapour, which, as just stated, 

 may be easily ascertained to pass off at certain times, from the 

 surface of the living plant. 



257. Now a few simple experiments will show, that there is 

 a strong probability that this rapid transpiration takes place 

 through the stomata. If a piece of glass be held near the upper 

 surface of the leaf of a Yine, actively growing in a hot-house, 

 little effect will be produced upon it; but if it be held near the 

 under surface, the glass will soon be dimmed by the vapour ; 

 and in a short time longer, this will accumulate so as to form 

 drops. As the upper surface of a vine-leaf is nearly destitute of 

 stomata, whilst the lower is thickly covered with them, the dis- 

 proportion in these effects is at once explained, if the transpira- 

 tion really take place through these apertures. Similar experi- 

 ments on other plants lead to the same general result. Where 

 the stomata are equal in number on the two surfaces, both seem 

 to transpire alike ; and when neither possess stomata capable of 

 action, the transpiration is scarcely to be observed. Again, if a 

 plant, actively transpiring under the influence of sun-light, be 

 carried into a dark room, its transpiration is immediately and 

 almost entirely checked ; and if its stomata be then examined, 

 they will be found to have closed. Thus it appears almost un- 

 questionable, that the rapid loss of fluid from the whole vege- 

 table surface, but especially from the leaves, which constitutes a 

 most important part of the economy of the living plant, is regu- 

 lated by the number of stomata which each part contains, and 

 by the degree in which light acts upon them (. 94). 



258. Still, this kind of transpiration (which, to distinguish 

 it, may be termed Exhalation) is not altogether different in its 

 character, from the common evaporation first described. It will 

 be recollected, that the stomata open into large passages chan- 

 nelled out, as it were, in the fleshy substance of the leaf; and 

 that the walls of these are everywhere composed of a very soft 

 tissue, which is constantly kept moist by the crude sap conveyed 

 so plentifully into the leaves. If, therefore, the atmosphere be 

 admitted into these passages, a very large amount of evaporation 

 must take place from their sides, which resemble, in the want of 



