hnportance of Cleanliness to Plants. 55 



He employed the same sort of apparatus for this purpose, 

 which has ah-eady been mentioned, as being used in estimat- 

 ing the evaporation of moisture from the upper and lower 

 surfaces of leaves ; namely, two circular glass vessels, which 

 being applied respectively to the upper and under sides of 

 the leaf, enclosed a circular disk of the leaf between them, 

 in a way very convenient for the purposes of the observer. 

 In these experiments, however, in place of using dried chlo- 

 ride of calcium, a portion of lime-water was used, and the 

 chalk formed in it by the absorption of carbonic acid subse- 

 quently weighed. The general results of these experiments 

 are, that in the shade or diffused daylight, no carbonic acid 

 is evolved by the leaves ; that in the night it is given out by 

 both surfaces, but in greater quantity from the lower than 

 from the upper surface ; and that when exposed to the influ- 

 ence of a very bright and hot sunshine, so that the rate of 

 evaporation is very rapid, carbonic acid is likewise given off. 

 In the latter case, the quantity of gas thus evolved is com- 

 paratively small, and by far the larger portion of it is given' 

 off by the lower surface of the leaves. It also appears that 

 the proportion of carbonic acid given off by different leaves, 

 bears a closer relation to the number and size of the stomata 

 than the proportion of water which the leaves are able to 

 evaporate does. 



All these experiments of M. Garreau teach us this fact, 

 that under ordinary circumstances the growth of a plant 

 causes the formation and deposition of certain substances, 

 which in time fill up its pores, check perspiration, and con- 

 sequently interfere with the nourishment and further growth 

 of the plant. On the one hand, there can be little doubt 

 that in very hot weather these matters must sometimes be 

 useful in checking extreme perspii'ation, and in diminishing 

 for the time the powers of the plant to absorb too much food 

 from the air, or to part with water and carbonic acid too 

 rapidly. On the other hand, the effect of rain must be to 

 wash away a portion of these deposits, and so to favor the 

 perspiration and consequent growth of the plant. Lastly, as 

 the more heat a plant is exposed to, the more it perspires, 



