54 Importance of Cleanliness to Plants. 



arranged that the leaf formed the division between the two 

 glasses — the upper surface was in the one glass, whilst the 

 under surface of the leaf was in the other glass. The quan- 

 tity of moisture given off was ascertained by placing in each 

 glass a weighed portion of dry chloride of calcium, which 

 being hygroscopic, or very greedy of moisture, would absorb 

 all the vapor as fast as the surface of the leaves gave it out. 



The result of these experiments is, that the lower surface 

 of the leaves gives off, from an equal quantity, three times 

 as much as the upper surface does ; sometimes the proportion 

 is as high as five to one ; and the ratio is quite independent 

 of the position of the leaf itself This exhalation of water 

 has some connection with the number and size of the stom- 

 ata, but is by no means wholly dependent on it, as there is 

 evidently a large quantity of water given off independently 

 of them. The evaporation is most abundant along the course 

 of the nerves, and in those parts of the epidermis, on which 

 there is the least quantity of oily matter. 



As by the experiments already mentioned it has been shown 

 that the transmission of water is greatly checked by the 

 accumulation of oil and resin in the epidermis, it might nat- 

 urally be expected that the perspiration or evaporation of 

 water would likewise be diminished by the same cause. 

 The experiments, made with a view of ascertaining the cor- 

 rectness of this supposition, consisted chiefly in exposing 

 leaves of different plants, the exact weight of which is 

 known, to the air, and by subsequently weighing them again, 

 observing the quantity of water which they had lost, and 

 consequently the rate at which they were able to give off 

 water. It was found in every case that cleansing, or wash- 

 ing the leaves with soap and water, increased their power of 

 evaporation to a very considerable extent. 



The emission of carbonic acid by the leaves of plants, 

 under certain conditions, was first ascertained by Saussure ; 

 but since his first experiments, made half a century ago, no 

 one has attempted to measure exactly the quantity of this 

 gas which leaves give out ; the careful and exact experiments 

 of M. Garreau on this point are, therefore, of great value. 



