105 



ON THE AQUEOUS EMANATION, OR EXHALATION OF 

 VASCULAR VEGETABLES.* 



(Continued from page 91.) 



The state of the atmosphere appears also to exercise an influence on this 

 phenomenon ; the facts altogether tend to prove that plants exhale more in a 

 dry than in a humid air, and probably more in a rarified than in a condensed 

 one ; but I am not aware of any proofs at all precise on this subject ; nor do I 

 know more especially in what exact degree we can distinguish from these facts, 

 what belongs to the deperdition or to the exhalation. 



Independently of exterior causes ; the age of the parts of vegetables destined 

 for exhalation, influences considerably the intensity of this function : thus, in an 

 equal light and temperature, leaves exhale more in spring than in summer, and 

 more in summer than in autumn. Guettard has observed that in winter the 

 exhalation of evergreen trees is extremely feeble : according to him a laurel 

 exhales during two days of summer, as much as during two months in winter. 



Senebier has made a great number of experiments, in order to determine the 

 relation which exists between the quantity of water absorbed and the quantity 

 exhaled in a given time. To effect this, he steeped a branch in water, the weight 

 of which he knew ; he introduced the extremity of the branch into a spacious 

 receiver ; weighed the water which was found in the receiver at the termination 

 of some hours, and compared its quantity with that which was deficient in the 

 vessel from which the branch absorbed its nourishment. These experiments left 

 always something to desire, because of the water which remained in suspension in 

 the receiver, or which was deposited on the leaves ; they offered also some 

 variations with respect to the surfaces of the branch, and the leaves, and in the 

 intensity of the light, &c. The result of a number of trials tends to prove, that 

 the water absorbed is, to the water exhaled, as three is to two, or in other terms, 

 that one third of the water absorbed remains in the plant, and that the two other 

 thirds exhale in the air. 



The same physiologist has also sought to compare the nature of the water 

 exhaled, with that of the water absorbed by the plants. He steeped some 

 branches in infusion of cochineal ; the colour of the injection penetrated even to 

 the summit of the plant, but the water exhaled was perfectly transparent. 

 Having steeped some branches in water, mixed with a little sulphuric or muriatic 

 acid, the water exhaled has been sometimes perfectly pure ; sometimes it has 

 presented some traces of the acids ; which may be attributed to the disorganization 

 which resulted from its progress. Hales and Duhamel have remarked that the 

 liquids exhaled by the vine, the apple, the peach, rhubarb, parsnip, and even by 

 rue and cabbage, do not offer any difference of taste, and only differ from common 

 water, in having a slight colour, which might have been communicated while in 



p 



