89 



ON THE AQUEOUS EMANATION, OR EXHALATION OF 

 VASCULAR VEGETABLES.* 



Every one is aware that fresh vegetables exposed to the air yield to it a 

 notable portion of their humidity. Mariotte appears to have been the first who 

 endeavoured to account for this phenomenon. He placed a leafy branch in a 

 closed vessel, and, in two hours, he collected two spoonsful of water deposited on 

 the sides of the vessel. Hales has measured with greater exactness the tran- 

 spiration of the Helianthus annuus, or annual sunflower. He planted one, three 

 feet high, under a vase, the orifice of which was closed by a metal plate pierced 

 with two holes, through one of which the stalk passed, and through the other the 

 plant was watered. The pot and plant 'were weighed morning and evening, 

 during fifteen days. The result of this observation was, that the plant lost by 

 evaporation a quantity equal to twenty ounces daily. A Cabbage of moderate 

 size, lost, under the same circumstances, nineteen ounces. Plenk admits that a 

 stalk of Maize exhales seven ounces of water a day ; a Cabbage 23 oz. ; a 

 Heliotrope 24 oz. &c. ; and Guettard calculates that a branch of the Dog-wood, 

 weighing five drachms and a half, plunged at its base into water, exhales in 

 twenty-four hours a quantity of water equal to one ounce, three drachms, and 

 three quarters. From very complicated calculations, Hales has proved that this 

 evaporation of the Sunflower, or the Cabbage, is, in an equal measurement of 

 surface, seventeen times greater than that which the human body experiences 

 from insensible perspiration. Were it possible that this calculation could be 

 contradicted, there can be no doubt but that vegetables evaporate a great quantity 

 of water ; but upon a nearer examination of the facts, it may be clearly perceived 

 that there are various phenomena connected with them which must be necessarily 

 distinguished. 



1st. If fleshy fruits are placed in the open air, such as apples or grapes, or 

 tubercules, such as potatoes, it is perceptible, at the termination of a longer or 

 shorter time, that they have lost something of their weight, and deposited a little 

 humidity on the sides of the cavity which encloses them ; but this effect is very 

 slow, and may continue many months before the loss (deperdition) becomes very 

 sensible. It takes place in a very gradual manner, and is more active from heat 

 than from any other cause. This slow operation, which tends to deprive 

 gradually all the cellular parts of vegetables of water, and which works through 

 their tissue without visible pores, is what I call the insensible deperdition. It is 

 perhaps a phenomenon independent of life, or which at least does not appear to 

 act an important part in the vital phenomena. This deperdition is explained by 

 the permeability of the tissue, and the tendency of water to evaporate when it is 



* From Decandolle's " Physiologie V^getale." 

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