246 Vegetable Physiology. 



Various experiments have been made at different time 

 ascertain the quantity of fluid thus exhaled by plants, and tn° 

 results of some of them are very interesting. There is no g 

 difficulty in ascertaining the amount upon a small scale • f v 

 a plant be supplied with a known weight of water, and in 

 weight it has gained during a certain time be deducted fr 

 this, — allowance being also made for the evaporation fromth 

 surface of the water in which its roots are immersed, the quan- 

 tity of which may be easily estimated, — the difference must b 

 the proportion exhaled. This differs much in different plant 

 according to the rapidity of their growth. It has been ascer- 

 tained that the young leaves and shoots of the Wild Cornel 

 exhale twice their own weight of water daily. A common 

 sized cabbage was ascertained by Hales, (one of the best 

 experimenters upon this interesting subject) to exhale from 

 fifteen to twenty-five ounces daily, according to the light and 

 warmth to which it was exposed. This is more in proportion 

 to the surface, than is given off" by the skin of man in the same 

 time. The transpiration of a Sun-flower in full growth 

 during fifteen days and nights, was carefully observed by 

 Hales. This plant was three and a half feet in height, three 

 pounds in weight, and the surface of its leaves was estimated 

 at five thousand six hundred and sixteen square inches, or 

 about two and a half times that of the human body. The 

 average transpiration during the whole period was found to be 

 twenty ounces per day; but in one warm dry day it was us 

 much as thirty ounces. During a dry warm night, it lost three 

 ounces — probably by simple evaporation j when the dew was 

 sensible, though small, it neither lost nor gained ; and by 

 heavy dew, it gained two or three ounces. When this amount 

 is compared with that perspired by man, it may be shown 

 that, if their surfaces were equal, the man would perspire fifty, 

 and the plant fifteen ; but that, for equal weights, the plant 

 exhales seventeen, while the man perspires one. Experiments 

 upon single leaves, when not too long separated from the plant 

 so as to lose their vitality, yield fully as striking results. Thus 

 a leaf of the Sun-flower, weighing thirty-one and a half grains, 

 absorbed in four hours, by its petiole immersed in water, 

 twenty-five grains of that fluid $ the leaf had increased in 



