48 VITAL ACTIONS. 



says his sunflower perspired seventeen times more 

 than a man. There is, however, this important 

 peculiarity in vegetable perspiration, that it takes 

 place only or principally in sunlight. The last expe- 

 riment shows that, while the sunflower was losing 

 from twenty to thirty ounces of water daily during 

 the day, it lost only three ounces during the night 

 without dew, and that there was no loss whatever if 

 a slight dew were present. Here it is probable that 

 the small amount which was lost at night was parted 

 with by the sides of the garden pot, and that the 

 plant itself lost nothing ; for it is in evidence that the 

 perspiration of plants is in proportion to the quantity 

 of sunlight that strikes them, and that in darkness 

 they perspire little or not at all.* It is no doubt 

 true, that in a dry atmosphere plants will lose their 

 water day and night ; but it is equally certain that 

 under such circumstances they will lose very much 

 more by day than by night. They will, however, 

 lose much more by day in a dry atmosphere in a 

 given time, than they will in an atmosphere abound- 

 ing in moisture. 



to be thrown on the roots of the plant ; but neither this nor any 

 other plan that I eonld deTise, prevented the sap from flo-wing, 

 which it continued to do with so much force as to burst the bladder 

 in about forty-eight hours after the operation was performed; the 

 weather continuing the whole time warm and geniaL — (Sort. Trans., 

 V. 202.) 



* M. De Candolle distinguishes between exhalaison or perspira- 

 tion, which is a vital action, deperdition or evaporation, which is 

 merely physical. But the latter is too small in amount to be worth 

 taking into account for practical purposes. 



