PERSPIRATION. 67 



with a surface of 561"6 square inches above the ground, had 

 perspired as follows : — 



OtTHOES AVOIKDDPOIS. 



In twelve hours of a very dry warm day . . .30, 



On stnotlier day ; 20, 



In a dry warm right without dew . . . -3, 

 In a night with some small dew . . . . ; 



and that when the dew was copious, or there was rain during 

 the night, the plant and pot were increased in weight two or 

 three ounces. Other persons hav^ instituted other experiments 

 of a similar nature, the result of all which is, that the insensible 

 perspiration of plants is very considerable. Hales 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 

 experiment shows that, whUe the sunflower was losing from 

 twenty or 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 perspi- 

 ration 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 abounding in moisture. 



Although perspiration thus appears to be principally excited 

 by the solar rays, and to be in a given plant in proportion to 

 their intensity, yet we are not authorised in concluding that 

 perspiration is not increased or diminished by the medium in 



* M. De Candolle distinguisheB between exhalaison, or perspiration, whioh is a 

 vital action, and deperdUion or evaporation, which is merely physical. But the 

 latter is too small in amoant to be worth taking into account for practical purposes. 



F 2 



