133 PROCESS OF NUTRITION. CHAP. III. 



the pot and plant weighed for fifteen successive 

 days in the months of July and August ; hence he 

 ascertained, not only the fact of transpiration by 

 the leaves, from a comparison of the supply and 

 waste ; but also the quantity of moisture transpired 

 in a given time, by subtracting from the total waste 

 the amount of evaporation from the pot. In a dry 

 and hot day it transpired the most, and in a damp 

 and wet day it transpired the least ; the mean rate 

 of transpiration being 1 Ib. 4oz. 17 times more in 

 proportion than that of the human body. In a hot 

 and dry night without dew it transpired 3oz. ; in 

 a dewy night it did not transpire at all ; and in a 

 rainy night, or night of much dew, its weight was 

 increased by 3oz. 



Hales suspected that the quantity transpired was 

 in proportion to the extent of the surface of the 

 leaves, which he regarded as the principal organs 

 of transpiration ; and ascertained also the relative 

 proportion of the capacity of the leaves for trans- 

 piration as compared to the capacity of the root 

 for absorption. The surface of the leaves and stem 

 of the plant which was the subject of experiment 

 was found to be equal to about 56 1 6 square inches ; 

 and the surface of the root of the same plant, or 

 rather, as I believe, of a plant of nearly the same 

 size, was found to be about 2280 square inches, the 

 latter being to the former in the proportion of two 

 to five ; from which it follows that the absorbing 

 power of the root is greater than the transpiring 



