29 



transpiration or giving off of water from 

 tiie leaf, when exposed to drought, is very 

 rapid, and the communication from the 

 root to the leaf very rapid and constant ; 

 since on felling trees of thirty or forty 

 feet high, while they were shooting in the 

 summer, I have observed the shoots lose 

 their turgescence, that is, droop, in the 

 course of a few minutes from the time that 

 the stem is divided from the root. The 

 accurate and admirable Hales found that a 

 sunflower, in dry, hot, weather, gave off two 

 pounds and a half, that is, two pints and a 

 half of water in twelve hours. At night, 

 and in moist weather, the quantity was much 

 less. Senebier supposes that plants give off 

 two thirds of the water which they absorb. 



Many physiologists imagine that the 

 great use of the leaf is to absorb mois- 

 ture. M. Bonnet tells us that the leaf 

 is formed to absorb, chiefly from the 

 lower part, because dew ascends. But as 

 dew is a condensation of moisture sus- 



