Chap. II.] 
COURSE OF TIIE SAP. 
53 
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 tur- 
gescence, 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 moisture. 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 which is 
suspended in the atmosphere, it cannot be said 
to ascend more than to descend ; and the phy¬ 
siologist forgets that rain descends. But I do 
not think that M. Bonnet’s facts are more to be 
depended on than his reasoning. He states 
that, if leaves are floated on water on their upper 
surfaces, they will die as soon as if they were 
not put in water, but if they are floated on their 
Leaves supposed 
by some to ab¬ 
sorb the food of 
plants. 
