298 PRINCIPLES OF BOTANY, ETC. 
and soonest from drought, and I have seen its leaves 
on this account often drop off very early. 
That plants imbibe the moisture of the atmo- 
sphere and rain, is proved by a very simple ex- 
periment made by Bonnet. He placed a leaf of the 
white mulberry-tree, Morus a/ba, with its upper sur- 
face upon water, and it remained six days fresh and 
green. Another leaf of the same tree, laid with its 
under surface upon water, remained six months fresh. 
This I think shews, that plants rapidly imbibe by 
the under surface of their leaves the dew of the 
night and the moisture of the atmosphere. 
This office is performed by hairs or pubescent 
points, which are on the surfaces of plants. The 
under surface is therefore never quite without them, 
and in many plants this hair is a hollow tube con- 
structed for that purpose. When leaves have no such 
_ pubescence, small apertures are found in their place. 
§ 280. 
Carbon and hydrogen are the stibstances of which 
the food of plants chiefly consists, and they theretore 
form the two chief constituent parts of vegetables. 
By various organs and glandular bodies they are, 
according to the power of assimilation, combined 
with other substances, and changed in form and ap- 
pearance, so that different parts have likewise a quite 
different smell or taste from others. The roots, for 
instance, of Mimosa ni/otica, smell like gum asa- 
foetida; the sap of the stem is of a very sour, 
astringent taste, the well known gum arabic exudes 
from it, and the flowers possess a very sweet smell. 
, In 
