278 Mr. Knight’s Account of some Experiments 
by winds or other agents, capillary attraction, and probably 
something in the conformation of the vessels themselves, which 
renders them better calculated to carry fluids in one direction 
than in another. I shall begin with a few observations on the 
leaf, from which all the descending fluids in the tree appear to 
be derived. This organ has much engaged the attention of 
naturalists, particularly of M. Bonnet : but their experiments 
have chiefly been made on leaves severed from the tree ; and, 
therefore, whatever conclusions have been drawn, stand on very 
questionable ground. The efforts which plants always make to 
turn the upper surfaces of their leaves to the light, have with 
reason induced naturalists to conclude, that each surface has a 
totally distinct office; and the following experiments tend strongly 
to support that conclusion. 
I placed a small piece of plate glass under a large vine leaf, 
with its surface nearly parallel with that of the leaf; and, as 
soon as the glass had acquired the temperature of the house in 
which the vine grew, I brought the under surface of the leaf 
into contact with it, by means of a silk thread and a small wire, 
adapted to its form and size. Having retained the leaf in this 
position one minute, I removed it, and found the surface of the 
glass covered with a strong dew, which had evidently exhaled 
from the leaf. I again brought the leaf into contact with the 
glass, and, at the end of half an hour, found so much water 
discharged from the leaf, that it ran off the glass when held 
obliquely. I then inverted the position of the leaf, and placed 
its upper surface in contact with the glass : not the slightest 
portion of moisture now appeared, though the leaf was exposed 
to the full influence of the meridian sun. These experiments 
were repeated on many different leaves ; and the result was, in 
