90 
to appear on both their surfaces when the receiver 
was exhausted, and again to disappear, when the 
equilibrium was restored ; from which he concluded 
that the air had been first drawn out of the leaf, and 
had afterwards re-entered it. in the oteajragranSj 
the upper surface of whose leaves is destitute of pores, 
the inferior surface alone was covered \v ith bubbles, 
which disappeared in the same manner when the at- 
mospheric equilibrium was restored *. It must 
however be observed, that the air, in these experi- 
ments, might be derived from the \\ ater, and its dis- 
appearance be caused by its re-attraction by that 
fluid, when the pressure of the atmosphere was re- 
stored. That air, however, does escape from leaves 
in vacua, is rendered certain by the experiments of 
M. Senebier, who found a leaf of sedum to sink in 
boiled water placed under an exhausted receiver, and 
again to swim after being placed for a few minutes 
in water charged with carbonic acid f- 
327. Besides entering vegetables through the me- 
dium of solution in water, air seems to gain admis- 
sion into them by other means. According to Dr 
Bell, Dr Hill first shewed the cuticle of vegetables 
to be an organised vascular substance, which, in 
trees and shrubs, has external openings, but not in 
herbaceous plants. When a portion of a tree is 
placed under -an exhausted receiver, air enters 
through the cuticle and issues from the wood, and 
We may therefore conclude that the proper entrance 
* Philosophical jragazine, v. xvi. p. 4. 11, 110. 
f Thysiol. Veget, v, iii. p. 24?. 
