91 
of air into plants is through these cuticular vessels *. 
Many experiments of a similar nature were made by 
Dr Hales, who concluded from them that air enter- 
ed plants not only \\ith the nutrient matter by the 
roots, but also through the bark and the leaves f. 
These experiments, however, as Dr Smith observes, 
prove only that the vegetable body is pervious to air 
when emptied of its sap, or when pressed by the 
weight of the atmosphere J ; but they afford no 
proof that the transmission of air through the vege- 
table structure is a natural and necessary function of 
vegetation. 
328. Some experiments of Dr Priestley furnish 
evidence of a different nature respecting the entrance 
of air into plants. He found that the willow plant 
(cpilobium hirsutum), while growing in water and 
confined in jars of hydrogen gas, greatly diminished 
the bulk of gas, and rendered it more pure ; so that 
it was -affected by nitrous gas, and exploded, when 
fired, like a mixture of common and inflammable air. 
The leaves continued green, and appeared to be al- 
ways loaded with air bubbles, which were continual- 
ly detached, and their place supplied by others* 
These bubbles, he supposed to be oxygen gas, and 
to have been the cause of the purification of the hy- 
drogenous atmosphere ; and he believed them to 
proceed from the plant itself, and not to be separated 
from the water in which it grew . 
* Manch. Mem. v. ii. p. 3 
t Veg. Statics, vol. ii. 
J Introduce, to Bot. p. 200. 
Obs. on Air, vol, v. p 9. 10. 
