67 
vessels, which correspond in their office to the lacteals and pul- 
monary arteries of animals, and are distributed in minute ram- 
ifications over the surface of the leaves. In its passage through 
this organ, which may be termed the lungs of a plant, the sap 
is fully exposed to the agency of light and air, experiences 
a change by which it is more completely adapted to the wants 
of the vegetable economy, and then descends through the 
inner layer of the bark in another system of tubes called the 
proper vessels, yielding in its course all the juices and prin- 
ciples peculiar to the plant. This leads to the consideration of the 
109 Chemical Changes in Plants. The chemical changes which 
take place during the circulation of the sap are in general of 
such a complicated nature, and so much under the control of 
the vital principle, as to elude the sagacity of the chemist. One 
part of the subject, however, namely, the reciprocal agency of 
the atmosphere and growing vegetables on each other, falls 
within the reach of chemical enquiry, and has accordingly been 
investigated by several philosophers. For the leading facts 
relative to what is called the respiration of plants, or the chem- 
ical changes which the leaves of growing vegetables produce on 
the atmosphere, we are indebted to Priestly and Igenhousz, the 
former of whom discovered that plants absorb carbonic acid 
from the air, under certain circumstances, and emit oxygen in 
return ; and the latter ascertained that this change occurs only 
during exposure to the direct rays of the sun. When a healthy 
plant, the roots of which are supplied with proper nourishment, 
is exposed to the direct solar beams in a given quantity of 
atmos|)heric air the carbonic acid after a certain interval is 
removed, and an equal volume of oxygen is substituted for 
it. If a fresh portion of carbonic acid is supplied, the same 
result will ensue. In like manner, Sennebier and Woodhouse 
observed that when the leaves of a plant are immersed in water, 
and exposed to the rays of the sun, oxygen gas is disengaged. 
That the evolution of oxygen in this experiment is accompa- 
nied with a proportional absorption of carbonic acid, is proved 
by employing water, deprived of carbonic acid by boiling, in 
which case little or no oxygen is procured. Such are the chan- 
ges induced by plants when exposed to sunshine; but in the 
dark an opposite effect ensues. Carbonic acid gas is not ab- 
sorbed under these circumstances, nor is oxygen gas evolved; 
134 ATJCTARIVM. 
