VEGETABLE RESPIRATION. 81 
_ by their roots, we should be enabled to form a rational 
_ theory of vegetable respiration. It is possible that this may 
_be the case, and that, in some of the experiments which have 
been made, there may have been a deficiency of carbon in 
the sap of the plant, which might lead to the absorption of 
carbon from the air, while this might not be the casé in 
others. 
278. The late Professor Burnett has proposed a new view 
_of the subject. The very anomalous condition of the plant, 
evolving at one time pure oxygen, and at another carbonic 
acid, or, in other words, performing at certain, and often ir- 
regular periods, functions so diametrically opposed to each 
other, having led to more minute researches on this question, 
it was observed that a small portion of carbonic acid was 
disengaged by leaves both day and night. He therefore 
conceives that the commonly so-called function of respira- 
tion, includes another office, that of digestion. The respi- 
ratory process, analogous to the same in animals, is always 
going on, during the night as well as during the day, and 
accordingly will always be attended with the production of 
carbonic acid gas. So far then the theory of the evolution 
of the carbonic acid gas is sufficiently precise. : 
But how is the production of the oxygen to be explained ? 
This he conceives, depends in part on the decomposition of 
the water, but also and principally on the decomposition of 
carbonic acid, whether taken up by the roots along with wa- 
_ ter, or simply applied to the leaf in the form of gas. 
The same difficulties, however, present themselves to this 
_ theory, as we already adverted to in alluding td the common- 
- ly received theory of vegetable respiration; for the diges- 
tive process, depending on the direct action of the solar 
rays, must necessarily be a very precarious event in the 
_ plants of a country such as Great Britain. The theory, 
_ however, has the merit of getting rid of the very singular — 
maly of oxygen being expired at one time, and carbonic 
acid gas at another, an anomaly which appears the more 
_ Singular, if we were to conjecture such a condition in the 
