44 HOW CROPS FEED. 
These opposite changes obviously cannot take place at 
the same points, but must proceed in different organs or 
cells, or in different parts of the same cells. They further- 
more tend to counterbalance each other in their effects on 
the atmosphere surrounding the plant. The processes to 
which the absorption of oxygen and evolution of carbonic 
acid are necessary, appear to go on at all hours of the day 
and night, and to be independent of the solar light. The 
production of carbonic acid is then continually occurring ; 
but, under the influence of the sun’s direct rays, the oppo- 
site absorption of carbonic acid and evolution of oxygen 
proceed so much more rapidly, that when we experiment 
with the entire plant the first result is completely masked. 
In our experiments we can, in fact, only measure the pre- 
ponderance of the latter process over the former. In sun- 
light it may easily happen that the carbonic acid which 
exhales from one cell is instantly absorbed by another, and 
likewise the oxygen, which escapes from the latter, may 
be in part imbibed by the former. 
In total darkness it is believed that carbonic acid is not 
absorbed and decomposed by the plant, but only produced 
in, and exhaled from it. In no case has any evolution of 
oxygen been observed in the absence of light. 
When, instead of being exposed to the direct rays of 
the sun, only the diffused light of cloudy days or the soft- 
ened light of a dense forest acts upon them, plants may, ac- 
cording to circumstances, exhale either oxygen or carbonic 
acid in preponderating quantity. In his earlier investiga- 
tions, Corenwinder observed an exhalation of carbonic acid 
in diffused light in the cases of tobacco, sunflower, lupine, 
cabbage, and nettle. On the contrary, he found that let- 
tuce, the pea, violet, fuchsia, periwinkle, and others, evolv- 
ed oxygen under similar conditions. In one instance a 
bean exhaled neither gas. These differences are not pe- 
culiar to the plants just specified, but depend upon the in- 
tensity of the light and the stage of development in which 
