44 



HOW cnoi'S FRED. 



These opposite clianges obviously cannot take place at 

 the same points, but must proceetl in different organs or 

 cells, or in different parts of the same cells. They further- 

 more tend to counterbalance each otlier in their effects on 

 the atmospliere 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 tlie solar liglit. 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 

 absoi-bed 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 



