Still, with full-grown leaves in the open air, and with abundance of light, we but rarely find them exhaling even a very small amount of carbonic acid; though—as was shown by M. Corenwinder ten years ago—if we transfer them to a room lighted only by side-windows, and to which the direct rays of the sun do not penetrate, they generally, under these conditions, exhale carbonic acid in the daytime, the proportion varying according to the nature of the plants, the weakness of the diffused light, and the temperature. Of all the plants experimented on in this way by M. Corenwinder, the common nettle appeared to give out the largest amount of carbonic acid, when kept in a room.
These facts M. Corenwinder announced ten years ago, but he did not then venture an interpretation of them, as he does now.
M. Boussingault has shown that leaves placed in a bell-glass containing pure hydrogen mixed with a little carbonic acid, and kept in an ill-lighted room, give off traces of oxygen. This proves that even under the most unfavorable conditions the assimilation of carbon does not entirely cease: this act is completely suspended only in total darkness.
Now, as it is likewise certain that, under the same conditions as to light, leaves kept under a bell-glass filled with air give out carbonic acid, and inhale oxygen, it follows inevitably, from these two classes of observations, that the functions inherent in plants, respiration and assimilation of carbon, are simultaneous; the latter function is, however, reduced to such a degree that it no longer completely masks the effect of the former; in other words, the chlorophyll can no longer absorb all the carbonic acid produced by respiration.
Here, then, we have a fact analogous to that which we see in the earliest stage of vegetation, with this difference, that in the case of the bud the reducing action is insufficient, owing to the relative inferiority of its carbonaceous matters, while in the other case the insufficiency results from the reduction of their power.
These facts are undoubtedly very strong arguments for the theory of two simultaneous functions in leaves. M. Corenwinder still further confirms it with sundry observations, which are easily made.
Place perfectly green, full-grown maize-leaves under the bell-glass of the apparatus already described, and it will be seen that in the daytime they do not exhale the slightest trace of carbonic acid. If we could deprive these leaves of their green matter, which to all intents and purposes performs the assimilative function, we could doubtless discover the other function which this conceals, viz., respiration. Now, this very experiment Nature makes easy for us. As all are aware, there is a species of maize with striped leaves, which often bears white leaves without a trace of green. If we examine closely the striped leaves, we shall see that their white portions have absolutely no chlorophyll within. This is not the case with the leaves of a purple or of a black color; these, in addition to the coloring matter characteristic
