nous intensity in the spectrum, but is more prolonged toward the more refrangible rays. Here, again, is a preferred intensity, beyond which chlorophyl is formed less easily. Experiment has also shown that the production of oxygen takes place only where there are grains of chlorophyl. If we resolve with a prism a ray of white light that we have passed through a solution of chlorophyl, we will remark a strong absorption-band in the red, and two others in the blue and violet. These represent the rays that have been absorbed by the green substance, and which effect the reduction of carbonic acid.
In his experiments on fermentations, M. Pasteur distinguished the minute microscopic beings into aërobes and anaërobes; the former respire and are developed in the presence of the oxygen of the air, while the others are killed by oxygen. If we examine the aërobes of a liquid with the microscope, we will find them collected around bubbles of an*, where they can find oxygen. If the liquid is deprived of air-bubbles and incloses a filament of a green alga, the bacteria will distribute themselves indifferently in the medium so long as it is illuminated with a very weak light, or, which is better, with a light that has been filtered through a solution of chlorophyl. In white light, the bacteria will be seen to precipitate themselves upon the grains of chlorophyl, to get the oxygen disengaged from them. They thus constitute a very delicate reagent. To witness the effect of the different colors, we let the microscopic spectrum fall upon a filament of conferva, or a transverse leaf-section. The bacteria will collect upon the plant in the red, at the point of maximum absorption, next in the blue, and the density of the population will nearly follow the absorption-curve of the coloring-matter.
I will not insist too long upon these facts in natural history; but I must add that great specific differences exist in the proper color of different plants, resulting from unequal absorption by their coloring matters. One example of this kind will be enough. The color of seawater varies according to the thickness through which we observe it, on account of the unequal absorption of the different rays; hence a marine plant will find itself in a condition more or less favorable, and be better or less equipped for the struggle for existence, according to the depth of the soil on which it rests. If we examine a bottom which the tide has just left, we will find blue sea-weeds on the edges of the deepest waters, farther down green sea-weeds, beyond these brown ones, and, lastly, red plants in the places which are least frequently uncovered. From the top of a bank we may thus perceive a series of concentric bands of different colors defining the limits within which each species, better fitted to the physical conditions, has overcome and eliminated the neighboring species. This is not a question of depth, because we find red sea-weeds at the water-level in sheltered places, the hollows of rocks, and deep caves, like the one at Capri, where the light comes in weakened.
