690 THE POPULAR SCIENCE MONTHLY. 



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 sub- 

 stance, and which effect the reduction of carbonic acid. 



In his experiments on fermentations, M. Pasteur distinguished the 

 minute microscopic beings into aerobes and anaerobes ; 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 aerobes of 

 a liquid with the microscope, we will find them collected around bub- 

 bles 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 illu- 

 minated 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 dif- 

 ferent 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 absorp- 

 tion-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 sea- 

 water 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. 



