PLANTS — THEIR NATURE AND PHENOMENA. 41 



circumstance that plants may be said to breathe only 

 in the light, and that to the action of light they are 

 indebted in a great measure for their colors. By 

 " breathing " is meant that absorption of carbonic acid 

 gas and that exhalation of oxygen, the recent satisfactory 

 demonstration of which has led to the formation of the 

 Aquarium. This vital operation, it is insisted by some 

 observers, is partially, if not wholly, suspended at night, 

 and proceeds rapidly only when the plants are exposed to 

 the sun, and hence their enjoyment in such an exposure. 

 Hence, also, the importance of sunlight, and the disadvan- 

 tage of gloom, in successfully encouraging the vegetable 

 life inclosed in an Aquarium. 



As regards color, even Aristotle observed that plants 

 were colored by the sun. Senebrer found that when they 

 were put in a dark place their green leaves first became 



- > 



yellow on the surface and then white, while young plants 

 which had grown up in the dark, when brought gradually 

 to the light, exchanged their white color for a yellow, 

 which, after a time, darkened, exhibited green spots, and 

 finally assumed that general complexion. Van Mons 

 and Yasali assert that the light of a lamp and even of the 

 moon, exercises a coloring influence over plants as well as 

 that of the sun. 



How the original constituents of plants are absorbed by 

 light and heat ; how they are so united by the vegeta- 

 ble organization as to produce the various substances of 

 which plants are composed — the gum, starch, sugar, 



