ACTION UPON THE ENVIRONMENT. 151 



is the origin of tlie carbon dioxide given oft". It will appear 

 beyond that precisely the same action takes place in the respi- 

 ration of animals, and that all living things breathe or respire in 

 essentially the same way. 



It was for a long time believed that a leading difference between plants 

 and animals lay in the fact that the former give off oxygen and absorb 

 carbon dioxide, while the latter give off carbon dioxide and absorb oxygen. 

 But it is now known that both give off carbon dioxide and both require 

 oxygen, and that only the chlorophyll-bearing parts of green plants are en- 

 dowed with the special function of decomposing carbon dioxide and water 

 and manufacturing starch — as a result of which they do (but in tlie light 

 only) give off oxygen as a kind of incidental- or by-product. 



Interaction of the Fern and its Environment. 



The actions of the environment upon the fern have already 

 been sufticiently dwelt upon (p. 144). It still remains, however, 

 to consider the actions of the fern upon the environment. 

 These are partly physical, but mainly chemical. By pushing 

 its fronds into the air and slowly thrusting its rhizome, roots, and 

 branches through the soil, the atmosphere and the earth are alike 

 displaced. But it is by its chemical activity that it most pro- 

 foundly affects its environment. Absorbing from the latter 

 water, salts, carbon dioxide, and other simple substances, as well 

 as sunlight, it produces with them a remarkable metamorphosis. 

 It manufactures from them as raw materials organic matter in 

 the shape of starch, fats, and even proteids. These it gives 

 back to the environment in some measure during life, and sur- 

 renders wholly after sudden death. But the most striking fact 

 is that the fern is on the whole constnictive and capable of pro- 

 ducing and accumulating compounds rich in energy. In this 

 respect it is unlike the earthworm (p. lO-t) and is typical of green 

 plants in general. Thus, wdiile animals are destroyers of ener- 

 gized compounds, green plants are producers of them. Ani- 

 mals, therefore, in the long run are absolutely dependent on 

 plants ; and animals and colorless plants alike upon green plants. 

 But it must never be forgotten that most plants are enabled to 

 manufacture organic from inorganic matter by virtue of the 

 chlorophyll which they contain. Without this they are j^ower- 

 less in this respect. (See, however, p. 11>T). 



