5 i4 SCIENCE PROGRESS 



plant has the power of forming complex substances from simple 

 ones. In animals the food has to be reduced from the solid and 

 complex to the soluble condition before it can be assimilated. 

 On the whole, however, the substances absorbed by the entire 

 plant kingdom are the same as those absorbed by animals con- 

 sidered as a whole. Like the animal, also, a plant feeds partly 

 on nitrogenous substances and constructs proteids. If, how- 

 ever, plants absorb the same elementary substances as animals 

 they absorb them in different forms and combinations. Plants 

 are fed largely by means of the carbon dioxide existing in the 

 atmosphere, which they accumulate and which, though given 

 off by animals, cannot be breathed by animals, except in minute 

 quantities, without producing suffocation owing to the effect it 

 has of diluting down and excluding the necessary oxygen. For 

 although animals can take this gas into their stomachs, they 

 do not feed upon it directly. Notwithstanding the fact that plants 

 do, like animals, absorb and return oxygen, and exhale carbon 

 dioxide (probably what is not needed for the formation of starch 

 and other substances) it is known that the inhalation of an excess 

 of C0 2 does not kill them. 



These differences of functions in this important particular 

 constitute a gap between the two kingdoms. What is rejected 

 in the process of expiration by the one is received as an 

 alimentary necessity by the other in the form in which it is 

 rejected, and although in animals carbon is also an alimentary 

 necessity it is received in the food of animals in combination 

 with other substances and is not directly assimilated. Further, 

 although plants take in both carbon and water and reject what 

 they do not want of these substances, they absorb the carbon in 

 the form of gas and eject the surplus water in the form of 

 vapour. Animals, on the other hand, take in carbon in their 

 food and reject what they do not need chiefly in the form of gas, 

 eliminating surplus water mostly in a liquid state and nitrogen 

 in combination. Neither the plant nor the animal, however, can 

 live for any length of time without oxygen. Both need this 

 substance for the purpose of combustion and both eject it. It is, 

 as we know, by reason of the differences in the manner of nutri- 

 tion that the balance is maintained whereby life is possible on 

 earth, and they are of the highest significance in a comparison of 

 animal and plant life. 



The methods of reproduction are not all similar in the two 



