108 THE NUTRITIVE FUNCTIONS. 



an unique and special organ developed in one part of tte 

 organism. 



Another source of demand for oxygen, common to both 

 animals and plants, arises out of the changes which are 

 always going on in them, and the necessity for the re- 

 moval of all waste matter from the system. Plants are 

 without a nervo-muscular system, and are therefore ex- 

 empt from that interstitial waste and destruction which 

 takes place in animals, and which is the result of nervous 

 and muscular action. Although they have no organs de- 

 veloped expressly for the removal of effete matter, we never- 

 theless see them spontaneously throwing off those parts of 

 their fabric which are no longer of any service to them. 

 Thus the cotyledons perish as soon as the first pair of 

 atmospheric leaves are fully developed; the bud-scales, 

 which protect the young shoots of trees through winter, are 

 thrown off as soon as spring and warm weather comes. 

 The blossoms drop from the trees as soon as they have 

 contributed to the formation of their fruits ; and the trees 

 themselves shed those leaves which have contributed to 

 their nourishment through the spring'and summer months, 

 on the approach of winter, because their vitality is ex- 

 hausted, and they are useless to the plant. In all these 

 instances, oxygen is absorbed and carbonic acid evolved. 

 Thus, in both animal and plant, oxygen is the principal 

 agent employed in the removal of the waste matter. Is it 

 objected that waste matter is thrown off en masse from the 

 plant, and not in a gaseous condition, as in the animal ? 

 We reply that the evolution of waste matter in the gaseous 

 form, commences with the ordinary process which produces 

 the decay of leaves ; their decomposition after their separa- 

 tion is only a continuation of the same process. All the 

 waste matter thrown out of the animal system is not evolved 



