97 



in the bark, ministering to the growth and support of 

 the whole plant. 



Such is the close assimilaritj in the digestive and 

 circulatory processes of the two classes ; an assimi- 

 larity which obtains in all the other functions enjoyed 

 by them in common. In respiration, the air inhaled 

 through the mouth and nostrils proceeds immediately 

 to the lungs and acts upon the blood ; in plants, when 

 it is inhaled by their leaves, it operates instantane- 

 ously upon the sap. The changes that take place 

 have just been imperfectly noticed, and we have 

 no space to do more than add, that the oxygen 

 of the atmosphere is the gas essential to the exist- 

 ence of animals ; but it is its carbonic acid that is 

 quite as important to vegetables. They may be con- 

 sidered the vital airs of the two classes. If animals 

 are placed in a situation where they inhale pure 

 oxygen, their functions are highly and rapidly in- 

 creased ; but it is an exhilaration which would soon 

 terminate in exhaustion and death, if breathed by 

 them for any extended period. So plants will flourish 

 in an atmosphere containing 1-1 2th of carbonic acid, 

 but if it much exceeds this proportion, they are 

 rapidly destroyed. During sleep, animals respire less 

 carbonic acid than during their waking hours ; so 

 plants emit little or no oxygen during the night. 



After an animal has enjoyed the regular course of 

 its functions for a period varying in its duration, the 



H 



