in their Structure and Functions, 509 



elaborated, the growth of the plant is checked, and its fruit 

 falls off before it is fully ripened. 



The respiration of plants, although resembling that of ani- 

 mals, is attended with chemical results somewhat different. In 

 the respiration of animals, oxygen disappears, and carbonic acid 

 is produced: in that of plants, carbon is consumed, and oxygen 

 is thrown off. If an animal be immersed in carbonic acid gas, 

 it soon dies ; but if a plant be immersed in it, it thrives vigor- 

 ously, at least for a short time. Carbon is the proper food 

 of vegetables ; but if this material be allowed to accumulate 

 in the animal system, it puts a period to its existence. It 

 may be remarked, however, that although common air is not 

 so favourable to the growth of vegetables as an atmosphere 

 containing one twelfth of carbonic acid, yet, if the proportion 

 of this gas be greater than this, it impedes the process of 

 vegetation remarkably ; and it is found that an atmosphere, 

 one half or two thirds of whose volume consists of carbonic 

 acid, proves destructive of vegetable life in the course of a 

 few days. With the exception of these discrepancies, which, 

 when closely examined, are very trifling, there is a wonderful 

 analogy between the respiratory functions of vegetables and 

 those of animals. In so far as regards their ultimate effect, 

 it is particularly striking. The process to which the sap is 

 subjected in the leaves renders this fluid fit for the continu- 

 ance of all the vegetable functions, in the same manner as the 

 process performed in the lungs changes the blood into a state 

 proper to carry on all the vital operations of the animal 

 system. But, beyond this, the analogy can be considerably 

 extended. For example, we observe that plants and animals 

 both thrive in an atmosphere composed of oxygen and nitro- 

 gen ; and they both perish when deprived of it for any length 

 of time ; both, too, die in an atmosphere of pure oxygen ; by 

 means of their respiratory organs they both exhale and they 

 both absorb moisture ; both, likewise, imbibe nitrogen. Under 

 certain circumstances all plants absorb oxygen, like animals, 

 instead of exhaling it, and emit carbonic acid gas ; and there 

 are some parts of plants, such as the stem and branches, and 

 also the flowers, which take in oxygen at all times, and under 

 all circumstances, both when in the dark, and when exposed 

 to the full rays of the sun. Oxygen, then, which is the prin- 

 cipal agent concerned in effecting the proper change in the 

 blood, is likewise the agent which chiefly operates upon the 

 sap, and prepares it for being assimilated to the plant. So 

 that it appears that oxygen is as indispensable to the exist- 

 ence of vegetables as to that of animals. 



From what has been stated, it seems that the presence of 



