CHAP. II. FUNCTION OP NUTRITION'. 177 



growth. Besides the three elementary substances, oxygen, 

 hydrogen, and carbon, essential to the composition of all 

 organized matter, whether animal or vegetable, there 

 are other elements to be met with in slight proportion 

 in some vegetables. Azote is an element more espe- 

 cially essential to the formation of animal substances ; 

 but it seems probable, that it is also a fundamental 

 ingredient in certain vegetable compounds, in which 

 it exists in considerable abundance. As this gas 

 also forms a component part of the atmosphere, plants 

 may as readily be furnished with it, as with either 

 of the other three ingredients universally essential to 

 their nature. Whether the other elements occasion- 

 ally found in plants ever constitute an essential part 

 of their structure, is uncertain. Several of them exist 

 under combinations, such as common salt for example, 

 which appear to be useful to some plants ; possibly as a 

 stimulus necessary for the preservation of their health, 

 since they languish and die when wholly removed from 

 their influence. In all cases, however, whatever be the 

 nature of the various saline, earthy, metallic, and other 

 compounds found in small quantities in the ashes of 

 plants, they must have been introduced in a state of so- 

 lution through the spongioles. 



(l6l.) Cause of Absorption. This absorption by 

 the spongioles continues during the lifetime of the plant, 

 and it becomes a question for the physiologist to deter- 

 mine, upon what cause the action depends ; whether it 

 may be ascribed, for instance, to the known hygroscopic 

 powers of the cellular tissue, or whether it be wholly or 

 partly due to a vital action. This question can scarcely 

 be considered as satisfactorily settled. If we suppose 

 the plant capable of removing the imbibed fluid as fast 

 as it is absorbed by the spongioles, then we may imagine 

 the possibility of a supply being kept up by the mere 

 hygroscopic property of the tissue, much in the same 

 way as the capillary action of the wick in a candle 

 maintains a constant supply of wax to the flame by 



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