74 ASSIMILATION OF 



assimilate oxygen^ or rather oxygen enters 

 into combination with the salts they contain, 

 and gives them their acid taste ; such plants 

 are the common sorl^el and the vine, the one 

 yielding oxalic, the other tartaric acid ; but in 

 these combinations it must be distinctly un- 

 derstood, that although the acid seems to be 

 and is a necessary consequence of the absence 

 of light, and thereby a constituent of the plant, 

 still that it never exists of itself, but always in 

 connexion with some salt termed its base — 

 and it is desirable farther to notice that 

 whether that base be soda, potash, magnesia 

 or lime, that provided a base exists (which 

 ^hereafter will be noticed as a necessary con- 

 stituent of the vegetable economy) the oxygen 

 will unite with it to form its peculiar acid. 



From this it may be inferred, that the pre- 

 sence of the base is always necessary for the 

 assimilation of the oxygen or acidifying 

 principle, that without such base no oxygen 

 could be assimilated ; but the base may and 

 constantly does exist, where the plant is fully 

 developed without any such assimilation of 

 oxygen. 



That these bases are the causes of the 

 acidity of plants, may be demonstrated from 

 the fact that the acid is always in proportion 

 to its base, a definite quantity of the one com- 

 bining with a definite proportion of the other ; 

 thus, in the grape for example, the quantity 

 of potash contained in its juice is less when it 

 is ripe than when unripe, so likewise the pota- 

 toe plant contains more potash before blossom- 



