CONSTITUENTS OF PLANTS. 93 



tity, or are entirely separated when not suited for 

 assimilation. 



Phosphate of magnesia in combination with am- 

 monia is an invariable constituent of the seeds of 

 all kinds of grasses. It is contained in the outer 

 horny husk, and is introduced into bread along with 

 the flour, and also into beer. The bran of flour 

 contains the greatest quantity of it. It is this salt 

 which forms large crystalline concretions, often 

 amounting to several pounds in weight, in the 

 caecum of horses belonging to millers ; and when 

 ammonia is mixed with beer, the same salt sepa- 

 rates as a white precipitate. 



Most plants, perhaps all of them, contain organic 

 acids of very different composition and properties, 

 all of which are in combination with bases, such as 

 potash, soda, lime or magnesia. These bases evi- 

 dently regulate the formation of the acids, for the 

 diminution of the one is followed by a decrease 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 ; and the acids, 

 under the same circumstances, are found to vary 

 in a similar manner. Such constituents exist in 

 small quantity in those parts of a plant in which 

 the process of assimilation is most active, as in the 

 mass of woody fibre ; and their quantity is greater 

 in those organs, whose office it is to prepare sub- 

 stances conveyed to them for assimilation by other 

 parts. The leaves contain more inorganic matters 



