INORGANIC MATTER. 279 



the pure state, but combined either with organic or 

 inorganic acids, or else with chlorine. Albumen, 

 gluten, caseine, and fibrin, are always found to contain 

 a small quantity of certain phosphates, particularly 

 those of lime and magnesia; hence these substances 

 appear to be essential to the formation of those prin- 

 ciples in plants. 



733. It is not known whether plants have any 

 power of selection by their roots ; that is to say, 

 whether they are able to absorb from the soil only 

 those substances which they require, or whether they 

 absorb all the soluble matters present in it. To a 

 certain extent, they seem to have some power of 

 selection, but at the same time they are very fre- 

 quently injured by the absorption of poisonous 

 matters from the soil. 



734. When the roots of a healthy young plant are 

 plunged into a vessel containing water, in which two 

 different saline substances are dissolved in equal pro- 

 portions, the plant will not take up both salts in 

 similar quantity. If, after continuing this experi- 

 ment some days, we then evaporate the remainder of 

 the solution, we shall find that the plant has taken 

 up the two salts in very different proportions; per- 

 haps half of the one salt has been absorbed, but only 

 one-third of the other. 



735. The quantity of inorganic matter found in 

 plants is various at different periods of their growth. 

 In general, young- plants contain a larger proportion 

 than old ones; and as seeds contain a supply of all 



