THEIR EARTHV CONSTITUENTS. 



187 



rally presented to them in solution, but not, howeA'er, in the same 

 proportions, nor in proportion to the relative solubility of these 

 several substances ; while, on the other hand, the same species in 

 different localities, and also each of its particular parts or organs, 

 contains, or tends to contain, the same mineral constituents in nearly 

 the same proportion. One base, however, is often substituted for 

 another, equivalent for equivalent, as magnesia for lime, soda for 

 potash. The roots, therefore, appear to have a certain power of 

 selection in respect to these mineral materials. Nor is it a valid 

 objection to this view, that they absorb poisons which destroy them. 

 These are either organic products, such as opium ; or else are cor- 

 rosive substances, such as sulphate of copper, which disorganize the 

 rootlets. For mutilated roots or stems absorb all dissolved materials 

 of the proper density that are presented to them, not only in much 

 larger quantity (so long as the cut is fresh) than do uninjured root- 

 lets, but almost indifferently, and in the same proportion that they 

 absorb the water they are dissolved in. 



338. In the ashes, only the salts which resist the action of heat, 

 such as the phosphates, sulphates, and hydrochlorates, are in the 

 state in which they existed in the plant itself. A great part of the 

 bases were combined with organic acids, formed in the plant, and 

 most largely with the oxalic (8G) : these compounds are by incinera- 

 tion, or by exposure to the air, principally converted into carbonates. 



339. It being indispensable to its well-being that a plant should 

 find in the soil such mineral matters as are necessary to its growth, 

 we perceive why various species will only flourish in particular soils 

 or situations ; -why plants which take up common salt, &c. are re- 

 stricted to the sea-shore and to the vicinity of salt-springs ; why 



herbage, compared, in several cases, with the root or grain. The water was 

 previously driven off by thorough drying. 



