THEIR EARTHY CONSTITUENTS. 191 



naturally presented to them in solution, but not, however, in the 

 same proportions, nor in any close proportion to the relative solu- 

 bility of these several substances : while, on the other hand, the 

 same species in different localities, under generally similar cir- 

 cumstances, and also each of its particular parts or organs, con- 

 tains, 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 

 corrosive substances, such as sulphate of copper, which disorgan- 

 ize the rootlets, and are then indiscriminately imbibed by mere 

 capillary attraction. For mutilated roots or stems absorb all dis- 

 solved 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 the uninjured rootlets, 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 (90, 91) : these compounds are by 

 incineration, or by subsequent exposure to the air, principally con- 

 verted into carbonates. 



339. It being indispensable that a plant should find in the soil 

 such mineral matters as are necessary to its growth or perfect de- 

 velopment, we are enabled to understand why various species will 

 only flourish in particular soils or situations ; why plants which 

 take up common salt, &c., are restricted to the sea-shore and 

 to the vicinity of salt-springs ; why numerous weeds which grow 

 chiefly around dwellings, and follow the footsteps of man and the 

 domestic animals, flourish only in a soil abounding in nitrates 

 (their ashes containing a notable quantity either of nitrate of pot- 

 ash or of lime) ; why the Vine requires alkaline manures, to re- 

 place the large amount of tartrate of potash which the grapes con- 

 tain ; and why Pines and Firs, the ashes of which contain very 

 little alkali, will thrive in the thinnest and most sterile soil, while 

 the Beech, Maple, Elm, &c., abounding with potash, are only 

 found in strong and fertile land. 



