~~ } PHYSIOLOGICAL BOTANY. PART II. 



vetches, tares. &c.), prepare or improve the soil for 

 those of the Grammes (various kinds of corn, &c.). If 

 the farmer by further experimental research should 

 ever be able to establish an extensive series of facts of 

 this description, he may expect to grow a succession 

 of crops with comparatively little manure and without 

 ever being obliged to let his land lie fallow. In the 

 present state of this inquiry it would be idle to 

 say much upoh the possible advantages which may 

 be expected from the confirmation of this theory; 

 but it must be evident to the most prejudiced admirer 

 of old customs, that we cannot expect to make any real 

 progress in the various branches of human knowledge, 

 agriculture among the rest, until we have obtained 

 clearer notions and a sounder theory respecting the 

 fundamental principles upon which the successful prac- 

 tice of any pursuit depends. 



(219-) Extraneous Matters. Besides those numer- 

 ous products directly secreted by plants, and which are the 

 immediate results of vegetable action, there are many 

 others which have either been accidentally absorbed 

 with the water that enters through the spongioles and 

 pores, or else have resulted from subsequent combin- 

 ations chemically effected between matters so introduced 

 and the undoubted products of vegetation. All matters 

 however which are accidentally introduced, form only 

 a very slight per eentage of the weight of the whole mass. 

 They compose the various earthy, saline, metallic, and 

 other ingredients found in the ashes of plants, after 

 combustion has dissipated all the purely vegetable pro- 

 ducts. They generally exist in the greatest quantity in 

 those plants, or parts of plants, where the process of ex- 

 halation has been carried on with the greatest rapidity. 

 Hence they abound more in the leaves than in other 

 parts, and more in the bark than in the wood. Herba- 

 ceous plants for similar reasons furnish more ashes 

 than trees. 



(220.) Earths. Lime is the earth which is most uni- 

 versally present in the ashes of plants, generally in tin 



