136 SUPPLY OP MINERAL INGREDIENTS. 



199. The sources of what may be regarded as the essential 

 ingredients of the food of Plants, having now been fully con- 

 sidered, (the more fully on account of the practical importance 

 of the subject,) we shall inquire into the influence of certain 

 other materials, which particular kinds require for their health- 

 ful growth ; an increased supply of which tends greatly to their 

 productiveness, and the influence of which ought, therefore, to 

 be fully considered, in the tempering of soils, or the application 

 of manures. These materials consist of solid particles of various 

 kinds, which are contained in the earthy portion of the soil, and 

 which, being dissolved in the water, are taken up by the roots. 

 Of these, some are imbibed by almost all plants alike ; whilst 

 others are retained only by particular kinds, so that they are 

 either not taken up at all by plants of other kinds, or are 

 secreted again into the soil, not being deposited in their tissues. 



200. There is considerable variety in this respect, among the 

 different tribes of Plants ; each seeming to grow most advan- 

 tageously, when supplied with a certain kind of mineral matter, 

 but being capable of taking up other forms in place of it, if it 

 should be deficient. Thus the Rhododendron, like most othei 

 plants, deposits in its leaves and stem a large quantity of calca- 

 reous matter (lime combined with an acid, usually the carbonic,) 

 when freely supplied with it. When grown in a calcareous soil, 

 the ashes of its leaves have been found to contain 43^ parts in 

 ]00 of carbonate of lime, and only f of silex or flinty matter ; 

 the ashes of the stem of the same plant contained 39 of calca- 

 reous earth, and ^ of silex. But when grown in a soil in which 

 silex predominated, the leaves of a similar plant contained 16J 

 per cent, of earthy matter, and 2 parts of silex; whilst the stem 

 contained 29 parts of calcareous earth, and J9 of silex. 



201. It is curious to observe that, whilst calcareous matter 

 seems principally deposited in the softer tissues, silex is found 

 much more abundantly in the stem. This is especially the case 

 in the Grasses, nearly all of which require for their healthy 

 growth, a large proportion of silex; and this substance it is, 

 which, being deposited in the slender tissue of the hollow stem, 

 imparts to it a strength, that seems disproportionate to the 

 quantity of matter it contains. The silex may be melted by 



