THE ROOT 89 



the root. Thus, for instance, substances that enter 

 into the composition of ash cannot exist under normal 

 conditions in a gaseous state ; it is for this reason that 

 after complete combustion they remain in the ash and 

 do not volatilise with the rest of the vegetable matter. 

 Consequently, all that is found in ash must be looked 

 for in the soil ; and therefore we conclude that such 

 substances enter the plant through its root. As to the 

 other elements, however, they may be in the soil as 

 well as in the air. We remain as yet uncertain with 

 regard to them : we cannot tell, until we get clear 

 evidence from an experiment, whether they are obtained 

 from the soil or from the air, and so whether they pene- 

 trate into the plant through its root or through its 

 leaves. 



Let us turn our attention first of all to the root ; 

 let us see what it absorbs from the soil, how it absorbs it, 

 and why it absorbs precisely that which is indispensable 

 to the plant. 



But before we begin the study of the root and its 

 functions, we must become acquainted with the environ- 

 ment in which it manifests its activity, and cast a 

 passing glance at the soil itself and its composition. 



Every kind of soil which is covered with vegetation 

 has two classes of components, which are sharply 

 differentiated ; these are the combustible and non- 

 combustible, the organic and inorganic substances. 

 The organic part, which is merely the residue of decayed 

 plants, gives the soil its black colour. This black 

 matter, which is burnt away when soil is calcined, is 

 called humus. Even the blackest soils, however, the 

 real moulds, contain comparatively small quantities of 

 humus, rarely more than 10 per cent. Soil which has been 

 thoroughly calcined and therefore deprived of decaying 

 organic matter no longer appears black, but yellow or 

 red. The mineral parts of the soil which remain after 

 calcining, and are in bulk its main constituents, can be 



