PASSAGE OF MATTERS INTO THE ROOTS. 71 



in mass ; for as it is certain that a plant will not grow 

 if no food is offered to it, so it is equally certain that it 

 will absorb no nutriment if the external conditions are 

 not favourable to growth. Yet the view given above 

 would force us to conclusions which are not founded in 

 nature ; such as, for instance, (1) that there is actually 

 around the roots a solution containing all the ash con- 

 stituents of the plants ; and (2) that the roots of all 

 plants have a similar structure, and their sap is of the 

 same nature. 



With regard to the roots, the most common observa- 

 tions appear to show that they possess the power of 

 selecting the proper mineral nutriment for the plant 

 from the matters presented to them. All plants do not 

 thrive equally well in 'the same soil ; one kind succeeds 

 best in soft water, another in hard water, or water 

 abounding in lime ; another only on marshy ground ; 

 many on fields rich in carbon and carbonic acid, such 

 as the turf-plants ; others again on soil containing large 

 quantities of alkaline earths. Many mosses and lichens 

 will grow only on stones, the surfaces of which they 

 sensibly change ; others, like Koleria, possess the faculty 

 of extracting from silicious sandstone potash and the 

 phosphoric acid so sparingly present in it. Roots of 

 grass attack the felspar rocks, accelerating their disin- 

 tegration. Rapes and turnips, sanfoin and lucerne, as 

 also the oak and beech, receive the chief part of their 

 food from the subsoil poor in humus ; while the cereal 

 and tuberous plants thrive best in the arable surface 

 soil, and in soil abounding in humus. The roots of 

 many parasitic plants are absolutely unable to extract 

 froni the soil their necessary food ; but this is prepared 

 for them by the roots of the- plants on which they grow. 

 Others again, as certain fungi; grow only on vegetable 

 and animal remains, whose azotised and unazotised sub- 

 stances they use for their own construction. 



These facts, accepted in their true significance, seem 

 sufficient to remove all doubt respecting the different 

 action of the roots of plants upon the soil. We know 

 that common Lycopodium (club-moss) and ferns absorb 



