60 THE PLANT. 



such as, for instance, (1) that there is actually around the 

 roots a solution containing all the ash constituents 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 

 hme ; 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 alkahne 

 earths. Many mosses and Hchens will grow only on 

 stones, the surfaces of which they sensibly change ; others, 

 like Koleria, possess the faculty of extracting from sili- 

 cious sandstone potash and the phosphoric acid so sparingly 

 present in it. Eoots of grass attack the felspar rocks, 

 accelerating their disintegration. Eapes and turnips, san- 

 foin 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 from 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 un- 

 azotised substances 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 alu- 

 mina ; yet we also know that this substance, in the form 



