CONDITIONS OF PLANT GROWTH. 171 



therefore requires the presence of bodies capable of furnishing 

 carbon, nitrogen, "svater and its elements, and the mineral 

 matters Avhich are likewise essential to vegetables. The latter 

 can only be derived from the soil, but the supply of the ele- 

 ments which form the organic parts may be quite independent 

 of the soil. 



Lichens, which at one time were supposed to be destitute of 

 roots, have been shown to be possessed of these organs with 

 seemingly greater power than those of trees and herbS, for 

 they are able to disintegrate and take up the necessary por- 

 tions of the rocks to which they are attached, and fix them as 

 part of their constitution. Their organic constituents under 

 these circumstances can only come from one source, namely, 

 the atmosphere. 



The different substances necessary to the growth of a plant, 

 or the different articles of their food, are all of equal value ; 

 that is to say, if one out of the whole number be absent, the 

 plant will not thrive ; the soils which are proper for the culti- 

 vation of all sorts of plants, contain all the mineral constit- 

 uents necessary for them. 



But of two soils containing equal quantities of these mineral 

 ingredients, one may be considered rich and fertile, and the 

 other poor or barren, if in the former case they are pres- 

 ent in a form soluble in water, and in the latter are in- 

 soluble. 



All soils adapted for culture contain the mineral food of 

 plants in these two states ; the quantity of the soluble ingre- 

 dients can be increased from the insoluble ; in other words, the 

 soil made more productive by mechanical means ; but land in 

 which the necessary mineral constituents are not present in 

 any form, cannot be rendered fertile by ploughing. 



As different plants require for their development in some 

 cases the same mineral substances, but in unequal quantities, 

 a soil may become barren for one kind of plant, when by a 

 series of crops one only of these constituents, as for example, 

 soluble silica, has been so far removed, that the remaining 

 quantity is no longer sufficient for a crop ; but it may still 

 contain sufficient mineral constituents for another kind of crop 

 which does not require soluble silica ; a third sort of plant 

 may thrive on the same soil after the second, if the remaining 



