ITS USE EXPLAINED. 77 



and require no food from the soil when their exterior 

 organs of nutrition are formed. This class of plants 

 is even at present ranked amongst those which do 

 not exhaust the soil. 



The necessity of the existence of plants such as 

 these at the commencement of vegetation, must now 

 be apparent. Humus is a product of the decay of 

 vegetable matter, and therefore could not have ex- 

 isted to supply the first plants with the food neces- 

 sary for the development of the more delicate kinds. 

 Hence the plants capable of flourishing under such 

 circumstances could only be those which receive their 

 nourishment from the air alone. By their decay, 

 however, the soil in which they grew became sup- 

 plied with vegetable matter, and the progress of 

 vegetation must have furnished to the earth materi- 

 als adapted for the development of those plants, 

 which depend upon the nutriment contained in the 

 soil, until those organs are formed which are des- 

 tined for the assumption of nourishment from the 

 atmosphere. 



The plants of every former period are distinguished 

 from those of the present by the inconsiderable de- 

 velopment of their roots. Fruit, leaves, seeds, near- 

 ly every part of the plants of a former world, except 

 the roots, are found in the brown coal formation. 

 The vascular bundles, and the perishable cellular tis- 

 sue, of which their roots consisted, have been the 

 first to suffer decomposition. But when we examine 

 oaks and other trees, which in consequence of revo- 

 lutions of the same kind occurring in later ages have 

 undergone the same changes, we never find their 

 roots absent. 



The verdant plants of warm climates are very often 

 such as obtain from the soil only a point of attach- 

 ment, and are not dependent on it for their growth. 

 How extremely small are the roots of the Cactus^ 

 Sedum, and Sempervivum, in proportion to their 

 mass, and to the surface of their leaves 1 Large for- 

 ests are often found growing in soils absolutely des- 



7 # 



