60 ORIGIN AND ACTION OF HUMUS. 



principal nutriment of young plants at a time when, 

 being destitute of leaves, they are unable to extract 

 food from the atmosphere. 



In former periods of the earth's history, its sur- 

 face was covered with plants, the remains of which 

 are still found in the coal formations. These plants 

 the gigantic monocotyledons, ferns, palms, and 

 reeds, belong to a class, to which nature has given the 

 power, by means of an immense extension of their 

 leaves, to dispense with nourishment from the soil. 

 They resemble, in this respect, the plants which we 

 raise from bulbs and tubers, and which live while 

 young upon the substances contained in their seed, 

 and require no food from the soil, when their exte- 

 rior organs of nutrition are formed. This class of 

 plants is, even at present, ranked amongst those 

 which do not exhaust the soil. 



The plants of every former period are distin- 

 guished from those of the present, by the inconsi- 

 derable development of their roots. Fruit, leaves, 

 seeds, nearly 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 tissue, of which then: roots con- 

 sisted, have been the first to suifer decomposition. 

 But when we examine oaks and other trees, which 

 in consequence of revolutions of the same kind oc- 

 curring in later ages have undergone the same 

 changes, we never find their roots absent. 



The verdant plants of warm climates are very often 



