NOT INDISPENSABLE FOR PLANTS. 61 



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 ! Again, in 

 the most dry and barren sand, where it is impossible 

 for nourishment to be obtained through the roots, 

 we see the milky-juiced plants attain complete per- 

 fection. The moisture necessary for the nutrition 

 of these plants is derived from the atmosphere, and 

 when assimilated is secured from evaporation by 

 the nature of the juice itself. Caoutchouc and wax, 

 which are formed in these plants, surround the 

 water, as in oily emulsions, with an impenetrable 

 envelope by which the fluid is retained, in the same 

 manner as milk is prevented from evaporating, by 

 the skin which forms upon it. These plants, there- 

 fore, become turgid with their juices. 



Particular examples might be cited of plants, 

 which have been brought to maturity, upon a small 

 scale, without the assistance of mould ; but fresh 

 proofs of the accuracy of our theory respecting the 

 origin of carbon would be superfluous and useless, 

 and could not render more striking, or more con- 

 vincing, the arguments already adduced. It must 

 not, however, be left unmentioned, that common 

 wood charcoal, by virtue merely of its ordinary well- 

 known properties, can completely replace vegetable 

 mould or humus. The experiments of Lukas, which 



