34 ON THE ORIGIN AND ACTION OF HTTMUS. 



in proportion to their mass, and to the surface of their leaves ! 

 Large forests are often found growing in soils absolutely desti- 

 tute of carbonaceous matter : and the extensive prairies of the 

 Western Continent show that the carbon necessary for the suste- 

 nance of a plant may be entirely extracted from the atmosphere. 

 Again, in the most arid and barren sand, where it is impossible 

 for nourishment to be obtained through the roots, we see the 

 milky-juiced plants attain complete perfection. 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 emul- 

 sions, with an impenetrable envelope by which the fluid is 

 retained, in the same manner as milk is prevented from evapo- 

 rating by the skin which forms upon it. The plants become 

 turgid with their juices. 



sidered very palatable, although strangers usually find it insipid. The 

 hills of Palermo covered with the Cactus correspond to our corn-fields. 

 It is a very important plant for such districts, because its roots easily enter 

 into the cracks and crevices of the volcanic rocks. These, although 

 destitute of humus, soon acquire it by the decay of the leaves, and thus 

 fertile soils are gradually formed for other plants. (Ait$lande> S. 274, 

 3d October, 1842.) 



