VEGETATION. 343 



forms in the minute roots absorbent mouths to take ma- 

 terials from the earth, and other mouths in all the leaves 

 to absorb material from the air. This absorption from 

 earth and air begins indeed as soon as the root and germ 

 are at all formed, but it is not established in full until all 

 the nutriment of the seed is taken into the plant. 



474. Source of Carbon in Plants. You have already seen, 

 in 408, the source of a large part of the carbon in plants. 

 Then there is some carbon introduced by the root, for there 

 is always carbonic anhydride in the soil, as the product 

 of decompositions going on there, and this is absorbed 

 with other materials by the innumerable mouths in the 

 minute fibres of the root. What proportion of the car- 

 bon comes from the air is not known, but it is probably by 

 no means always the same. It is supposed that generally 

 more is taken in by the leaves than by the roots. 



475. Sources of the Oxygen and Hydrogen. With the car- 

 bon there must be united oxygen and hydrogen to form 

 the various structures of the plant the wood, bark, leaves, 

 etc., and also the substances contained in the plant the 

 starch, sugar, gum, etc. From whence, then, does it get 

 the oxygen and hydrogen ? Probably mostly, and some- 

 times wholly, from water. As this fluid, absorbed by the 

 root, carries up in the channels which life has constituted 

 various materials gathered from the earth, some of it is de- 

 composed in order to furnish oxygen and hydrogen to unite 

 with carbon in forming the various compounds alluded to. 

 It is not a union, you observe, of water and carbon, but of 

 the elements of water and carbon, and to effect this the 

 water must be decomposed into its elements. This is done 

 by the vital force which alone can build up organized 

 structures. Man can not effect this decomposition except 

 by applying strong heat, and at the same time presenting 

 some substance to the water which has a decided affinity 



