SECT. VI. ELABORATION OF OXYGENE. 173 



very small proportion, is retained also and assimi- 

 lated along with the carbon. 



Hence the most obvious influence of oxygene, as 

 applied to the leaves, is that of forming carbonic 

 acid gas, and thus presenting to the plant elements 

 which it may assimilate; and perhaps the carbon 

 of the extractive juices absorbed even by the root 

 is not assimilated to the plant till it is converted by 

 means of oxygene into carbonic acid. 



But as an atmosphere composed of nitrogene and 

 carbonic acid gas only is not favourable to vegetation, 

 it is probable that oxygene performs also some other 

 function beyond that of merely presenting to the 

 plant, under the modification of~ carbonic acid, ele- 

 ments which it may assimilate. It may effect also 

 the disengagement of caloric by its union with the 

 carbon of the vegetable, which is the necessary re- 

 sult of such union. 



But oxygene is also beneficial to the plant from Influence 



-, i -i r i i ... ofoxvgene 



its action on the soil; for when the extractive juices O n soil, 

 contained in the soil have become exhausted, the 

 oxygene of the atmosphere, by penetrating into the 

 earth and abstracting from it a portion of its carbon, 

 forms a new extract to replace the first. Hence we 

 may account for a number of facts observed by the 

 earlier phytologists, but not well explained. Du 

 Hamel remarked that the lateral roots of plants are 

 always the more vigorous the nearer they are to the 

 surface,;* but it now appears that they are the 

 * Phys. <les,Arbres, liv, i. chap. v. 



