FUNCTION.] NITROGEN. 309 



this action is promoted by the influence of light, but that it 

 continues to be exerted, although more slowly, even in the 

 dark. Secondly, that carbonic acid is never disengaged 

 during the healthy condition of the leaf. Thirdly, that the 

 fluid so abundantly exhaled by plants in their vegetation is 

 pure water, and contains no trace of carbonic acid. Fourthly, 

 that the first portions of carbonic acid gas contained in an 

 artificial atmosphere, are taken up with more avidity by 

 plants than the remaining portions ; as if their appetite for 

 that pabulum had diminished by satiety. In like manner 

 Dr. D. P. Gardner speaks in the most positive manner to the 

 absence of free carbonic acid in the interior of the green parts 

 of plants at from 11 12 A.M., " during vigorous existence 

 in the presence of bright diffused lights in summer." In other 

 cases, however, he found 2'4 per cent.; and Calvert and 

 Ferrand always found minute quantities of it at night in the 

 hollow stems of Phytolacca decandra. 



Although nitrogen is, as has already been shown, an im- 

 portant and constant element in vegetation, when dissolved 

 or obtained by separation from the atmosphere, yet in a 

 pure gaseous state it seems incapable of affording any sup- 

 port to the development of plants, as proved by Theodore 

 de Saussure, who found that, five days after immersion in 

 pure nitrogen, the buds of poplars and willows were in a state 

 of decay. But he inclined to ascribe the apparent inca- 

 pability of leafy plants to absorb nitrogen to the artificial 

 conditions under which the experiments were conducted. 

 And this is probable, considering the nature of modern dis- 

 coveries with respect to the action of nitrogen in vegetation . 



There can be no doubt that nitrogen is a product of vege- 

 table respiration. Mr. Bigg's experiments, already mentioned 

 (p. 274), are conclusive upon that point, and are confirmed 

 by those of Dr. D. P. Gardner (Phil. Mag. xxviii. 430). " By 

 overlooking/' he observes, "the laws of penetration, De 

 Candolle, Saussure, Ingenhouz, and Plenck are thrown into 

 contradictory positions in their experiments on the action of 

 the green parts of plants on artificial atmospheres. Thus 

 De Candolle (Phys. Veg. t. i. p. 133), ' Les parties vertes 

 laissent moins de gas oxigene dans le gas azote ; elles ne 



