144 PROCESS OF NUTRITION. CHAP. III. 



siderable change, first in its mixing with the juices 

 already contained in the plant, and then in its 

 throwing off a portion at the leaf. Perhaps it is 

 also further aifected by means of the gases entering 

 into the root along with the moisture of the soil, 

 but certainly, by means of the gases inhaled into 

 the leaf; the action and elaboration of which I 

 shall now proceed to elucidate. 



SECTION V. 

 Elaboration of Carbonic Acid. 



Carbonic THE utility of carbonic acid gas as a vegetable 

 inhalecTby fd nas heen already shown in the preceding 



chapter, in which plants were found not only to 

 absorb it by the root along with the moisture of 

 the soil ; but also to inhale it by the leaves, at 

 least when vegetating in the sun or during the day. 

 But how is the elaboration of this gas effected? Is 

 it assimilated to the vegetable substance imme- 

 diately upon entering the plant, or is its assimilation 

 effected by means of intermediate steps ? The gas 

 thus inhaled or absorbed is not assimilated imme- 

 diately, or at least not wholly : for it is known 

 Evolved that plants do also evolve carbonic acid gas when 

 nigh". 8 1 1C vegetating in the shade, or during the night. The 

 circumstance that led to this discovery was as fol- 

 lows : About the year 1771, Priestley in his experi- 

 ments on air found that a cabbage leaf which was 



