PROCESS OF NUTRITION. CHAP. III. 



their leaves ; for they are then found to vitiate com- 

 mon atmospheric air, whether in the sun or shade, 

 without changing the volume of their atmosphere ; 

 replacing the oxygene which they consume by an 

 equal quantity of carbonic acid, and by consequence 

 not assimilating it immediately. Branches of Salia: 

 alba, Populus nigra, and Quercus Robur consumed 

 in the space of 24 hours, in the spring and summer, 

 at 15 of Reamur, a quantity of oxygene, equal to 

 more than half their volume ; while branches of the 

 Apple and Pear consumed, in equal circumstances, 

 two or three times their volume. 



The oxygen which the stem and branches inhale 

 in the shade they give out again in the sun, in pro- 

 portion to the quantity of green vegetable substance 

 contained in their bark ; by means of which they 

 perhaps assimilate a small quantity of oxygene in 

 decomposing the carbonic acid which they form 

 with that of their atmosphere, though the effect is 

 not perceptible. 



But if a portion of the stem remains in the re- 

 ceiver whilst the root remains in the soil, and the 

 leaves in the open ?: r, then the oxygene gas which 

 the stem consumes is not replaced by an equal 

 quantity of carbonic acid ; because the carbonic 

 acid, after being formed, follows the course of the 

 branch, and is decomposed by the leaves in the open 

 air. Into a glass tube, containing 6'3 cubic inches 

 of atmospheric air, Saussure introduced the extremity 

 of the branch of an Apple tree, stripped of its leaves, 



