THE SOURCES OE THE NITROGEN OE VEGETATION, ETC. 



491 



Experiihents, Series 5. 



This set of experiments was arranged to show how far the reduction of the carbonic 

 acid, with the evolution of oxygen, was due to the action of the sunlight, in conjunction 

 with the fluids of the plant, at the moment of the passage of the gas through the walls 

 of the cells. 



If the decomposition of the carbonic acid resulted from a physico-chemical action, in 

 the presence of sunlight, upon this gas only as it passed through the cell-walls, then 

 there might be no oxygen liberated in the growing cell. If, on the contrary, it were 

 decomposed before passing out of the cell, free oxygen would exist within the latter. 



To settle this question, a set of experiments was made exactly similar to those the 

 results of which are given in Table IV., with the exception, that now the time of the 

 exhaustion, and of the action of the sunlight, was reduced to four or five minutes, and 

 the quantity of plant operated upon was increased, so as to give sufficient gas for 

 analysis during this short period. The following Table gives the results obtained. 



Table V. — Showing the amount and composition of the Gas evolved into a Torricellian 



vacuum, during four or five minutes only, by duplicate quantities of plant, both 



kept in the dark for some time before commencing the exhaustion, then one still 



kept in the dark, and the other exposed to sunlight during the short period of the 



operation. 



(1858.) 



The above results show that the carbonic acid can pass through the cell-wall, in the 

 presence of sunlight, without suffering decomposition. It would hence appear that 

 the free oxygen which a plant yields after it has been for some time under the influence 

 of the direct rays of the sun, existed as such in the cells before the exhaustion. The 

 slight preponderance of oxygen observed in the gas exhausted in sunlight is doubtless 

 due to its action upon the carbonic acid within the cell, during the short period of its 

 operation upon it before it passes out; precisely analogous to the action when the 

 plant is subjected to ordinary atmospheric pressure. 



