THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. AQ] 
Experiments, 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. 
TaBLeE 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.) 
Per cent. 
Date. [Description] Conditions during |; Total Gas -. Oxygen and 
of Plant. Exhaustion. collected. Nitrogen. | Oxygen. ee Carbonie 
acid. 
cub. cent. 4 6 
In dark......... 41°7 242 3 23°98 27°58 
July 30. |Oats...... In sunlight ...|| 42-5 72-23 4-71 | 23:06 || 27:77 
In dark......... 557 71-46 3°23 | 25°31 || 28-54 
July 30. |Oats...... {te sunlight ...| 43:3 69-98 3°23 26°79 30°02 
In dark......... 37°9 83°11 6°86 10°03 16°89 
July 30. |Oats...... In sunlight ...|| 38° 4714 9-09 13°77 22-86 
In dark......... 34°4 78°49 T27 14°24 23°51 
July 31. |Oats...... In sunlight oe 41°83 75:84 7°89 16:27 24°16 
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. 
