THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 493 
2. Living vegetable cells, &c., which are in the dark, or are not penetrated by the 
direct rays of the sun, consume the oxygen they contained very rapidly after being 
placed in such circumstances, carbonic acid being formed. 
3. There can hence be little or no oxygen in the living cells of plants during the _ 
night, or during cloudy days. The presence of oxygen in the cells of thick-leaved 
plants, or in the deeper layers of fruit, is also very problematical. 
4. With every cloud that passes over the sun, the oxygen of the living cell will oscil- 
late under the influence of the reducing-force of the carbon-matter, forming carbonic 
acid, on the one hand, and of the reducing-forces of the associated sun’s rays, liberating 
pure oxygen and forming a carbon-compound containing less oxygen than carbonic acid, 
on the other. 
5. The idea is suggested by the above considerations, that there may be in the outer 
cells, which are penetrated by the sun’s rays, a reduction of carbonic acid, and a fixation 
of carbon, with the evolution of oxygen, at the same time that, in the deeper cells, the 
converse process of the oxidation of carbon and the formation of carbonic acid is taking 
place. If such be the case, the oxygen of the outer cells would, according to the laws 
in conformity with which the diffusion of gases and their passage through tissues are 
known to take place, be continually penetrating to the deeper cells, and there oxidizing 
their carbon-matter into carbonic acid; whilst the carbonic acid thus formed would 
pass in the opposite direction to be decomposed in the sunlight of the outer cells. As 
the process of cell-formation went forward, and the once outer cells became buried 
deeper by the still more recent ones above them, they would gradually pass from the 
state in which the sunlight was the greater reducing-agent, to that in which the car- 
bon-matter of the cell became the greater—from the state in which there was a flow 
of carbonic acid to them and of oxygen from them, to that in which the reverse action 
took place. The effect of this action may be the formation of oxidized products—acids, 
or saccharine matter, &c.—in the deeper cells, whilst the great reducing-power of the 
sun’s rays may form more highly carbonized substances in the outer cells, which in their 
turn become subject to oxidation when buried deeper. The physical and physiological 
phenomena of such interchanges are obviously worthy of a closer study; but the subject 
is too wide for any further development here. 
6. The very great reducing-power operating in those parts of the plant where ozone 
is most likely, if at all, to be evolved, seems unfavourable to the idea of the oxidation of 
Nitrogen into nitric acid by its means—that is to say, under circumstances where the 
much more readily oxidizable substance, carbon, is not oxidized, but on the contrary its~ 
oxide, carbonic acid, is reduced; whilst, as has been seen, when beyond the influence 
of the direct rays of the sun, the cells seem to supply an abundance of the more easily 
oxidized carbon, in a condition of combination readily available for oxidation by: free 
oxygen, or ozone, should it be present. The conclusion that free Nitrogen would not 
be likely to be oxidated into nitric acid within the structures of the plant, Seems to 
be borne out by the well-known fact, that nitrates are as available a source of - Nitrogen 
