THE SOUECES OF THE NITEOGEN OF VEGETATION, ETC. 493 



2. Living vegetable cells, &c, which are in the dark, or are not penetrated by the 

 direct rays of the sim, 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 



