BY THE ACTION OF SUNLIGHT 75 



dioxide. Now, since 44 grms. of carbon-dioxide gas measure at 

 normal temperature and pressure 22,000 c.c., this means that 3 c.c. 

 of carbon dioxide weigh approximately 6 mgrms. ; 6 mgrms. of 

 carbon dioxide in 10 litres, therefore, represents a concentration of 

 6 parts in 10,000,000 parts of water. It is the carbon in the carbon 

 dioxide which is utilised in formation of the organic carbon com- 

 pounds of the plant, and this stands to carbon dioxide in the 

 relationship of 12 to 44, so that the concentration of assimilable 

 carbon becomes reduced to about 1 part by weight in 6,000,000 

 parts of water. 



So far as nitrogen assimilation is concerned, the amount of 

 nitrogen required by plants is not on the average more than about 

 5 per cent, of the required carbon assimilation, so that other factors, 

 such as coefficients of distribution between air and water, being 

 taken as equal, a concentration of nitrites or nitrates in the 

 atmosphere or cell sap about equivalent to 1 part of nitrogen in 

 120,000,000 parts of water ought to supply sufficient for assimilative 

 purposes. 



In a series of twenty experiments it was shown that both rain 

 and dew invariably contain nitrites, but these slowly diminish, so 

 that water drawn from a reservoir by a service tap gives a quite 

 negative result when tested by the diazo-reaction for nitrites. If, 

 however, another portion of this same sample of water be exposed, 

 either in the open or within a transparent quartz container, to sun- 

 light or an artificial source of light rich in short wave-lengths, in 

 one or two hours a strong reaction is given for* nitrites. In this 

 manner, by testing water before and after exposure, the presence and 

 relative amounts of nitrite and nitrate may be determined. Similar 

 changes, only greatly diminished quantitatively, are obtained when 

 the exposures are made in glass vessels, showing that it is the ultra- 

 violet waves which are most potent. That this result is due to 

 conversion of nitrates into nitrites, and not conversion of dissolved 

 nitrogen and oxygen, is shown by the fact that it does not occur with 

 distilled water holding air in solution; but if a small amount of 

 potassium nitrate say, 1 part by weight in 10,000 parts be added, 

 an intense reaction occurs on insolation. In several experiments it 

 was shown that the presence of green leaves from different types of 

 plants diminished the amount of nitrites present after exposure, as 

 contrasted with control flasks alongside containing no green leaves. 



It is clear from these experiments that nitrates taken up by the 



