108 STUDIES OF PHOTO-SYNTHESIS 



on purely circumstantial evidence, or upon very weak and uncertain 

 positive experiments. 



The uptake of nitrogen from the atmosphere is shown by the 

 difference between Nos. 1 and 2 with nothing whatever added, only 

 tap water and a slight insemination with the alga) present, the sole 

 difference being that No. 1 is screwed up airtight, while No. 2 is 

 open to the air, The difference in nitrogen- content, 1-6-0-3= 

 1 3 mgrms. , is quite unmistakable. The limiting factor to growth here 

 is unquestionably deficiency in phosphate in the tap water, as is 

 clearly proven by Nos. 5 and 6, where this defect is supplied. This 

 experiment again in a still stronger way shows the absorption of 

 nitrogen from the air. 



The fillip of the phosphate in No. 6, with screwed- down lid, has 

 caused a light commencing growth until carbon dioxide became 

 exhausted, amounting to 1 mgrm. ; but when this is contrasted with 

 No. 5, differing only in access of air i.e., available carbon dioxide 

 and nitrogen the figure runs up to 8-1 mgrms., an increase of 

 7-1 mgrms. Now taking this No. 5 as it stands by itself, and not con- 

 trasting it with any of the others, the proof is given that in 200 c.c. 

 of tap water, plus 2 c.c. of 5 per cent, solution of disodium hydrogen 

 phosphate, plus a minute insemination of chlorella, there is an 

 abundant growth, and the absorption from the air of 7 to 8 mgrms. 

 of nitrogen. The controls are abundant and the conclusion seems 

 to us inevitable. 



Next observe the effects of a luxus addition of easily available 

 nitrogen as nitrite, as shown by No. 7. This differs from No. 5 

 only in the extra addition to the culture medium of 0-05 per cent. 

 of sodium nitrite, and the crop goes up by 50 per cent, from 8-1 to 

 12-3 mgrms. Similarly in the effect produced by manuring with 

 ammonium salts or nitrates, a luxury supply produces a luxus 

 crop. 



The small growth in No. 8 with lid screwed down shows that 

 available phosphates and sources of nitrogen may be present, but 

 when the supply of carbon dioxide is cut off, and no substitute 

 supplied, no photo-synthesis can occur. This is obvious, but No. 8 

 is a necessary control to No. 7, where C0 2 is supplied from the air, 

 and to Nos. 9 and 11, where the air is shut out, but a supply of 

 excessively dilute vapours of formaldehyde and methylic alcohol 

 respectively was obtained from these substances contained in narrow 

 test-tubes placed slantingly in the jars before the lids were screwed 



