IN FRESH- WATER ALG^ J13 



carbon dioxide is supplied from the sodium bicarbonate, but no 

 oxides of nitrogen are given to the air, so that the only source for the 

 fixation of nitrogen 1 is the elemental nitrogen of the air enclosed 

 in the jars, this figure rises to (540 - 2-50) 2-90 mgrms. This result 

 is of high importance, for it shows that, given supplies of carbon 

 dioxide, nitrogen, and oxygen, in presence of sunshine, the cell can 

 form its own oxides of nitrogen and build these into amino-com- 

 pounds, although at a much slower rate than when oxides of nitrogen 

 are supplied. 



This is confirmed again by the experiments on marine algae to be 

 detailed in the succeeding chapter. In the sea water a source of 

 carbon dioxide already exists in the bicarbonate of magnesium 

 and calcium dissolved in it. Hence no side-tube is necessary, and 

 a marine alga simply shut up airtight photo-synthesises and fixes 

 both carbon and nitrogen. The stimulating and growth- quickening 

 effect of traces of oxides of nitrogen, passing from the air to dissolve 

 in the aqueous medium bathing the green cell, is shown by the great 

 rise when these oxides are supplied, as in Nos. 7 and 8. 



The amount of nitrogen here fixed is 11-60 mgrms., as compared 

 with 2-90 when elemental nitrogen is the sole source. 



SUMMARY. 



1. The primeval living organism, like the inorganic colloidal 

 systems which were its precursors, must have possessed the power 

 of fixing carbon and nitrogen and building these up into reduced 

 organic compounds with uptake of energy. The source of the 

 energy was sunlight. 



2. This power is still possessed by the lowliest type of synthesising 

 cell existing namely, the unicellular alga. 



3. A synthesising cell must have existed prior to bacteria and 

 other fungi, since these can only exist upon organic matter, and 

 the primeval world before the advent of life could contain no 

 organic matter. 



4. Their specific reactions show that even the ultra-microscopic 

 filter-passing organisms are highly organised products on the path 

 from the inorganic towards life, and it hence follows that there is a 



1 As we have shown previously, this air contains a trace of nitrites, but 

 the amount in the few hundred cubic centimetres of air contained in the jar 

 is infinitesimal and may be neglected. 



8 



