316 Mr Saunders, Photosynthesis and Hydrogen Ion Concentration 



tion of the dissolved carbon dioxide out of the water. The hydrogen 

 ion concentration of the Cambridge tap-water which I used for 

 these experiments was 7-15 when the water was tested immediately 

 after being drawn from the tap. On standing at a temperature of 

 13° C. the hydrogen ion concentration rises to l-i. After boihng 

 and rapidly cooling the hydrogen ion concentration was 7-9 and 

 bubbling through air free from carbon dioxide produced the same 

 result. By incubating tap-water for 36 hours at a temperature of 

 40° C. and then cooling the hydrogen ion concentration could be 

 made to rise to 8-15, but in no case did the value of the control 

 tap-water approach near that of the tap-water containing Spiro- 

 gyra filaments. 



The following is a record of a typical experiment. The Spirogyra 

 was placed in 25 c.c. of tap- water in a boiling tube and exposed to 

 light at a window. Control boiling tubes containing tap-water 

 only were used. All these tubes were half immersed in a glass bowl 

 of running water so that the temperature was maintained fairly 

 constant. 



I have tried using Elodea instead of Spirogyra and it gives 

 much the same result. 



Both in darkness and in daylight the contents of the living cell 

 of Spirogyra show an acid reaction when stained with neutral 

 red. When Spirogyra is killed by heating to 40° C. and then placed 

 in tap-water the hydrogen ion concentration falls considerably 

 since the cell membranes are broken or dead and the contents of 

 the cell are now free to pass out into the water. 



In a large pond the mass of the plants in proportion to the 

 water is not sufficiently great to affect the hydrogen ion concen- 

 tration very much. I have however found slight variations. On 

 one occasion I noticed a fall in the hydrogen ion concentration of 

 0-1 after several dull days and a subsequent rise of 0-2 after sunny 

 days. This variation may possibly be due in some degree to the 

 photosynthetic activity of the plants present. 



