112 THE BIOLOGY OF FLOWERING PLANTS 



In water overlying mud rich in organic matter, carbon 

 dioxide is supplied by the processes of decomposition which 

 continually go on. The presence of bicarbonates in many 

 natural waters also increases the supply. In fresh water 

 calcium bicarbonate is most important. It is hydrolysed 

 and dissociated into several ions, carbon dioxide being 



set free. 



The amount of free carbon dioxide present in solution 

 depends on the concentration of bicarbonate ; but if the 

 water is in contact with air, carbon dioxide passes into the 

 air until the solution is in equilibrium with the partial pres- 

 sure of the gas in the air. In contact with the atmosphere 

 this point is reached when the water contains about 0-03 

 per cent, of the gas. As the gas passes into the air the 

 splitting of the bicarbonate goes on, with the result that, 

 in the end, calcium bicarbonate in solution is almost com- 

 pletely converted into the insoluble carbonate which is 

 precipitated. If plants are present they utiHse the carbon 

 dioxide, and, as they use it up much more rapidly than it 

 could diffuse into the air, they appear to take an active part 

 in breaking up the bicarbonate. It is this effect which 

 explains the conclusion reached by Angelstein (191 1), 

 that the plant actively splits bicarbonate. Wilmot (1921) 

 has shown that, in a solution of bicarbonate of given strength, 

 Elodea assimilates at the same rate as in water containing 

 that amount of carbon dioxide which should, on theoretical 

 grounds, be present in a bicarbonate solution of the strength 

 employed. There is, therefore, no active splitting. Ruttner 

 (1921) has found that the plant can convert the bicarbonate 

 completely into carbonate, and this does not take place 

 spontaneously. The reason evidently is that the sponta- 

 neous splitting ceases when the solution has reached 

 equilibrium with the atmosphere as regards the carbon 

 dioxide content, while the plant goes on using up the carbon 

 dioxide till no more is formed and the conversion, in a closed 

 vessel, is complete. The presence of bicarbonates in natural 

 waters is certainly of great importance in increasing the 

 carbon dioxide supply to aquatics. The employment of 



