AQUATIC PLANTS 23 



per hour, however, there is no further increase with an aug- 

 mented carbon dioxide supply unless the light intensity be 

 increased. In other words, light intensity in this connection 

 is a limiting factor. 



In all such experiments with aquatic plants, it is to be 

 remembered that the conditions are not precisely the same as 

 for a subaerial plant, since the resistance to the diffusion of 

 carbon dioxide to the chloroplast is much less in the aquatic 

 habitat, being only the liquid diffusion phase of the land plant, 

 i.e. the passage of carbon dioxide in solution from the solution 

 in contact with the surface of the plant to the chlorophyll 

 surface. Further, in normal conditions the amount of carbon 

 dioxide dissolved in the water surrounding the assimilating 

 plant depends on the rate of diffusion from the source of supply ; 

 it is, in general terms, slow and is dependent on such factors 

 as temperature, relative gaseous pressures, degree of agitation, 

 and so on. The work of Blackman and Smith has been fol- 

 lowed by others, Harder for instance, to whose observations 

 allusion has already been made, some of whom used a bicar- 

 bonate solution as a source of carbon dioxide. The results 

 obtained are not always comparable, and the question arises 

 whether the carbon dioxide in the free state only is assimilable 

 by the plant and not — HC0 3 ions and the undissociated 

 fraction of such salts as sodium hydrogen carbonate. A com- 

 parison between the two types of solution has been made by 

 James * working with Fontinalis antipyretica in controlled 

 laboratory conditions. He found that a flow of 400 c.c. of 

 sodium bicarbonate solution per hour resulted in a higher rate 

 of carbon assimilation as compared with a solution of pure 

 carbon dioxide of an equal partial pressure when no other 

 factor was limiting ; but with a flow of 600 c.c. of bicarbonate 

 solution per hour and with a low intensity of light, and there- 

 fore a slow rate of carbon assimilation, the two solutions gave 

 the same assimilation values. An increase in the light in- 

 tensity resulted in the bicarbonate solution giving a greater 

 rate of carbon assimilation as compared witli the carbon 

 dioxide solution. It thus appears that in bicarbonate solutions 

 * James: " Proc. Roy. Soc," B, 1928, 103, 1. 



