THE NATURE OF PHOTOSYNTHESIS 79 



dioxide-content at the surface 0.0534 to 0.284 volume per cent, above 

 the leaves 0.0401 to 0.0674 and at a height of one meter 0.0375 to 0.0720. 

 From analyses of this nature it has been concluded that the available 

 carbon dioxide for many crop plants may be very much higher than that 

 of the free atmosphere, and since the concentration of carbon dioxide, 

 perhaps more than any other factor, determines the rate of photosynthesis, 

 it is apparent that the production of carbon dioxide by the soil may 

 exert a decided influence on the development of plants. 



Boussingault -' showed definitely that plants grown in an atmosphere 

 lacking carbon dioxide did not increase in carbon content, while Moll '* 

 was able to demonstrate that plants growing in a carbon dioxide rich 

 humus soil formed no starch in leaves which were kept in an atmosphere 

 free of carbon dioxide. The leaves growing in normal air produced 

 starch as usual. It is conceivable, however, that conditions may exist 

 in which the carbon dioxide absorbed by the roots may contribute to 

 the total amount of carbon dioxide reduced by the plant.^^ Neverthe- 

 less, the chief source of carbon dioxide for the plant must be recognized 

 as being the atmosphere. 



In aquatic plants the conditions of carbon dioxide-absorption are in 

 some respects different from those obtaining in land plants. The exten- 

 sive anatomical investigations of aquatic plants have disclosed that in 

 submerged leaves there are generally no stomata, though there appear 

 to be some exceptions to this. In floating leaves the stomata are usually 

 confined to the upper surface. In submerged aquatic plants the ingress 

 of carbon dioxide into the leaf must therefore be accomplished by dififu- 

 sion through the epidermal cells to the cells containing chloroplasts. 

 In the mosses which are without stomata similar conditions must exist. 

 The mechanism of this diffusion has been more closely studied by 



Devaux.^* 



Ordinarily the carbon dioxide-content of pure water is, of course, 

 determined by the partial pressure of the carbon dioxide in the atmos- 

 phere over the water. Under certain circumstances the carbon dioxide- 

 and oxygen-content of water are greatly influenced by the presence of 

 decaying organic matter on the one hand and by active photosynthesis of 

 aquatic plants on the other. 



Of special importance to the carbon dioxide supply of natural waters 

 is the presence of carbonates and bicarbonates. It has been known for 

 a long time that aquatic plants form incrustations of calcium carbonate 

 and that these deposits arise from the conversion of the calcium bicar- 

 bonate into the more insoluble carbonate by the withdrawal of carbon 

 dioxide through photosynthetic activity. Draper -' first showed that 



^' Boussingault, Ann. Chim. et Phys., (5), 8, 433 (1876). 

 ~Moll, Arbeit, hot. Inst. Wiirzburg, 2, 105 (1878). ^ .„ ^ 



="Pollacci G., Bull. Soc. Bot. Ital. Genoa, 208 (1918). Cailletet, Compt. rend., 

 152 1215 (1911). Maquenne, ibid., 1811. Moillard, ibid., 154, 291 (1912). 

 ^"Devaux, Ann. Set. Nat., (7) 9, 35 (1890). 

 "Draper. Ann. chim. et phys., (3) 11, 223 (1844). 



