156 PHOTOSYNTHESIS 



tions in their metabolic activities in which organic acids, such as malic 

 acid, play an important role. While there is no reason for believing that 

 in these plants, the chemical reactions constituting the photosynthetic 

 process are different from other plants, owing to the modified gaseous 

 exchange the study of photosynthesis in xerophytes offers specially complex 

 conditions.^"* 



But water supply is of importance to the photosynthetic process not 

 only on account of its indirect influence through affecting the stomatal 

 openings but also in a very direct manner. This question is more ex- 

 haustively discussed in the chapter on the Chemistry of Photosynthesis. 

 Suffice it to recall that de Saussure demonstrated that water enters into 

 the chemical reactions constituting photosynthesis. He showed that of the 

 dry material synthesized by a plant less than half could be ascribed to the 

 weight of the carbon in the carbon dioxide taken up by that plant. The 

 rest he regarded as coming from the water. 



The reaction for photosynthesis is usually written in the following 

 manner with the molecular weight relations as indicated : 



6 CO, + 6 HoO -> CeHxoOe + 6 O2. 

 264 108 180 192 



The basis for this is that the ratio of the volume of carbon dioxide ab- 

 sorbed to that of oxygen emitted is very close to unity which would indi- 

 cate the formation of a compound of the composition CnHanOn. The value 

 of n is, of course, not established thereby, but from the widespread oc- 

 currence of hexoses in leaves the assumption is frequently made that the 

 equation represents empirically the course of the reaction. Similarly the 

 relation of the weight of carbon dioxide absorbed to that of material 

 formed in photosynthesis in many instances supports this view. From 

 the equation it is apparent that the molecular ratio of water to carbon 

 dioxide in photosynthesis is : H2O : CO, = 108 : 264. It is rather sur- 

 prising that in much of the writing on photosynthesis the authors disregard 

 the fact that we are dealing with carbonic acid and not simply with CO,, 

 that the water actually enters into the chemical reaction and is not only a 



solvent. 



From theoretical as well as experimental considerations, therefore, we 

 can conclude that water is essential for the photosynthetic process. Each 

 100 grams of material synthesized, calculated as CsHioOg requires 60 



grams of water. 



Interesting in this connection are also the osmotic relations of the 

 photosynthetically active cells. Treboux ^'^^ has studied the effect of dif- 

 ferent concentrations of various substances which are considered as non- 

 toxic to the photosynthetic activity of Elodea. A noticeable effect was 

 first observed with a 0.1 per cent solution of KNO3. Solutions of KCl, 



*" Richards, H. M., Pub. No. 209, Carnegie Inst, of Wash., 1915. Spoehr, H. A., 

 Biochem. Zeit, 57,95 (1913). „. ,0^ ,ionox 



"'Treboux, Flora., 92, 53 (1903). Jacobi, Flora, 86, 326 (1899). 



