THE CHEMISTRY OE PHOTOSYXTHESIS 309 



illuminated in the presence of carbon dioxide, it is claimed the gas in 

 the flasks containing the preparations, when compared to the unilluminated 

 preparations, is higher in oxygen content. 1'he dif¥erence in oxygen 

 content between the illuminated ])reparations and those kept in the dark 

 is 0.9 to 4.5 j)er cent. Should these findings be verified the method may 

 prove a valuable one for further investigation. 



5. The Role of Water in Photosynthesis 



The assumption is almost universally made that photosynthetic re- 

 action may be represented by the equation : 



6CO2 + 6H2O > CeHioOc + 6O2 



This conclusion is based largely upon the relation of the amount of car- 

 bon dioxide absorbed to that of oxygen emitted. That water enters into 

 the reaction was already assumed by Senebier, although with the excep- 

 "tion of two experiments carried out by de Saussure in 1803 no experi- 

 mental work has been carried out to establish this assumption. It is, 

 of course, largely based upon the fact that carbohydrates are the first 

 products which accumulate as a result of photosynthesis and that the 

 dry material of the plant is largely of carbohydrate composition. 



The ex^^eriments of de Saussure '"'^ on the utilization of water during 

 photosynthesis gave the following results. Seven plants of Vinca Minor 

 L., grown in atmospheric air, absorbed 431 cc. carbon dioxide; this accord- 

 ing to the constants available to de Saussure, he calculated as equivalent 

 to 217 mg. of carbon. The plants increased their dry weight by 531 mg. 

 De Saussure concluded that during photosynthesis the plants had fixed 

 315 mg. of water. It is evident that there are in de Saussure's calcula- 

 tions some minor errors due largely to the fact that the density of car- 

 bon dioxide had not been determined accurately. But there is a fair 

 agreement between his results and the values which can be calculated 

 on the basis of the usual equation. With 431 cc. of carbon dioxide 

 absorbed the increase in dry weight calculates to 514 mg. as compared 

 to the 531 mg. found by de Saussure. 



A second experiment with Mentha aqimtica showed that 309 cc. of 

 carbon dioxide were absorbed, which de Saussure makes equivalent to 

 159 mg. of carbon. The plants gained 318 mg. ; therefore it is concluded 

 that 159 mg. of water had been fixed. Here the agreement with the 

 theoretical values is not very good, for from 309 cc. of carbon dioxide 

 there should be formed 360 mg. of dry material on the basis that this is 

 carbohydrate of the molecular weight 180. 



Experiments of this nature are rather difficult to carry out and there 

 are a number of factors which must be considered which were omitted 

 in de Saussure's experiments. It is essential that the plants be not in- 

 jured in the slightest degree which is very difficult to avoid. The rela- 



"*"de Saussure, Ostimld's Klassikcr, 15, 26; 16, 36 (1890). 



