36 



OVER-ALL REACTION OF PHOTOSYNTHESIS 



CHAP. 3 



the assimilation of a certain quantity of carbon dioxide, is easier; we 

 remember that de Saussure used this method in 1804 to prove the par- 

 ticipation of water in photosynthesis. Quahtatively, the proof was suc- 

 cessful; but quantitatively, the two experiments performed by de Saussure 

 disagreed. In the first of them, seven Vinca plants assimilated 314 mg. 

 water together with 217 mg. carbon, corresponding to a molecular ratio 

 X : y = 1.03; in the second, two Mentha plants assimilated 159 mg. water 

 together with 159 mg. carbon, corresponding to x : y = 1.50. Similar 

 experiments were carried out almost one hundred years later by 

 Krasheninnikov (1901), who determined the total increase in the dry 

 matter of illuminated detached leaves of five species, and the amount of 

 absorbed carbon dioxide, and obtained x : y ratios between 0.87 and 1.23. 

 Bose (1924) compared the increase in dry weight and the oxygen pro- 

 duction of several plants of Hydrilla, and obtained x : y ratios of 0.92 

 or less. Smith (1943) made careful determinations of the carbon dioxide 

 consumption and dry matter production by sunflower leaves, after illu- 

 mination periods of the order of 1-3 hours. Table 3. II shows some 

 typical results. 



Table 3.II 



Carbon Assimilation and Increase in Dry Weight of Sunflower Leaves 



(after Smith) 



The average ratio AC/AW (0.415), corresponds to x : y = 1.06 and 

 thus agrees well with the theoretical ratio for a simple sugar (0.40, 

 X : y = 1.00) and even better with that for a disaccharide (0.42, 

 X : y ^ 1.09). In the leaf as a whole, the proportion of carbon is con- 

 siderably larger than in the newly formed " photosynthate " (about 0.51 

 if referred to dry weight without ash). 



The most satisfactory method of determining the nature of the 

 products of photosynthesis is direct analysis. However, when Sapozh- 

 nikov (1890) first determined the difference between the carbohydrate 



