44 OVER-ALL REACTION OF PHOTOSYNTHESIS CHAP, 3 



was at that time investigated by Curtius and Franzen (c/. page 252). 

 Hexenaldehyde has the formula CeHioO, and its formation by photo- 

 synthesis should lead to a photosynthetic quotient of 1.33. In addition 

 to this high photosynthetic quotient, not confirmed by experiments, two 

 other arguments speak against Meyer's interpretation. In the first 

 place, the quantity of hexenic aldehyde, found by Curtius and Franzen, 

 is much too small to account for the large volume of Meyer's "photo- 

 synthetic secretion." In the second place, even this small quantity has 

 recently been proved to be of a secondary origin, being apparently 

 formed during the steam distillation of the leaf material (page 254). It 

 seems probable that Meyer's ''oil drops" were the so-called "grana," 

 whose occurrence in chloroplasts was first postulated by Meyer himself 

 in 1883 and recently confirmed by many other observers (c/. Chapter 14). 



3. The First Products of Photosynthesis and Their Transformations 



Having satisfied ourselves that no direct photochemical formation of 

 fats or proteins needs to be postulated on the basis of available experi- 

 mental material, we may now return to the problem of the "first carbo- 

 hydrate," mentioned on page 38. This role has variously been claimed 

 for glucose, sucrose, inositol and starch, usually on the basis of experi- 

 ments on the absolute and relative concentrations of these carbohydrates 

 in plants at different times of the day and season of the year. Both the 

 concentration of soluble sugars in the cell sap and the quantity of solid 

 starch in the chloroplasts, undergo wide variations with the intensity 

 of photosynthesis. They may be reduced to zero by starvation, and can 

 rise to 20 or 30% of the total dry weight after a period of intense photo- 

 synthesis, particularly if translocation is interrupted, as in detached 

 leaves. Brown and Morris (1893) found, in the leaves of Tropaeolum 

 majus attached to the plant, 9.7% sugars and 1.2% starch at 5 a.m. and 

 9.6% sugars and 4.6% starch at 5 p.m.; but if the leaves were detached 

 at 5 A.M. and left in sunlight until 5 p.m., the concentration of sugars 

 increased to 17.2%, while that of starch was 3.9%. 



Numerous authors have determined the relative quantities of glucose, 

 fructose, and sucrose in leaves, and the changes in these ratios caused by 

 starvation and illumination; and several of them, e. g. Perrey (1882), 

 Brown and Morris (1893), Parkin (1911), Mason (1916), Davis, Daish 

 and Sawyer (1916), Davis and Sawyer (1916), Gast (1917), and Venezia 

 (1938) have arrived at the conclusion that the disaccharide sucrose pre- 

 cedes the monosaccharides in the order of synthesis. They based this 

 conclusion either on the more widespread occurrence and larger absolute 

 quantity of sucrose in leaves, or on the observation that the concentration 

 of sucrose follows more closely the diurnal cycle of photosynthesis. 



