FIRST PRODUCTS OF PHOTOSYNTHESIS 45 



However, the primary formation of a disaccharide seems implausible 

 a priori, and authors who argued in its favor, have neglected the rapidity 

 with which the primary product of photosynthesis may undergo enzymatic 

 isomerizations and polymerizations in leaves which are equipped with 

 invertase, diastase, maltase and other carbohydrate-transforming en- 

 zymes. Priestley (1924), Stiles (1925), Spoehr (1926), and Barton-Right 

 and Pratt (1930) stressed the fact that the way in which the leaves are 

 killed (by freezing, drying, boiling, or immersion into alcohol) affects the 

 analytical results, thus proving that extensive enzymatic transformations 

 can take place even during the preparation of the material. Dixon and 

 Mason (1916), Priestley (1924) and Spoehr (1926) pointed out that a 

 mechanism for rapid enzymatic conversion of primary products (e. g., 

 hexoses) into storage materials (and sucrose may be a soluble storage 

 material) can keep the concentration of the primary products approxi- 

 mately constant, while that of the storage materials rises and falls with 

 the intensity of photosynthesis. Contrary to the experimental results 

 of the above-mentioned investigators, others — notably Weevers (1924), 

 Tottingham, Lepkovsky, Schulz and Link (1926), Clements (1930), 

 Barton-Right and Pratt (1930) and Kretovich (1935)— have obtained 

 analytical evidence favoring the conclusion that the monosaccharides 

 precede the more complex sugars in organic synthesis. Weevers (1924), 

 for example, found both glucose and sucrose in the green {i. e., photo- 

 synthetically active) spots of variegated leaves, and only sucrose in the 

 yellow spots. The same author observed that when a leaf of Pelargonium 

 was deprived of all its sugars by starvation for 48 hours, the first sugar 

 to appear upon illumination was glucose, which was only later followed 

 by sucrose and starch. Clements (1930) and Barton-Right and Pratt 

 (1930) found by hourly analyses extending from sunrise to sunset, that 

 glucose predominates in leaves early in the morning, while sucrose begins 

 to accumulate (and often surpasses glucose in concentration) later in the 

 day. These experiments support the plausible assumption that disac- 

 charides are secondary products formed by condensation of simple 

 hexoses. On the other hand. Smith (1944) has again found, in extending 

 to several hours the duration of his experiments on the fate of carbon 

 assimilated in sunflower leaves (c/. page 37), that sucrose (and starch) 

 are formed immediately upon the beginning of illumination, while the 

 relative quantity of monosaccharides is at first very small, and increases 

 with time (e. g., from 4% of total carbohydrates after 27 minutes of 

 illumination to 22% after 146 minutes). He concluded that the primary 

 product of photosynthesis is a common precursor of sucrose and starch 

 (perhaps a hexose monophosphate), and suggested that the free mono- 

 saccharides found in the cell sap are secondary products, formed by the 

 hydrolysis of sucrose. 



