THE PRODUCTS OF PHOTOSYNTHESIS 321 



spaces is continuous with that of the outside atmosphere, and carbon dioxide 

 can diffuse with little impediment from the outside air into the intercellular 

 spaces. 



The Products of Photosynthesis. — The products of photosynthesis are 

 simple carbohydrates and oxygen. Most of the latter diffuses out of the 

 cells in which it is liberated and plays no further part in the metabolism 

 of the plant. Some of the oxygen, however, may be utilized in respiration 

 within the plant. The products of importance in plant nutrition are the 

 energy-rich carbohydrates which are built up in the chloroplasts. 



Sachs termed starch the "first visible product" of photosynthesis, but it 

 has long been recognized that the first substances to appear in living green 

 cells during this process are certain sugars, and that the starch is produced 

 as a result of a secondary and entirely independent reaction. 



The three sugars almost universally found in leaf cells during or im- 

 mediately after photosynthesis are the hexoses : ^-glucose and J-fructose, and 

 the disaccharide: sucrose. Numerous attempts have been made to determine 

 which of these sugars is the first to be produced in photosynthesis. Analyses 

 of the leaves of a number of species of plants have shown that the quantity 

 of hexose remains fairly constant throughout the day, while the amount of 

 sucrose present increases during periods of active photosynthesis, and decreases 

 rapidly upon cessation of photosynthesis (Parkin, 19 12, and others). This 

 has been interpreted by some investigators to signify that sucrose is the first 

 sugar produced in photosynthesis. The relatively small fluctuation in the 

 hexose content of leaves may, however, be explained upon the probably more 

 reasonable assumption that all of the hexoses present above a certain concen- 

 tration are rapidly converted into sucrose or starch (Priestley, 1924). 



This latter interpretation is supported by the work of Weevers (1924) 

 who showed that the non-green portions of variegated leaves usually contain 

 only sucrose, while the green, photosynthesizing portions contain both sucrose 

 and hexoses. The same investigator also showed that if geranium plants, the 

 leaves of which had been depleted of sugars by keeping them in the dark, 

 were allowed to photosynthesize for a wery short period, only hexoses accumu- 

 lated. A longer exposure was required for the appearance of sucrose, and 

 a still longer one for the appearance of starch. 



As the foregoing discussion indicates it seems reasonably clear that hexoses 

 are the first sugars produced in photosynthesis. A further problem remains, 

 however; which of the hexoses, glucose or fructose, is the first product? No 

 positive answer can be given to this question. It seems probable that either 

 both glucose and fructose are synthesized in photosynthesis, or that a very 

 active form of one of these sugars is produced which can be readily trans- 



