THE PRODUCTS OF PHOTOSYNTHESIS 213 



tion of soluble carbohydrates in the cell sap greatly increases the resistance 

 of leaves to freezing. Aliyake reports that in the winter many evergreen 

 leaves in the middle and southern parts of Japan contain more or less 

 starch, while in the colder regions the leaves are found to be starch-free. 

 Henrici has demonstrated the effect of low temperatures on starch forma- 

 tion during photosynthesis. She has shown that below 0°, with the alpine 

 plants used, starch is never formed. With super-optimal illumination 

 there takes place an increase in the internal temperature of the plant which 

 results in some starch formation. Henrici's investigations have been dis- 

 cussed in the section dealing with the effect of temperature on photo- 

 synthesis. 



Czapek ^^ has shown that, at low temperatures the sugar concentration 

 in the cells of leaves must be very much higher in order that starch 

 formation may take place, than at higher temperatures. In none of the 

 leaves used by Czapek did starch formation occur at low temperatures 

 with sucrose concentrations under 7 per cent, while at ordinary tempera- 

 tures a 1 per cent solution produced starch. Tollenaar "" found that 

 tobacco leaves placed on sucrose solutions formed starch (detected with 

 iodine), in one day at 28°, about the same amount in two days at 17°, and 

 in three days at 12°, He finds that the minimum sucrose concentration 

 which will produce starch is %o per cent at 28°. 



In considering the photosynthetic activity of leaves not only is it neces- 

 sary to regard the rate of formation of any one carbohydrate but at least 

 three other factors are of importance, namely the rate of respiration, that 

 of transformation into other carbohydrates and the rate of migration of 

 the carbohydrates from the leaf to other portions of the plant. It is im- 

 possible to determine all of these factors with any degree of accuracy in 

 a living plant. It must be apparent, therefore, that it is impossible to ob- 

 tain even an approximation of the rate of photosynthesis from determina- 

 tions of the changes in the starch or carbohydrate content of leaves. 

 Starch is a transitory reserve material in the leaf ; it appears when the 

 supply of mono-and disaccharides is in excess of the metabolic and trans- 

 locating power of the leaf or when the concentration of these sugars has 

 reached a certain degree. 



That the disappearance of starch in leaves is due to the action of 

 diastase was demonstrated by Brown and Morris. They found diastase in 

 leaves of many different species of plants. The amount of the enzyme 

 vanes greatly in different plants. It is interesting that those plants which 

 ordinarily form no or very little starch also contain little diastase. The 

 tannin in the leaf apparently inhibits the activity of the diastase. Brown 

 and Morris also come to the conclusion that leaves contain "far more 

 diastase than is requisite to transform within a moderate amount of time 



(1882). Schultz, Flora, 71, 223 (1888). Henrici, Verhaud. Naturf. Gcs Basel 

 32, 107 (1921). 



^Czapek, Ber. bot. Ges., 19, 120 (1911). 



" Tollenaar, Dissertation, Wageningen, 1925, p. 42. 



