214 PHOTOSYNTHESIS 



all the starch which the leaf can ever contain." They also noted that 

 there exist decided periodic variations in the diastatic activity of leaves. 

 This they explained on the basis of their "starvation hypothesis" by which 

 they meant that the cell protoplasm elaborates the enzyme according to 

 the needs of the cells. Based upon the observations of Sherman,^^ that 

 amino acids exert a stimulating action on diastatic activity Spoehr and 

 McGee ^'~ have suggested that the variation of the activity of this enzyme 

 may be associated with variations in amino acid content of the leaves. 



Mention has already been made of the intiuence of temperature on 

 the rate of starch formation in leaves. Tollenaar, in the work previously 

 cited, has also studied the influence of temperature on the disappearance 

 of starch in leaves in the dark. Tobacco leaves, for example, were ex- 

 posed to an illumination of 24,000 M.C. for 4.5 hours and were then placed 

 in the dark at different temperatures. The time required for the leaves to 

 lose their starch at the different temperatures was as follows : 28°, 3 hours ; 

 17°, 5 hours; 10°, 12 hours; and 1.5°, 25 hours. 



Of equal importance with temperature in the transformation of carbo- 

 hydrates in leaves is the water-content. It has been observed that leaves 

 lose starch more slowly in an atmosphere saturated with water vapor than 

 under conditions in which the leaves wilt. The greater rate of loss of 

 starch in the drier atmosphere is apparently directly related to the water- 

 content of the leaves and has little to do with the rate of transpiration.^^ 

 Lundegardh also observed that in certain seeds rich in oil, the starch was 

 converted into oil, with soluble sugar as an intermediate product, when 

 the seeds were desiccated. Tollenaar is of the opinion that the relatively 

 rapid conversion of starch into soluble sugars on desiccation of leaves is 

 due to the fact, that under these conditions the enzyme which synthesizes 

 starch is inactivated while the hydrolyzing enzyme is more resistant. The 

 carbohydrate equilibrium, starch ^ sugar is therefore shifted in the di- 

 rection of soluble sugars, sucroses and hexoses. Schroeder and Horn, on 

 the other hand, consider that there is no such thing as a starch «=± sugar 

 equilibrium. Their reason for this opinion, that an equilibrium between 

 an insoluble substance such as starch and a soluble one such as sucrose 

 cannot exist, is however, not tenable. 



It is probably of considerable importance in considering these facts, 

 that in the experiments in which ordinary thin leaves were subjected to 

 drying, the desiccation was carried to an extreme point, so that in many 

 cases the leaves were wilted, though probably still living. In organisms 

 which are adapted to such extreme desiccation, as the cacti, Spoehr ^* 

 found that the ratio, polysaccharides; total sugars, was greater in the 



«J Sherman and Walker, Jour. Amer. Chem. Soc, 41, 1866 (1919); 43, 2461 

 (1921). 



''Spoehr and McGee, Carnegie Inst, of Washington, Pub. No. 325, p. 74 (1923). 



^'Molisch, Ber. bot. Ges., 39, 339 (1921). Schroeder and Horn, Biochem. Zeit., 

 130 165 (1922). Tollenaar, /. c, 80. Lundegardh, Jahrb. wiss. Bot., 53, 421 

 (19'l4). Ahrens, Bot. Arch., 5, 234 (1924). 



"Spoehr, Carnegie Inst, of Washington, Pub. No. 287, p. 57 (1919). 



