STORAGE OF POTENTIAL ENERGY 437 



to ultraviolet light or to sunlight in solutions containing salts of 

 Uranium. 



We have seen that the rate of assimilation of Carbon Dioxide is 

 governed, not primarily by the velocity of the photochemical reaction, 

 birt by the velocity of a subsequent reaction which removes its products. 

 This is shown by the fact that the temperature-coefficient of carbon- 

 dioxide assimilation is of the usual chemical magnitude and not unity, 

 as would be the case in a purely photochemical process. If the prod- 

 uct of the photochemical reaction is in truth formaldehyde, as Baeyer's 

 hypothesis assumes, then its accumulation would very evidently be 

 injurious and we can readily understand how its removal, which 

 presumably does not require the agency of illumination, would be an 

 essential condition of the continuance of the reaction and would " set 

 the pace" of the whole process. It is not certain, however, at exactly 

 what stage of carbohydrate-synthesis the necessity for light ceases. 

 Thus W. Loeb has obtained not only the formation but also the partial 

 polymerization of formaldehyde with the silent electric discharge. 

 The fact that starch-formation will go on in tubers and other plant- 

 tissues which are not exposed to the light throws no light upon this 

 question, for the starch in these instances is not formed from formalde- 

 hyde but from hexoses or other comparatively complex carbon com- 

 pounds. 



In regard to the nature of the earliest carbohydrate to arise in 

 photosynthesis the most natural supposition would appear to be the 

 formation of Glucose 



6HCHO = CeHizOe 



or some other hexose, since this synthesis has actually been performed 

 in the laboratory. As a matter of fact, however, there is much evi- 

 dence tending to show that the first carbohydrate to be produced in 

 photosynthesis is actually Cane-sugar (sucrose). This view, which 

 was first put forward by Brown and Morris, has received very strong 

 support from the investigations of Parkin. This observer employed 

 for his experiments the leaves of the snowdrop, Galanthus nivalis, 

 which are peculiar in that they do not form Starch during photosyn- 

 thesis, so that the analyses of sugar-content are not complicated by 

 the possible presence of sugars, Maltose or Glucose, derived from the 

 hydrolysis of starch. As a matter of fact it was found by Parkin 

 that the leaves of the snowdrop contain only three carbohydrates, 

 namely Sucrose, Fructose (levulose) and Glucose. Of these the per- 

 centages of hexose remain very constant throughout any given twenty- 

 four hours, not increasing during the illumination of the day, nor 

 decreasing during the night, while the percentage of sucrose rapidly 

 increases during the day and decreases decidedly at night. Moreover 

 the proportion of sucrose to the other sugars is greatest at the apical 

 portions of leaves where assimilation is most active, and decreases 

 toward the base. Two interpretations of this result, however, may 



