62 PLANT PHYSIOLOGY 



others have helped much to clarify our ideas of this matter. As 

 a result of their work, the present tendency is to consider that 

 the photosynthesis is over comparatively early in the process. 

 The dark or Blackman reaction and all that follows is post- 

 photosynthetic, in that light is no longer necessary, but the exact 

 division point between the photochemical and the dark reactions 

 still remains to be settled. 



It is also a disputed point as to what sugar is formed first from 

 the formaldehyde. A few workers like Brown and Morris held 

 that cane sugar was the first to be formed and that hexoses arose 

 later for translocation or respiration purposes; but the majority 

 of workers favor the idea that hexoses are formed first. Not only 

 is this theoretically sound, but Weevers (1924) found that in 

 variegated leaves the green parts contained both hexoses and 

 sucrose, while the yellow portions contained only the higher sugar 

 (sucrose). Likewise when Pelargonium was kept in the dark and 

 then exposed to light, chemical analyses showed that the first 

 sugars to appear were hexoses, followed by sucrose, and then 

 starch. Clements (1930) came to the same conclusions after study- 

 ing the hourly variations of the carbohydrates in leaves and 

 petioles. 



An objection raised to the hexose theory is that if one hexose, 

 e. g., dextrose, is formed, then there must also be formed fructose 

 in order for sucrose to be formed later, since (Chap. XII) cane 

 sugar (sucrose) is made from these two hexoses. The sugars, how- 

 ever, are notoriously unstable, and Nef found that if one begins 

 with dextrose and NaOH, no less than 93 substances are ultimately 

 in equilibrium in the solution ! Hence the problem of the origin of 

 another hexose is not a very serious one. 



Two molecules of formaldehyde may possibly unite to form gly- 

 colic aldehyde, a sugarlike substance: CH 2 0+CH 2 = CH 2 OH — 

 CHO. This in turn may combine with formaldehyde to form 

 glyceric aldehyde, a those, with distinct properties of sugar: 



CH 2 OH— CHO + CH 2 = CH 2 OH— CHOH— CHO. 



Further condensations of trioses to hexoses are not at all difficult 

 in the laboratory. None of these condensations requires any con- 

 sumption of energy, and the change from formaldehyde to sugar 

 is probably nothing more than the sum of the various reactions 

 already well known in vitro. 



