Theories of Plioto-synthesis. 205 



Willstatter and Stoll have, however, overlooked the principle of 

 limiting factors established by Blackmann and his pupils. In 

 leaves rich in chlorophyll the limiting factor is usually the supply 

 of carbon dioxide and an increase of illumination beyond that 

 necessary to assimilate the carbon dioxide available in a given unit 

 of time is naturally without effect. 



An increase of temperature on the other hand accelerates all 

 plant functions, including respiration. In leaves rich in chloro- 

 phyll this may not be more than -"^ the activity of CO^- assimila- 

 tion, but in leaves poor in chlorophyll it represents a large frac- 

 tion of the COo assimilated. Respiration may be 20 to 40 times as 

 active as 35"^ C. to 40°C. as at 0"C., and this would be sufficient to 

 prevent any accelerating action of a rise of temperature upon the 

 photo-synthesis of leaves deficient in chlorophyll being shown. 



Willstatter and StoU's results are, therefore, capable of a simple 

 and natural explanation, and do not support the construction they 

 place upon them. 



Theories of Photo-synthesis. 

 From Jorgensen and Kidd's criticismi of the results obtained by 

 Usher and Priestly, by Wager and by myself, it would appear as 

 though the work of these avithors was completely antagonistic, and 

 mutually contradictory, and therefore of no value. This is not 

 quite a correct view, as the work of each investigator led on to or 

 directly gave rise to that of the next. Thus Usher and Priestley 

 (I.e.) in supposing that they produced photo-synthesis outside 

 the cell, drew attention to facts which otherwise might have been 

 overlooked, and it was their work which led me to determine that 

 the formaldehyde was a direct product of the photo-oxidation of 

 chlorophyll, and was formed in the absence of carbon dioxide. It 

 was a further investigation of their results which enable me to de- 

 termine that carbon dioxide decomposes chlorophyll in darkness as 

 well as in light. I was in error in concluding that xanthophyll was 

 one of the products ol this decomposition, and this error has lieen 

 corrected by Willstatter as well as by Jorgensen and Kidd, thus 

 bringing our understanding of the changes possible in photo-syn- 

 thesis a stage further. The direct action of carbon dioxide is to 

 remove the magnesium from the chlorophyll, forming phaeophytin, 

 and I have shown that if zinc dust is present during this reaction, 

 the zinc steps into the place vacated by the magnesium, forming 

 the stable green zinc chlorophyll. 



1 Proc. Roy. Soc, vol. 89, B., p. 342, 1917. 



