CHAPTER VII 



THE PHYSIOLOGICAL FACTOR IN PHOTOSYNTHESIS 



The anabolic and catabolic processes A and D — External manifestations 

 of A and D effects — Photosynthetic and phototropic curves — De- 

 pendence of maximum response on the intensity of stimulus — Effect 

 of variation of temperature — D effect due to shock-stimulus — Pheno- 

 menon of alternating response — Dual effects of stimulus of light — 

 Unmasking of the A effect — Effect of chemical agents — Effect of tonic 

 condition on response. 



The process of photosynthesis begins with a photochemical 

 reaction in which C0 2 and H 2 are synthetised into some 

 form of organic matter such as formaldehyde. To what 

 extent this is effected, as a purely physico-chemical 

 process, by the chlorophyll proper is still uncertain. 

 Baly and Heilbron have recently shown that ' in the 

 presence of a suitable basic coloured substance, such 

 as malachite green, with which the carbonic acid can 

 combine loosely, the formation of formaldehyde can be 

 demonstrated in visible light, the malachite green acting as 

 a photocatalyst for the reaction.' x There can, however, be 

 no doubt that the physiological factor — the reaction of the 

 protoplasm with which the chlorophyll is associated in the 

 chloroplast — plays a very important part in the process 

 of photosynthesis. I hope to establish this in the present 

 and succeeding chapters by showing that photosynthesis 

 is modified by variations of external conditions just as are 

 other modes of physiological response. 



The protoplasmic response to external change may be 

 looked at as being the expression either of storage or of 

 depletion of energy. The positive or anabolic response A 

 is connected with the uphill work, increasing the potential 



1 Heilbron, Nature, April 14, 1923- 



E 



