PHOTOSYNTHESIS 5 



category as other phenomena of irritability, which manifest 

 themselves by mechanical response to stimulus or by 

 autonomous response. The important factors which modify 

 photosynthesis are the C0 2 -concentration, the temperature, 

 and the intensity of light. We have to determine the effect 

 of increasing C0 2 -concentration on photosynthesis : the 

 effect of rise of temperature has also to be observed, and 

 the minimum and the optimum points determined. 



The most important condition for the maintenance of 

 uniform photosynthesis is the constancy of the light inten- 

 sity ; but there is at present no means available for quick 

 determination of the intensity of light, or of the hourly 

 variation of daylight. An Automatic Radiograph and an 

 Electric Photometer had therefore to be devised to meet 

 these special requirements. 



In investigating the action of light in photosynthesis 

 we have to determine the effect of varying intensity from 

 the minimum to the maximum, so as to obtain a complete 

 photosynthetic curve. It is also necessary to determine 

 the relation between the quantity of light and the amount 

 of photosynthetic product. 



The investigation of certain other features of photo- 

 synthetic action are of very great theoretical importance ; 

 for example, the period of photosynthetic induction and its 

 characteristic variations under definite conditions. Another 

 interesting investigation is that of the relative photosynthetic 

 effectiveness of intermittent and of continuous light. 



Photosynthesis is found to be affected in a characteristic 

 manner by various anaesthetics and poisons. One of the 

 most unexpected results was the discovery of the influence 

 of infinitesimal traces of certain chemical substances in 

 inducing an extraordinary enhancement of C0 2 -assimilation. 



The photosynthetic products are carbohydrates, and 

 there is no means at present available of direct measure- 

 ment of the rate of their formation under light. An account 

 will be given of a sensitive method by which this can be 

 determined in the living plant during exposure to light. 



