THE PHYSIOLOGICAL FACTOR IN PHOTOSYNTHESIS 267 



strong electric shock causes an arrest of photosynthesis, 

 the period of arrest being prolonged with increased intensity 

 of stimulus. This is analogous to the temporary abolition 

 of the power of response in Mimosa after an electric shock, 

 the irritability not being fully restored until after the com- 

 pletion of protoplasmic recovery. 



The photosynthetic curve, showing the relation between 

 the activity and the intensity of light which induces it, is 

 similar to the phototropic curve. 



Light induces both the anabolic A and the catabolic D 

 reactions, the resultant being A— D. The C0 2 -assimilation 

 by the green leaf of Hydrilla is essentially an anabolic pro- 

 cess, as exhibited by an electric response of galvanometric 

 positivity (p. 78). In certain cases where the D reaction 

 is predominant, the A is unmasked on the cessation of the 

 action of light, when it is exhibited either as a positive after- 

 effect, or an overshooting response in the positive direction. 



Effect of variation of temperature. — The various activities 

 of the plant are arrested at a definite minimum temperature 

 characteristic of the species ; the thermometric minimum 

 is also modified to some extent by the conditions to which 

 the plants had become habituated. In the tropical Des- 

 modium gyrans, the pulsation of the leaflets is arrested 

 at 17 C. in summer, and at n° C. in winter. The minimum 

 temperature for the arrest of photosynthesis in Hydrilla 

 varies from 9 to 12 (p. 137). Growth and the ascent of 

 sap are similarly arrested at a definite thermometric 

 minimum. 



The various activities of the plant are enhanced by 

 rise of temperature, and reach a climax at an optimum 

 temperature, which is nearly the same in a large number 

 of tropical plants. The optimum temperature for photo- 

 synthesis under sunlight is 33 : the optimum temperature 

 for growth and ascent of sap is also 33 . The excitability 

 of Mimosa is at its highest at 34 C. The growth-curve 

 under variation of temperature is practically a replica of the 

 photosynthetic curve (p. 140). 



