324 



LIFE MOVEMENTS IN PLANTS 



increasing intensity of light. The shortest latent period 

 found by Czapek, as stated before, was 7 minutes. But by 

 employing high magnification for record, I find that the 

 latent period of phototropic action under strong light to be 

 a question of seconds. 



Determination of the latent 2^^1'iod : Experiment 122. — I 

 give a record of response (Fig. 116) of the terminal leaflet 

 of Erythrina inidca to light acting from above. The 

 recording plate was made to move at a fast rate, the 



Fig. 116. — Latent, period for photic stimulation at vertical line. Successive dots at 

 intervals of 2 seconds. {Erythrina indica). 



successive dots being at intervals of 2 seconds. The latent 

 period in this case is seen to be 35 seconds. By the 

 employment of stronger light I have obtained latent period 

 which is very much shorter. 



The term latent period is used in two different sense. 

 It may mean the interval between the application of 

 stimulus and the initiation of response. In the experi- 

 ment described above, the latent period is to be understood 

 in this sense. But in the extract given above, Jost uses 

 the terra latent period as the shortest period of exposure 

 necessary to induce phototropic reaction as an after-effect. 

 What then is the shortest exposure that will induce a 



