THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 357 



incident • light, was positive from the beginning, and in 

 which the curvature reached a maximum beyond which 

 there was no subsequent reversal. If the intensity of the 

 stimulus be feeble or moderate, the quantity of light inci- 

 dent on the responding organ at the beginning may fall 

 below the critical value, and thus act as a sub-minimal 

 stimulus. This induces as we have seen (p. 344) a nega- 

 tive tropic curvature ; continued action of stimulus, how- 

 ever, converts the preliminary negative into the usual posi- 

 tive. The prelimijiary negative curvature may be detected 

 by the use of a moderately sensitive recorder with a magni- 

 fication of about 30 times. It is comparatively easy to 

 obtain the preliminary negative response in specimens 

 which are in a slightly sub-tonic condition. 



Semi-conducting tissues exhibit under continued stimu- 

 lation, a neutralisation and reversal into negative (p. 331). 

 Since this reversal into negative usually takes place under 

 prolonged exposure to exceedingly strong light, it is diffi- 

 cult to obtain in a single curve all the different phases 

 of transformation. I have, however, been fortunate in 

 obtaining a complete phototropic curve which exhibits in a 

 single specimen all the characteristic changes from a 

 preliminary negative to positive and subsequent reversal to 

 negative. I shall describe two such typical curves obtained 

 with the terminal leaflet of Desmodium gyrans and the 

 growing seedling of Zea Mays. 



Complete phototropic curve of a jjulvinated organ : 

 Evperiment 135. — A continuous record was taken of the 

 action of light of a 50 c. p. incandescent lamp, applied 

 on the upper half of the pulvinus of the terminal leaflet 

 of Desmodium gyrans. This gave rise : (1) to a negative 

 curvature (due to sub-minimal stimulus) which lasted for 

 3 minutes. The curve then proceeded upwards, at first 

 slowly, then rapidly ; it then rounded off, and reached a 



