DIURNAL MOVEMENTS IN PLANTS 527 



The combined eflEects of different factors will evidently 

 be very numerous. This will be understood from considera- 

 tion of the number of possible combinations with only 

 two variables, geotropism and phototropism. The effect of 

 geotropism may be strong G, or feeble, g. Similarly we 

 may have strong effect of light L, or feeble effect of light 

 I. Light may exert positive phototropic action +L or 

 negative action - L. Thus from two variables we obtain 

 the following eight combinations : 



G + L ;G-L ; G + l ; G-l ; 



g-ifL ; g -L ; g-tfl ; g-l. 



The number of possible variables are, however, far 

 more numerous as will be seen from the following : 



Geotropism. — The effect of geotropic stimulus on hori- 

 zontally placed organs is one of erection. But this stimu- 

 lus, which is constant, cannot by itself give rise to 

 periodic movements. It has however been shown that 

 variation of temperature has a modifying influence on 

 geotropic curvature (p. 519), 



Phototropism, — The action of unilateral light is to in- 

 duce a tropic curvature, which in some cases is positive, 

 in others negative (p. 386), In addition to these effects 

 induced during the incidence of light, we have to take 

 account of the after-effects on the cessation of light. 



After-effects of light. — I find two very different effects, 

 depending on the intensity and duration of previous illu- 

 mination. Of these the most important is the phenomenon 

 of ' overshooting' which occurs on the cessation of light 

 of long duration. This particular reaction, to be fully 

 described, will be found to offer an explanation of certain 

 anomalous effects in diurnal movement. 



Periodic variation of turgor. — I have shown ( p. 39) 

 that artificial enhancement of turgor in the plant induces 



