3] EFFECT OF EXTERNAL AGENTS UPON GROWTH 485 



attuned, so that, if the intensity rises above that optimum, neg- 

 ative tropism will occur, and, if the intensity falls below that 

 optimum, positive tropism may be expected. This optimum - 

 this intensity to which the organism is attuned -- differs, how- 

 ever, in different species. Thus, according to WORTMANN, as 

 we have seen, the turning-point of the radicle is for 



Zea mais, 37; 



Ervum lens, 27; 



Phaseolus multiflorus, below 22. 



But why is the turning-point so different in different species, 

 or why are species attuned to different intensities ? This dif- 

 ference is for the most part more or less closely correlated with 

 the usual intensity of the agent to which the growing organ- 

 ism is subjected. To certain minds the phrase "Natural Selec- 

 tion " will be considered sufficient to stifle further inquiry ; 

 the known facts of self-adjustment, however, have thrown the 

 burden of proof that Natural Selection has acted in any case 

 upon those who assert it. Meanwhile we may seek for a 

 cause more consistent with sound physiology. 



In the first place, the facts of " after-effect " may be consid- 

 ered. It has been shown, with reference to the effect of many 

 of the agents considered, that this effect does not endure only 

 so long as the agent acts, but that it persists for a longer or 

 shorter period after the stimulus has ceased to be applied. 

 Thus, if a stem is placed horizontally, so that gravity stimu- 

 lates it and causes it to grow up, and it is then placed verti- 

 cally, the growth continues for a time in the former (now hori- 

 zontal) direction ; or, a horizontally placed root, decapitated 

 after the lapse of one hour, curves geotropically, whereas a 

 root placed horizontally and decapitated at once does not do 

 so. Again, if a tendril is irritated by contact and the irri- 

 tating body is then removed, a thigmotropic curvature of as 

 much as 45 may occur. Still again, in an experiment of 

 DARWIN ('81, p. 463), a seedling of canary grass was placed 

 before a window for nearly two hours, during which the coty- 

 ledon turned towards the glass ; the light was now cut off, but 

 the cotyledon continued to bend in the same direction for one- 

 fourth to one-half an hour. It was kept in the dark for an 



