625 



can then see how this insensitiveness gradually disappears again ; 

 for this purpose the plant must be left in the dark for some time 

 and the slight residual sensitiveness which remains at that moment 

 must be determined by observing the magnitude of the reaction to 

 a given stimulus. In tables VI and VII (he values are given relating 

 to a preliminary illumination of 25 candle-meter power during 

 100 sees, and 20 mins. respectively (see also table III). 



From table VI we see that the possibility of obtaining positive 

 curvatures has returned after only one minute has elapsed between 

 the end of the omnilateral illumination and the beginning of the 

 after-illumination. After 1 hour the original sensitiveness for the 

 positive reaction has returned more or less completely. It is however 

 remarkable that at 4500 C. M. S. after an intervalof 5 mins. between 

 fore- and after-illumination no negative curvature occurs again, but 

 instead a strong positive one. We see that here also through the 

 omnilateral illumination during 100 sees, the adjustment process has 

 been put into action, which process has continued in the dark and 

 resulted in the large quantity of energy giving not a negative but 

 a positive curvature. But the intensity of this process also diminishes 

 in the long run, so that after 60 minutes the negative reaction again 

 beo-ins to be evident. 



In table VII we see the return of the sensitiveness for positive 

 curvature as well as the possibility of a negative reaction. In this 

 case, however, neither the original sensitiveness for positive curvatures, 

 nor that for negative ones is completely reestablished after 1 houi-. 



§ 6. Omnilateval after-illuinination. 



Following PiUNGSHEiM, I investigated together with omnilateral 

 fore-illumination, the influence of an omnilatei-al after-illumiualion. 

 The simplest case imaginable, with two successive illuminations, is 

 that of a brief illumination from one side followed by one of equal 

 strength from the opposite side. Then the result is that the [)lant 

 remains straight. If there is an interval between the two exposures 

 even of only 2 minutes, the curvatures occur separately, so that 

 there is first a curvature in the direction of the first exposure and 

 then in that of the second. 



TABLE VIII 

 105 C. M.S. (7X15) immediately afterwards in the opposite direction 105 CMS (7X15). 



No curvature. 



1 min. „ No curvature 



2 min. „ apex curves first one way, then the other way 

 4 min. „ first one way, then slightly the other way 

 8 min, „ first one way then strongly the other way 



