()2<S 



sides willi the saino iiiteiisilv, and when the illiiiniiialioii is tlien 

 C'oiiliiuied oil one side, resulls are ohlaiiied coiiipletely analogous to 

 tliose \vith oinnilateral instead of bilateral fore-illumination. It need 

 cause no surprise, that with a bilateral illumination, the excess 

 which must he given on one of the sides, to obtain an ipsilateral 

 curvature, must be greater, in proportion as the tendency to cur- 

 vature on the other side is stronger. This is the same phenomenon, 

 which we have observed after an omnilateral fore-illumination. The 

 quantity of energy, which had to be given in one direction, in order 

 to obtain a positive curvature, was greater in proportion as the 

 previous illumination was more intense. 



There is no reason to regard this so-called smaller sensitiveness 

 of a previously illuminated plant, which only depends on the necessity 

 of overcoming a tendency to curve, as an adjustment phenomenon. 

 Rather shouhl this name be reserved for the process which we 

 have here ahvays called adjustment [)rocess. We have been able to 

 observe liow it is affected both by unilateral and by omnilateral 

 illumination. 



Bilateral illumination can also give some explanation of the fading 

 phenomenon (§ 5). 



We saw that, as the intervals between the two opposite illumina- 

 tions become longer, the curvatures show better. This gives us 

 a new point of view with regard to the fading process, which the 

 omnilateral illumination enabled ns to study. 



Here, with the time which elapses between the first stimulation 

 (omnilateral fore-illumination) and the second one (unilateral after- 

 illumination), the i)Ower of the latter of becoming visible increases. 

 This manifests itself in the phenomenon that, the longer the interval 

 has lasted, the smaller is the amonnt of energy required to produce 

 a visible curvalnre. We must therefore assume that the gradual 

 return of the original sensitiveness is the result of the fact that a 

 tendency to curvature can express itself more strongly when a longer 

 period has elapsed since the last stimulation. 



Utrecht, Botanical Laboratory. 



Chemistry. — ''7Vie Allofropi/ of Copper' I. By Prof. Ernst 

 Cohen and ^Tr. W. D. Het.dkrman. 



- 1. In studying the earlier literature on copper we found certain 

 indications which justilied tlie presumption that this metal is capable 

 of existing in different allotropic modifications. This presumption had 

 been strengthened by the results of our investigations on tin, bismuth, 

 cadmium and zinc. 



