627 



side, the two curvatures occur scparntdi/ one after the other. Clark 

 makes no mention of the lirst occurrence of the curvature in the 

 direction of the first illumination, and this deprives the phenomenon 

 of its surprising feature. 



Let us finally consider table X for an omnilateml after-illumi- 

 nation. Although carried out with a somewhat weaker intensity, it 

 may very well be compared with Clark's fig. 4. In this case also 

 Clark makes no mention of the positive curvature which occurs 

 first and only gives the negative values. Had the after-illumination 

 here not been omnilateral, no new result would ha\'e been obtained, 

 but since all sides were afterwards exposed to an equal amount of 

 energy, the phenomenon is somewhat more complicated. We must 

 come to the very plausible conclusion, that after-illumination has 

 not the same effect on ail sides, but has a different effect on the 

 side which had already been illuminated unilaterally. This results 

 in a sepai-ate production of the curvatures, first in the direction of 

 the first illumination and then in the opposite one. There is not the 

 slightest reason to call a curvature, in a direction opposite to the 

 first illumination, negative. 



§ 7. Summary. 



In conclusion a few results of this investigation may be considered 

 in Iheir mutual relationship. 



The observations with bilateral illumination ((able VIII) show, that 

 when we apply to a plant two stimuli by illuminating first one side 

 and then the opposite side, each stimulus results in a visible ipsi- 

 lateral curvature, as long as a certain time intervenes between the 

 two exposures. This is very marked when the interval between the 

 two inductions is long and less so with progressively shoi'ter inter- 

 vals until, when the interval is very short, only very slight a[»ical 

 curvatures are seen. This suggests that also when the two sides are 

 illuminated simultaneously, both stimuli would produce a tendency 

 to curve, which tendencies are not expressed because they are 

 simultaneous, equal and opposite, and therefore annul each other. 



The phenomena of omnilateral illumination are in complete agree- 

 ment with this. Here also, under certain conditions, there may occur 

 a curvature towards that side, which has had no preliminary unilateral 

 illumination. An omnilateral illumination must therefore be regarded 

 as the summation of unilateral ones. 



A series of experiments, which are not described here, has shown 

 me, that when a plant is illuminated simultaneously from two opposite 



