( 338 ) 



l^earing in mind the great difference in sensitiveness between the 

 apex and the liase, I considered to what extent tlie greater sensitiveness 

 of the apex might be caused by its being in general more trans- 

 parent to light than the base, since the leaves which are found 

 within the coleoptile, frequently do not completely till the apex and 

 since they are also narrower and thinner than those in the l)ase; 

 lience at the apex more cells of the coleoptile receive direct unilateral 

 illumination than at the base, where the leaves are mucii thicker 

 and broader and, in addition to being more opaque, offer a greater 

 resistance to any curvatures. In order fui-ther to decide this question 

 the leaves when very young were removed from the coleoptile, so 

 that the base was then as transparent as the apex. However the 

 great trouble which this gave was not i-ewarded by a positive result : 

 the leafless colcoptiles behaved in exactly the same way as the 

 normal ones. 



We will now pass on to the description of some experiments, 

 of which the results may be able to tlii-ow new light on the 

 physiology of stimulus. This paper has always been concerned with 

 the polarity of the ti'ansmission of phototropic stimuli; but it must 

 be specially borne in mind, that our terminology is only based on 

 the outward visible effect, which we shall si)eak of as the phototi'opic 

 curvature effect; and it is the pohirity or rather the irreversibility 

 of the phototropic curvature effect which has been demonstrated in 

 the course of tiiis investigation. The question will now be answered 

 as to how far the apex, even though it does not curve when the 

 base is illuminated, is nevertheless influenced by the illuminated 

 basal portion. Here we obtain the result that the darkened apex 

 is indeed influenced by the ilinminated base, namely in the sense 

 that the base renders the ape.v more .sensitive. For this experiment 

 the apex is shut off by means of a closely fitting cap of tin-foil, 

 and subsequently the base is illuminated; immediately afterwards the 

 apex is freed and the base made immoveable by means of a closely 

 fitting cylinder of' tin-foil ; then the apex is illuminated in order to 

 determine the quantity of light which is necessary in order to 

 establish after half an hour a maximal curxature and at the same 

 time control plants are started in which the base had not been 

 illuminated before, but which, for the sake of more accurate 

 comparison, had been fitted with little caps and cylinders just as the 

 actual plants of the experiment. The result now v/as that the quantity 

 of light necessary to give a maximal apex curvature after half an 

 hour at 25° — 27° C, amounts to about 85 candle meter seconds 

 for the control-plants, wl'.ilst for the seedlings in which the base 



