165 



was, that the coleoptiles, whereof the basal part was previously 

 exposed, remained straight, whereas the others showed the usual 

 antiphototropic curvature. 



In my former communication I admitted that in coleoptiles previ- 

 ously submitted to an omnilateral illumination of a definite value, 

 an antiphototropic curvature might, perhaps, be obtained with the 

 aid of a rather weak after-illumination. This seems now not very 

 probable, as under these circumstances, the occurrence of a normal 

 curvature in the basal part, may hardly be expected. Therefore, 

 in this case neither of the causes hitherto discovered, by which an 

 antiphototropic curvature may be produced, is present. 



The relative importance of the two causes is as yet wholly un- 

 known, but that the cause discussed in this paper, must be very 

 efficient, follows from the experiments described in my former com- 

 munication ^l.c. p 182). The antiphototropic curvatures produced 

 by an illumination with a given quantity of light, showed but little 

 difference if the exposition-time varied between 1 and 256 or between 

 *|^ and 192 seconds. Now, as we have seen that with a very short 

 exposition-lime, the presence of the cause discussed in my earlier 

 work, is wholly excluded, we uiust conclude that its influence in 

 the experiments with a longer ex|)Osition, was here also rather weak. 



SUMMARY. 



The antiphototropic curvature which appears at the tip of the 

 coleoptile of Avena with a very short exposition, does not show 

 itself, if the illumination is limited to the tip, or if the basal part 

 has previously been exposed to a rather strong illumination. 



Therefore we should assume, that with an unilateral illumination 

 of the whole coleoptile, the rate of growth of the tip, is enhanced 

 by an influence proceeding from the basal part. This influence must 

 be greatest in the side, which underwent the greatest contraction, 

 that is to say in the side, which duiing the exposition faced the 

 lamp. The origin of an antiphototropic curvature of this kind is, 

 therefore, always connected with the origin of a normal curvature 

 in the basal part. 



