161 



The explanations of these negative curvatures given by other inves- 

 tigators are, however, hardly more convincing. 



Information about the influence of an illumination of the basal 

 part on the reaction of the tip, is to be found in papers by van 

 DER Wolk ^), Guttenberg ') and Auisz '), all dealing with the photo- 

 tropism of Avena. 



According to van der Wolk the results of an illumination of the 

 basal half of the coleoplile on the upper half, is to be seen in the 

 fact, that an illumination of 12 MCS gives in these seedlings a 

 curvature of the same strength as an illumination of 85 MCS in a 

 wholly etiolated coleoptile. This greater curvability of the upper 

 half might, perhaj)s, be explained by assuming that the contraction 

 of the tissue in this part was facilitated by the decrease of turges- 

 cence in the basal half: the expulsion of the water would find 

 here less resistance. 



GuTTENBERG ou the Contrary, tried to show, that the curvability 

 of the tip of the coleoptile is not altered by an illumination of the 

 basal part. In his experiments three sets of seedlings were compared. 

 They were all illuminated unilaterally with 22,2 or 33,3 MCS; but 

 in the second and third set the basal part was exposed previously 

 during one hour to an illumination with J 1,1 MC; in the second 

 set the seedlings rotated during this time round a vertical axis, 

 whereas in the third set they stood still. In this case the after- 

 illumination took place from the opposite side. Guttenbehg found 

 that the phototropic curvature in the third set was a little weaker 

 than in the other two. 



This result seems at first in flagrant contradiction to the statement 

 of VAN DER Wolk cited above, but it should be remembered that in 

 the experiments of van der Wolk, the light was very strong, and 

 the exposition only short, whereas Guttenbehg used light of rather 

 feeble intensity and a very long exposition. Therefore, in the seed- 

 lings of van der Wolk the decrease of turgescence in the basal 

 part, might have been greater, and consequently the effect on the 

 curvability of the lip more important than in the seedlings of 

 Guttenberg. This explanation would probably suffice, if there was 

 no difference at all between the curvatures in the three sets. Gut- 



J) P. G. VAN DER Wolk. Investigations of the transmission of light stimuli in 

 the seedlings of Avena. These Proceedings, Vol. XIV, p. 327. 



■^) H RiTTER VON Guttenberg. Ueber akropetale Reizleiting. Jhrb. f. wis. Bot. 

 Bd. 52 p. 333. 1913. 



2) W. H. Arisz. Untersuchungen über den Phototropismus. Rec. d trav. bot. 

 Néerlandais. Vol. XII p. 44. 1915. 



