163 



are vevy valuable for our supposition, tiiat I lie antiphototropic cur- 

 vatures of Avena might find their explanation in this way. 



In the experiments of Arisz (i.e. p. 97), the exposition of the 

 basal part gave a normal curvature ^j, which did not extend itself 

 beyond the limits of the part exposed. As the occurrence of an 

 antiphototropic curvature in the tip is never mentioned, we must 

 assume that under the circumstances of these experiments, the tip 

 remained perfectly straight. At first sight, this seems to clash with 

 our supposition, but we should remember, that in these experiments 

 the tip remained continually in the dark, so that its turgescence 

 underwent no decrease. Now in consecpience of this circumstance, 

 an increase of the rate of growth might be difficult or even im- 

 possible. 



In my own experiments I compared in the first place the reaction 

 of coleoptiles exposed at the tip only, vv'ith the reaction of coleo- 

 ptiles exposed in the whole of their length. The result was very 

 clear. Whereas in the first case antitropic curvatures were never 

 found, in the second case they could be obtained without difficulty. 



The etiolated seedlings used for these experiments, were planted 

 in a single row in oblong zinc boxes. Each box got about 15 seed- 

 lings, so orientated, that their plane of symmetry was parallel to 

 the small side of the box. During the exposition, the boxes were 

 placed perpendicular to the rays of light. The seedlings that should 

 be exposed at the tip only, stood with their basal part behind a 

 screen, so that only 2^ — 3 mm. of the tip protruded. This screen 

 was prepared in the following way. A feeble red light was placed 

 just in front of the experimental lamp, and the silhouette of the 

 coleoptiles caught on a piece of black paste-board standing just 

 behind them. The place of the tip was marked thereon with the 

 aid of a pencil. Above these marks the paste-board was cut away 

 and then the screen pushed 27,-3 mm. deeper in the earth. After 

 that the box was turned round and the red light removed. During 

 the illumination with the experimental lamp, in this way just 27, — 

 3 mm. of the tip was exposed. 



The intensity of the illumination was in all experiments 750 MC; 



^) In two experiments out of a very great number, Aeisz mentions to have 

 obtained antitropic curvatures in the part exposed, in one case (illumination during 

 1 minute with 330 MC), the curvatures are stated to have been feebly normal or 

 antitropic, in the other ease (illumination during 1 minute with 200 MC), they 

 were antitropic or absent. As these cases, however, stand wholly isolated among 

 the rest of his results, it seems probable that these antitropic curvatures are due 

 to some experimental error. 



