365 



rested on the grouiid and wliicli could give no occasion for contact 

 curvatures, because it coidd at no point touch the coleoptile. The 

 phenomena, which are here discussed, are Iherelbie exclusively the 

 result of illuniinatiug the apical zone of 1- 2 mm. length. 



The negative curvature can be observed very distinctly, but is 

 much more developed, when after illumination the plant is rotated 

 on a clinostat round a horizontal axis, so that no geotropic influence 

 can counteract the curvature. In the same way as formerly I now 

 also used an intermittent clinostat ') in which the positions of rest 

 were separated by J 80'^. In each of these positions the plant remained 

 for 2 minutes, so that there was enough time to trace the image of 

 the seedling projected on to transparent paper by means of a lens 

 and in this way to obtain a complete survey of the course of the 

 cnrvature. The drawings were of course made by red light. 



Pi 



H H 



I0.+5 II. 



11.15 



11.30 W.'fS 



\Z.- 



12.15 12..30 1.4-5 



Negative curvature of an Avena saliva seedling. 



At 10.15 the apical zone of 2 mm. was illuminated from tlie right 



with an intensity of 340 c. m. s. After stimulation on the intermittent 



clinostat temp. 25°. 



The strength of the curve which is now reached, is found to be 

 by no means inferior to that of a positive curvature, whilst, because 

 only the uppermost zone of the apex is illuminated, there is here 

 also a conduction of stimulus towards the basal zones. 



Apart from the direction of the curvature a. negative curvature is 

 in no single respect distinguishable from a positive one. 



Very remarkable and probably of some importance from a theoretical 

 standing-point are the complicated curvatures which occur with 

 illumination of about 9000 C. M. S. applied during 30 seconds. 



In this case a positive curvature tirst takes place, later a negative 

 one is visible and this, although only a length of 1 mm. of the 

 apical zone, is illuminated. It is evident from this that positive and 

 negative curvatures are perceived and find expression independently 

 of one another. 



') Prof. Moll in Groningen was so good as to allow me to make use for these 

 experiments in his laboratory of the clinostat of van Harreveld. 



