MOVEMENTS OF IRRITATION. 473 



water impregnated with Indian ink. The wedge-shaped cavity of 

 the bottle, about 18 cm. long, was only about 3 mm. wide at one 

 end, but more than 25 mm. wide at the other. If pots of seedlings 

 were brought into the box, and this was disposed in the open air 

 in such a way that the wedge lay horizontally, and the rays of 

 light entered the apparatus from above, then all the seedlings 

 curved towards that part of the box in which the intensity of the 

 light was greatest, and consequently at right angles to the entering 

 rays and towards that end of the box towards which was directed 

 the narrow angle of the wedge. When the box was placed in the 

 room with the wedge parallel with the window panes, and there- 

 fore approximately at right angles to the incident rays of light, 

 while the pot was placed behind the wedge about centrally, then 

 the hypocotyls did not curve exactly at right angles to the 

 incident rays of light ; neither did they turn directly towards the 

 light, but curved obliquely towards the place where the intensity 

 of the light was greatest. I employed Lepidium seedlings. 



The following experiments on heliotropic nutations are also of 

 great interest. 3 



We lay grains of Avena sativa in dishes containing a little 

 water. When germination has just begun, we select seedlings 

 very similar in appearance, plant them in good garden soil in 

 separate small flower-pots, and put them in the dark. After some 

 time we select for the experiments proper five to ten seedlings 

 very uniform in development, and with plumules about 2 cm. long. 

 We place the pots in suitable heliotropic chambers, which have in 

 the front wall a horizontal slit some centimetres in width. Owing 

 to their vigorous circumnutation the plants grown in the dark are 

 often not perfectly straight. We put the pots into the heliotropic 

 chambers in such a way that none of the seedlings are inclined 

 towards the light from the window. It can be made out that the 

 heliotropic nutation begins at the extreme tip of the sheathing 

 leaf of the plumule, and gradually proceeds downwards, while a 

 greater and greater length of the upper portion of the leaf, which 

 continues to incline forward, becomes straight. At last the 

 curvature, generally a very sharp one, is found at the base of the 

 sheath leaf, and the upper part of the leaf forms an angle of 

 60-90 with the base of the leaf .*f This is the case at the end of 



* The magnitude of the curvature can be determined by measuring. The 

 measurements, which Consist in determining the inclination of the inclined 



