MOVEMENTS OF IRRITATION. 469 



the cover and back can be folded back. The front of the box is 

 provided with an opening 1 cm. wide, and carries the tube R. 

 The tube is closed at the front by a plate provided with a slit. 

 This slit is in a straight line with the slit c of the box. We 

 raise seedlings of Sinapis or Lepidium in small flower-pots. When 

 the plants have just emerged from the soil, we put one of the 

 flower-pots into the heliotropic chamber, direct the slit b at the 

 window towards white clouds, or towards a white wall illuminated 

 by the sun, and even after a few hours we find that the plants have 

 curved heliotropically. (See Sachs, Lectures.) 



If seedlings of Lepidium sativum or other objects are very 

 feebly illuminated, they curve only slowly towards the light. 

 In investigations which I made with seedlings of Lepidium 

 hitherto grown in the dark, the stems exhibited heliotropic 

 nutations more rapidly when placed in diffuse light immediately 

 in front of: a window looking to the south, than when they were 

 placed several metres from the window and illuminated unilater- 

 ally. On the other hand it appears that high intensity of light 

 retards heliotropic nutations. (See also below.) 



If seedlings grown in flower-pots (I used Lepidium) are placed, 

 as described in 8, in a box in which they are exposed to mixed 

 yellow light transmitted through a solution of Potassium bichro- 

 mate, they do not curve to- 

 wards the incident rays of 

 light at all or only very 

 slowly. In the experiment 

 of which the result is 

 illustrated in Fig. 159, the 

 seedlings made no helio- 

 tropic curvatures at all. 



. FIG. 159. Heliotropic chamber. 



Wiesner observed, however, 



even in mixed ^yellow light, slowly appearing heliotropic nuta- 

 tions, a result justified by the conditions under which he experi- 

 mented. Under the influence of the mixed blue light transmitted 

 by an ammoniacal solution of Copper oxide vigorous heliotropic 

 nutations take place. 



In searching investigations as to the influence with respect to 

 heliotropic nutations of rays of light differing in refrangibility, 

 the use of the objective spectrum is strongly to be recommended. 

 A heliostat set up before the window (on a firm support, so as to 

 be free from vibrations) throws a beam of parallel rays through a 



