THE MOVEMENTS AND SENSITIVITY OF PLANTS 321 



£ 



second supposition by Buder's (1920) recent investigations. By- 

 means of a very clever device, which he calls a light probe and 

 which consists of a small inverted glass funnel with the inner walls 

 coated with silver and the end bent sideways (Fig. 130), Buder suc- 

 ceeded in illuminating from within the hollow plumule of oats, in 

 which he observed a curvature toward the illuminated side, though 

 the direction of light in this case was reversed as compared to the 

 usual one. In experiments carried out in 

 diffused light, however, it is impossible to 

 take into consideration a definite direction of 

 light rays. 



Not all the rays of the solar spectrum 

 produce an equal phototropic effect. Red 

 rays, as a rule, are least effective. A gradual 

 moving towards the blue end of the spectrum 

 shows an increased effect which attains its 

 maximum in the indigo-blue rays (465 nn) 

 and then again gradually decreases towards 

 the ultra-violet region. Thus, one finds here 

 a certain parallelism between the delay of 

 growth and the phototropic effect. 



If a plant is subjected to unilateral illu- 

 mination from any source of light for a short 



. i , c , , j • i ; i i Fig. 130. — Diagram of 



period or beiore the reaction has taken place, the light probe. Arrows 

 the curvature will nevertheless be formed indi cate the direction of 



/., . . , ,, , -, ,. ,. ,, light rays (redrawn after 



alter a certain period, called reaction time, Buder). 



has elapsed. The time of illumination 

 required to produce the reaction, is called the " presentation 

 time." Naturally, the length of presentation is directly dependent 

 on the intensity of illumination. The weaker the light, the longer 

 the time of presentation, the product of these two quantities 

 remaining constant. Thus, for instance, Blaauw obtained the 

 following figures for seedlings of oats: 



