INTRODUCTION AND HISTORICAL SKETCH 



9 



-f^ 



II 



-Coleoptile tip 



I- Rubber band 



on the front. These investigations were continued in Pfeffer's 

 laboratory in Leipzig in 1909 and were repeated later with the 

 same results in the plant physiology laboratory in Copenhagen. 

 The method of investigation (Boysen Jensen, 1910, 1911) was 

 briefly as follows: Avena seedlings were grown singly in small 

 glass vials (10 by 2.5 cm.) filled with soil at a temperature of 

 20°C. in a darkroom. When a length of 2 to 3 cm. was attained, 

 the coleoptiles were used; the experi- 

 ments were performed in a darkroom 

 with a humidity of 50 to 60 per cent. 

 The light source was a Nernst lamp 

 placed approximately 100 cm. from 

 the experimental plants. Rectangu- 

 lar pieces of black paper about 9 cm. 

 high were wrapped around the culture 

 dishes to darken the basal portion of 

 the plants (Fig. 3). Two screens 

 held in place with a rubber band 

 were placed around each glass; one 

 screen extended to the bottom of the 

 glass, while the other could be moved 

 up and down and adjusted to the 

 desired height. Even when these 

 were not closed at the top, control 

 experiments showed that light pene- 

 trating from above did not produce 

 a phototropic curvature. 



Transverse incisions were made 

 with a sharp scalpel 2 to 3 mm. 

 below the tip and extending about to the middle of the coleoptile. 

 The basal portion of the coleoptile was darkened, and only the 

 apical 1 to 2 mm. portion was illuminated vmilaterally. These 

 light exposures produced phototropic curvature in the darkened 

 basal portion only when the incision was on the front side of the 

 coleoptile and not when it was on the back (shaded) side. Next 

 it was shown that the absence of phototropic curvatures in 

 coleoptiles with an incision on the side away from the light was not 

 due to the fact that the experimental plants had lost either their 

 sensitivity to light or their ability to respond in the basal region. 

 Coleoptiles wounded on the side toward the light showed marked 



-Upper shade 



Lower shade 



Fig. 3. — Method of darken- 

 ing Avena seedlings. The cul- 

 ture dish is covered with 

 cylindrical paper shades. The 

 tip of the coleoptile protrudes 

 through an opening in the top. 



