PLANT PHYSIOLOGY 53 



horizontal," Comp. rend. acad. sci., 172, 608-610, 1921). This 

 worker found that the stems of lentils germinated in the dark 

 took up a horizontal position, and that if displaced into the 

 vertical they curve so as again to lie horizontally. If they are 

 illuminated, however, they soon bend into a vertical position 

 and grow upwards in the ordinary way. 



With regard to the reaction of plants and plant organs to 

 light, one of the questions that has received some attention is 

 whether it is the direction of the light, or differences in the 

 intensity of illumination, bringing about differences in growth, 

 that determines the phototropic reaction. The latter view, 

 due to Blaauw, has found supporters in W. Nienburg (" Uber 

 phototropische Kriimmugen an langseitig zum Teil verdunkelten 

 Avenakoleoptilen," Ber. deut. hot. Ges., 36, 491-500, 191 8) and 

 J. Buder (" Neue phototropische Fundamental versuche," Ber. 

 deut. hot. Ges., 38, 10-19, 1920). Nienburg repeated the experi- 

 ments of Charles Darwin of exposing etiolated and partially 

 blackened seedlings of Avena and Phajus to light, using instead 

 of the light from a window an artificial source of light, and 

 screens throwing a sharp shadow so as to prevent diffuse light 

 from reaching the shaded side. Out of 125 coleoptiles of 

 Avena used, 91 curved towards the illuminated side and away 

 from the darkened side, but only two actually curved towards 

 the source of light. This result is held to afford confirmation 

 of Blaauw's view that it is the quantity of light, and not the 

 direction of the light, which determines the curvature. Buder 

 also worked with Avena, and in addition with Phycomyces. 

 Thin pencils of light were allowed to fall from above on to one 

 side of the coleoptile apex of Avena, as a result of which 

 curvatures resulted which were independent of the direction 

 of the light. Also illumination of the coleoptile from either 

 side brought about the same curvature, although the direction 

 of the light is reversed in the two cases. Other writers on this 

 question are H. Sierp (" Uber den Einfluss geringer Lichtmen- 

 gen auf die Zuwachsbewegung der Koleoptile von Avena sativa," 

 Ber. deut. hot. Ges., 37, 123-128, I9i9)and H. Lundeg^rdh (" Die 

 Bedeutung der Lichtrichtung fiir den Phototropismus," Ber. 

 deut. hot. Ges., 37, 225-236, 191 9), the former a supporter to 

 some extent of Blaauw's theory, the latter a supporter of the 

 " light direction " theory. 



An interesting paper by A. Paal (" Uber phototropische 

 Reizleitung," Jahrb. f. wiss. Bot., 58, 406-458, 191 8) dealt with 

 the conduction of the phototropic stimulus in the coleoptile of 

 Avena. The upper 3 to 5 millimetres of an etiolated coleoptile 

 of Avena sativa 1-5 to 2 centimetres long was cut off, and then 

 fastened on again into its original position by means of 10 per 

 cent, gelatine ; the upper part was then illuminated on one 



