INTRODUCTION AND HISTORICAL SKETCH 



13 



This 



region even in plants that had not been subjected to light, 

 objection can then surely be overruled. 



The next question is whether a sufficient number of experiments were 

 carried out. I disregard here all incision experiments and confine 

 myself to the experiments mth removed and replaced tips. Thirty-five 

 plants were used for the experiments on phototropic stimulus con- 

 duction; of these 2.5 reacted positively, 9 remained straight, and 1 showed 



Fig. 4. — Phototropic curvature in decapitated Avena coleoptiles. The tips 

 were replaced upon the three plants at the left, while the two plants at the right 

 served as controls. The plants with tips curved toward light. 



a weak negative curvature. Furthermore, a geotropic stimulus con- 

 duction was demonstrated in 12 plants (number of experimental plants, 

 12). Information as to the strength of the curvatures is given by 

 photographs in my papers of 1911 and 1913 (Figs. 4 and 5). The 

 curvatures are so strong and the percentage of curved plants so high 

 that the published material is sufficient to prove a stimulus conduction 

 from the removed tip to the basal portion. 



Fig. 5. — Geotropic curvature iu decapitated Avena coleoptiles. The tips were 

 replaced upon the two plants at the left, while the two plants at the right served 

 as controls. The plants with tips curved away from the force of gravity. 



Relative to the conclusions that I drew from the experiments pub- 

 lished in 1911, I should like to cite the following from that paper: 



"We are able, therefore, to represent the facts more or less as follows. 

 Under the influence of the action of unilateral light there is produced a 

 differentiation between the front and the back faces at the tip of the 

 coleoptile (and not, as Fitting thought, in the individual cells of the 



