10 GROWTH HORMONES IN PLANTS 



stimulus conduction and phototropic curvature in tiie basal 

 portion, pro\'ing that they had not been affected by the incision. 

 Coleoptiles with cuts on the back showed curvature in the basal 

 region when the tips were darkened and the bases illuminated 

 from the front. 



Searching for an explanation of the disagreement between Fit- 

 ting's investigations and those of Boysen Jensen, the latter 

 repeated the experiments under the same conditions as those 

 used by Fitting. The plants were kept in a saturated room, and 

 their basal portions were darkened with the type of screen used by 

 Fitting. Boysen Jensen then obtained the same result as Fitting, 

 for the coleoptiles curved toward the light even when the incision 

 was on the back side. Curvature did not occur, however, if a 

 small piece of mica was inserted into the incision in the back side 

 of the coleoptile (Fig. 1). If a thin transverse section from a 

 Calamus stem was inserted in the cut instead of a piece of mica, 

 then the stimulus was conducted past the incision. 



These experiments may be summarized as follows: When a 

 transverse incision is made on the same side of the coleoptile that 

 is unilaterally illuminated, there is invariably a conduction of 

 the stimulus from the unilaterally illuminated tip to the darkened 

 basal portion. If, however, the incision is made upon the back 

 side, stimulus conduction takes place only when the coleoptile is 

 in a saturated atmosphere or after the wound surfaces become 

 closely pressed together. Even in a saturated atmosphere, con- 

 duction is checked by the insertion of a thin piece of mica ; it is not 

 checked by insertion of a thin section of the living Calamus stem, 

 which has large bundles and permits the passage of water and 

 dissolved substances. These results are readily explainable on 

 the assumption that the stimulus is conducted upon the shaded 

 side of the coleoptile and that it can be transmitted across an 

 incision. 



Pfeffer was sceptical of the correctness of this theory, and so 

 Boysen Jensen carried the investigations further. A deep 

 incision was made upon the back side of the coleoptile so that only 

 a very small connection remained between the tip and the basal 

 portion on the front side. In this case, also, a transmission of the 

 stimulus could be demonstrated, provided a close contact existed 

 between the cut surfaces. Pfeffer maintained that so long as the 

 tip is connected by any living substance with the basal region, 



