10 PHYTOHORMONES 



regions, which then react. "We must, therefore, conclude 

 that when seedlings are freely exposed to a lateral light, some 

 influence is transmitted from the upper to the lower part, 

 causing the latter to bend " (p. 474). In regard to geotropism 

 of roots, he concludes ''that it is the tip alone which is 

 acted on, and that this part transmits some influence to 

 the adjoining parts, causing them to curve downwards" 

 (p. 545). At first Darwin's statements met with much 

 opposition, but Rothert (1894), working with phototropism 

 of shoots, confirmed completely the separation between the 

 zones which perceive and those which react. The connection 

 between these processes was envisaged by Fitting (1907) as 

 being due to a polarity set up by the light stimulus, which 

 "spread out" from cell to cell. 



Fitting's work was closely followed by the experiments 

 of Boysen Jensen (1910, 1911, 1913) which showed that a 



Fig. 3. First experimental demonstration of transmission of the phototropic 

 stimulus across a wound gap. Five Avena coleoptiles were decapitated, and the 

 tips replaced upon the three plants to the left, the wound being covered with 

 cocoa-butter. Two plants to the right as controls. On illumination of the tips 

 only, from the left, the plants with tips replaced show curvature in the base, 

 the controls not. (From Boysen Jensen, Ber. d. hot. Ges. 3/ : 559-566, 1913; 

 and Growth Hormones in Plants, tr. Avery and Burkholder, McGraw-Hill, 

 1936.) 



phototropic stimulus can be transmitted across a wound gap. 

 Boysen Jensen cut off the tips of Avena coleoptiles and stuck 

 them on again with gelatin (see Figure 3). He then illumi- 

 nated the tip only and showed that curvature appeared 

 not only in the tip but also in the base. From this he con- 

 cluded that "the transmission of the irritation is of a mate- 

 rial nature produced by concentration changes in the coleop- 



