16 W. J. Robbins 



In 1910 Boysen Jensen (1) of Copenhagen reported some simple 

 but most illuminating experiments performed in a truly Darwinian 

 manner which clearly indicated that phototropisin resulted from the 

 movement of a water-soluble substance or substances from the illumi- 

 nated tip. He made horizontal cuts about halfway through the coleop- 

 tile tip 3 or 4 mm. from the apex. In some he inserted a thin piece 

 of mica or platinum and then illuminated the tip; others were left 

 with open cuts. If the cut was on the illuminated side, bending 

 toward the light occurred. If the cut was on the shaded side, there 

 was no response. The conclusion drawn was that something which 

 would not penetrate the platinum, mica, or a dry cut passed from 

 the illuminated tip down the shaded side, causing lengthening of 

 that side and the bending toward light. In addition to these experi- 

 ments, Boysen Jensen performed another still more critical and de- 

 cisive. He severed the tip completely, covered the decapitated base 

 with gelatin, and then replaced the tip. When the tip of such a plant 

 was illuminated, the plant bent toward the light. This proved that 

 the effect of the stimulus was transmitted over a discontinuity. 



It is worth noting that these experiments were performed, although 

 Fitting (5) had reported 3 years earlier some similar experiments but 

 with negative results, probably becatise the experimental plants were 

 kept in too moist an atmosphere. Under such conditions the cut 

 filled with exuded water through which the active material diffused. 



Paal (11, 12), Stark (22), Stark and Drechsel (23), Purdy (14), Sod- 

 ing (21), Snow (20), Seubert (19), Boysen Jensen and Nielsen (2), and 

 others confirmed and extended these significant observations. 



Although Boysen Jensen's experiments of 1910 now seem so con- 

 vincing, not everyone was walling to accept them and the interpreta- 

 tions placed upon them. Braimer (3) considered the process of bend- 

 ing in response to light to involve: (1) increase in permeability and 

 increase of growth inhibitors on the illuminated side, (2) movement 

 of a growth inhibitor down the lighted side to the growth zones, (3) 

 inhibition of growth on the illuminated side, (4) bending toward the 

 lighted side. 



Priestley (13) said, "It may be permissible to point out what a pyra- 

 mid of conceptions are struggling to maintain themselves upon one 

 general experimental fact — the phototropic response of a coleoptile 

 stump when its severed apex is replaced and alone laterally illumi- 

 nated." Priestley then points out the frequency of the exudation of 

 drops of water from coleoptile tips (guttation). He assumed the per- 

 meability of the apical tissues of the coleoptile to be increased by light 

 and, therefore, light falling on the apex to increase apical guttation. 

 Lateral light increased guttation on the lighted side, decreased the 



