PLANT HORMONES 205 



concepts were established. In order to present a brief historical picture, a 

 few of the outstanding contributions from the time of Darwin are cited. 



Darwin ^ in 1881 showed that the coleoptiles of Phalaris and Avena curved 

 toward the light and that when only the tip was unilaterally illuminated the 

 influence traveled downward. When the tip was shaded, the remaining 

 stump was unable to make a phototropic response. This definitely showed 

 that the tip of the coleoptile was a place of great importance in connection 

 A\'ith phototropic curvatures in plants, though Darwin did not recognize 

 the influence as being of a chemical nature. 



The beginning of the chemical substance idea with proof to support it 

 goes back to 1907 when Boy sen- Jensen i- 2. 3 started his classic experiments 

 to show that the stimulus (chemical substance) could cross a discontinuity 

 in the coleoptile of Avena. He found that when excised coleoptile tips were 

 replaced on the stump mth a layer of gelatin, phototropic curvatures 

 resulted after unilateral illumination of the tip, as with normal coleoptiles. 

 That is, a substance which was formed in the tip drained into the gelatin 

 and then diffused through this non-living material into the stump, where it 

 accelerated growth on the dark side, causing bending toward the lighted 

 side. Boysen-Jensen also showed that he could intercept the substance by 

 inserting a small piece of mica into the coleoptile. If the mica was inserted 

 on the illuminated side, phototropic curvature occurred normally; if in- 

 serted on the dark side, very little or no bending occurred. Similar experi- 

 ments were performed ^^-ith geotropically stimulated coleoptiles. If mica 

 was inserted in the horizontally placed organs on the upper side of the tip, 

 negative geotropism resulted, as in normal coleoptiles; if the mica was in- 

 serted on the lower side, little or no bending occurred. The interpretation 

 which Boysen-Jensen put upon the results of these experiments was that 

 the stimulus originating in the tip was of a chemical rather than a physical 

 nature, and that it acted in regulating gro^^'th. He thought there was an 

 increased transmission of the groArth-promoting substance on the dark 

 side — a view which is still tenable. 



Paal ^2' ^^ from 1914 to 1918 confirmed Boy sen- Jensen's results and fur- 

 ther showed that if an excised coleoptile tip was replaced on one side of the 

 stump, gro\\i:h was accelerated on that side, resulting in curvature. For 

 this response no special stimulation of the tip was necessary, thus showing 

 that the tip was continually making the growth hormone in the dark also. 

 Paal also demonstrated that the stimulus passed through an interposed 

 gelatinous membrane 0.1 mm in thickness between the tip and the stump 

 of the coleoptile. He concluded from his experiments that the transmis- 

 sion of the phototropic stimulus was brought about by means of a diffusible 

 substance. ^^' p-^^^ 



Stark " in 1921 made the next big advance by investigating the transmis- 

 sion of phototropic, traumatotropic, and haptotropic stimuli. He expressed 

 the sap from coleoptiles and mixed it A\ath agar. Out of the agar plate, 

 blocks were cut and placed unilaterally on decapitated coleoptiles. A sub- 



