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THIMANN 



actively in America. The beginning of the story goes back to Darwin, who 

 with his son Francis showed that the extreme tip of a grass seedling con- 

 trols its ability to curve toward light. Covering the tip with an opaque cap 

 made it insensitive (fig. 1). The coleoptile, a cylindrical sheath surrounding 

 the first leaves of the grasses and cereals, has been used in all these early ex- 

 periments. Many years after Darwin, Boysen Jensen, in Denmark (1910), 

 cut off the tip of the coleoptile and stuck it back on again with gelatin; the 



Fig. 1. First developments in study of growth hormones. 



rest of the plant could still curve toward light, although if the tip had been 

 completely removed it could not. It was this experiment, falling just within 

 the period of our fifty-year celebration, that gave the first evidence that a 

 material substance, rather than some ill-defined "stimulus," was in control 

 of growth. The explanation was given by Paal (1915) in Hungary, who did 

 not use curvature toward light but stuck the tip on a little to one side; then 

 the plant curved even in darkness (fig. 1). This shows, concluded Paal, that 

 the tip secretes a substance which promotes the growth of the part below it. 

 When the tip is in situ, of course, growth is promoted symmetrically. It 

 follows that the curvature toward light must be due to an unsymmetrical 

 distribution of this growth substance, a disproportionately large amount 

 becoming present on the side of the seedling away from the light, causing that 

 side to grow more and thus the plant to curve. It was Cholodny, in Russia 

 (1926), who formulated this clearly and proposed that all tropisms, that is, 

 curvatures caused by an external stimulus, are due to the displacement of 



