12 GROWTH HORMONES IN PLANTS 



side. A curvature developed toward the side with the mica 

 plate. By inserting platinum foil between the tip and the base, 

 transmission of the phototropic stimulus was inhibited. In 

 this way, Paal proved that the stimulus was not electrical. 



Stark and Drechsel. — An advance in technique was reported by 

 Stark (1921a) who appUed unilaterally to a coleoptile stump a 

 block of agar which contained expressed sap from Avena coleop- 

 tiles (Fig. 1). Curvature followed. Stark and Drechsel (1922) 

 perfected another method in which the coleoptiles were unilat- 

 erally cut a few millimeters from the tip with a sharp scalpel, the 

 tip was then removed, and the exposed primary leaf was carefully 

 pulled out, leaving the empty coleoptile attached to the seed. 

 By this method they investigated transmission of the phototropic 

 stimulus from a stimulated tip into the base of the same individ- 

 ual or different individuals of the same species or different 

 species and genera. The desirability of a chemical analysis of the 

 growth-promoting substance and a comparative study of photo- 

 and geotropic reactions as a means of obtaining a unified explana- 

 tion of the two responses was recognized. Stark's investigations 

 will be discussed at some length later, as will those of Seubert 

 (1925), who found that saliva, diastase, and malt extract were 

 growth promoting. 



Formulation of the Growth-substance Explanation. — After 

 Boysen Jensen's results had been confirmed, there was some 

 criticism of his experimental methods, and the suggestion was 

 made that he had only hypothecated the existence of a growth- 

 promoting substance in the Avena coleoptile, while Paal had 

 demonstrated it. Boysen Jensen discusses this as follows. 



It is certainly not superfluous to see whether or not my experiments 

 in the years 1909-1910 were without objection and what conclusions 

 were drawn from these investigations. 



With regard to the first point, Padl maintained that the general set-up 

 in my experiments with darkened coleoptiles was not reliable and that 

 the experimental plants were insufficient in number. 



In considering the first point, it must be said that over one-half of my 

 experiments were carried out with paper tubes as used by Rothert and 

 Fitting. These are more reliable than the method used by Paal, and 

 the usefulness of this method has been proved by controlled experiments 

 both by Fitting and by myself. In addition, I could demonstrate a 

 conduction of geotropic stimulus from the separated tip to the basal 



