Plant Qrowth'Suhstances 



were not peculiar to a limited range of plants: African 

 marigold, tomato, buckwheat, sweet pea, Windsor (broad) 

 bean, sunflower, Jerusalem artichoke, dahlia, sensitive plant 

 (Mimosa pudtca), and goosefoot {Chenopodium album). A 

 number of other unrelated plants were used in later experi- 

 ments. Amongst them was the tropical vine Cissus stcyoides 

 illustrated in Fig. i. 



Epinasty of leaves was induced by application of the 

 substances in lanolin or by injecting a solution by the capil- 

 lary-tube method. When the substance was applied in lanolin 

 to a narrow zone around the stem of an active plant, leaves 

 above and below the zone of application showed downward 

 bending. Epinasty of any single leaf was induced by applying 

 the lanolin paste lightly to a small region on the upper side of 

 the petiole. There was considerable similarity in the response 

 of plants to the cyclic compounds and to ethylene, propylene, 

 acetylene, and carbon monoxide. 



Tomato plant cuttings 5 ins. long with their basal ends 

 immersed in Knop's solution containing the synthetic 

 substances showed declination of the uppermost leaves 

 within a few hours. The use of Knop's solution does not 

 appear to be essential, as Hitchcock (1935) and others have 

 reported a similar response to aqueous solutions of indole- 

 propionic acid. Zimmerman and Wilcoxon (1935) did not 

 determine the absolute lowest concentrations effective in 

 eliciting this rapid response, but they found that the following 

 low concentrations were effective in that way: Phenyl-acetic 

 acid 125, a-naphthalene-acetic acid 20, and indole-butyric 

 acid 4 parts per million of solution. On the other hand, 

 a-naphthyl-acetonitrile did not cause an evident response in 

 tomato plant cuttings until the second day after its applica- 

 tion in solution to the basal ends. Whether applied thus, or 

 injected, or in lanolin, a-naphthyl-acetonitrile evoked final 

 results with tomato similar to those obtained with a-naphthyl- 

 acetic acid. 



Growth substances applied to one side of active shoots 

 caused negative or positive bending, according to the con- 

 centration of the chemical, and the plant species. 



28 



