Plant Qrowth'Suhstances 



merman, Hitchcock, and Wilcoxon (1936, p. 109). Though 

 this paper was mainly devoted to discussing the effectiveness 

 of esters of the cycHc acids, the mode of root-formation by the 

 acids, esters, and the one nitrile tested was similar. Fig. 2, 

 indeed, presents evidence of the root-initiating capacity of 

 both a-naphthyl-acetic acid (left) and its ethyl ester (right), 

 applied to a leaf and one side of the stem of each plant. Fig. 3 

 shows an especially beautiful demonstration, the plant on the 

 left being in this case untreated, that on the right having been 

 treated with a low concentration of methyl 3-indole-acetate 

 only ten days before the photograph was taken. The plant 

 shown in Fig. 3 belongs to an uncommon species — Kalanchoe 

 daigremontiana — which has the property (useful for teaching 

 purposes) of producing white roots in a dry atmosphere. 

 The initiation of roots on woody plants such as apple, 

 holly, and Cupressus is a problem which has not yet been 

 solved satisfactorily in all respects; there remains consider- 

 able scope for experimentation. The lanolin-smear method 

 appears to be less generally effective with the woody plants 

 than with such plants as tomato and Kalanchoe. F. W. Went 

 (1934) and Cooper (1935) suggested that the apical ends of 

 cuttings, and not the basal ends, should be treated with 

 preparations of the growth substances, in accordance with the 

 idea that "auxins" can move only downward in stems. Other 

 workers have shown that treatment of the basal ends of 

 cuttings with lanolin, water or dust preparations of synthetic 

 growth substances can induce root formation. 



Practical Methods of Using the Substances 



With respect to the practical use of solutions of growth 

 substances for inducing root-formation in woody cuttings, 

 Hitchcock and Zimmerman (1936) recommend leaving 

 the cut (basal) ends of cuttings for twenty-four hours or 

 several days in aqueous solutions containing relatively low 

 concentrations in preference to using higher concentrations 

 for periods shorter than twenty-four hours. Desirable con- 

 centrations of what they call the three principal substances 

 (indole-acetic, indole-butyric, and a-naphthyl-acetic acids) 



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