204 PHYTOHORMONES 



G. Practical Applications 



Study of the use of auxin for rooting cuttings of commer- 

 cially important plants has been begun by Cooper (1935). 

 He obtained excellent root formation on cuttings of lemon, 

 Acalypha, Lantana, and fig by apical application of auxin 

 in lanoline. Subsequently the fact, mentioned above, that 

 high concentrations of auxin cause root formation when 

 applied at the base has been utilized successfully by Hitch- 

 cock and Zimmerman (1936) and by Cooper (1936) for 

 cuttings of Ilex, Taxus, Hibiscus, Pachysandra, lemon, 

 Chrysanthemum, and some other plants (see Figure 57). Pri- 

 vate reports from a number of horticulturists have already 

 extended this list considerably. In general the highest non- 

 toxic concentration of indole-acetic acid, dissolved in water, 

 will give the best results. This concentration varies for dif- 

 ferent plants, and is lowest for green cuttings. A treatment 

 with 0.2 mg. per cc. for 12 to 24 hours can be recommended, 

 but before large-scale applications are made the toxic limit 

 for each species to be treated should be ascertained. Indolo- 

 butyric and naphthalene-acetic acids are also effective. For 

 treatment by the lanoUne method a concentration of about 

 1 mg. indole-acetic acid per gram of lanoline is satisfactory. 



It is clear that some of the procedures used by gardeners 

 to induce root formation have their foundation in the pro- 

 duction and movement of auxin (see Bouillenne and Went, 

 1933). A curious example of this is found in the insertion, 

 into the apical split end of cuttings, of a germinating wheat 

 seed, as practised in parts of Holland and Scotland. As to 

 more general principles, in non-deciduous plants leafy 

 cuttings are always used, probably because no auxin is 

 stored in their stems. In deciduous plants leafless cuttings 

 are preferred because of the difficulty with water supply to 

 the leaf, but here the bud acts as auxin supply. As one of 

 numerous examples might be mentioned the holly cuttings 

 of Zimmerman and Hitchcock (1929); leafless cuttings of 

 the deciduous Ilex verticillata will root, but those of the 

 evergreen varieties will not. 



