238 



PLANT GROWTH SUBSTANCES 



them in 50 per cent alcohol containing 2 mg. of IB per cc. also failed 

 to root (point C). However, when this auxin treatment was given to 

 leafy cuttings, roots were formed in proportion to the number of leaves 

 present on them (curve CD). Leaves, then, provide factors which 

 together with auxin cause root formation. At present we have also 

 learned something about the chemical nature of these factors. 



In order to investigate which factors were contributed by the Hibiscus 

 leaves to the cutting, defoliated Hibiscus cuttings were treated at the 

 base with a number of compounds which conceivably might be auxin 

 cofactors. This was followed by the usual IB treatment. One of the first 

 compounds to be tried as a possible cofactor was sucrose. It has been 

 known for years to stimulate root formation on cuttings (17), and was 

 indeed found active in the defoliated, IB-treated ////^/Va/^ cuttings (52), 

 Yet the number of roots thus formed was still far below that formed 



A 



LEAVES 



Figure 6. Graph showing how root formation on Hibiscus cuttings depends 

 upon both auxin and the presence of leaves. AB shows that without auxin 

 leaves are ineffective. CD shows that in auxin treated cuttings (IB treatments 

 as in Figure 5) the number of roots formed increases with the number of 

 leaves left on the cutting. F shows that the effect of leaves in auxin treated 

 cuttings can be replaced by treatment at the base with 4 per cent sucrose and 

 0.1 per cent ammonium sulfate; sucrose and 10 ppm. arginine has a similar 

 effect. AE (dotted line) shows that without auxin treatment sucrose and 

 ammonium sulfate are ineffective, even in the presence of leaves. (52). 



