J. VAN OVERBEEK ' 237 



this powder before being placed in the propagator. The author has 

 found the alcohol dip method (13) also highly practical. The cutting 

 ends are dipped in 50 per cent alcohol containing a relatively high 

 concentration of a suitable auxin, such as 2 mg. of IB per cc. 



Recently attention has been called to the effectiveness of some of 

 the chlorinated phenoxy acids (25). In view of the striking results 

 that this class of synthetic auxins gave in the citrus and fig industry 

 in the regulation of fruit production, it is not unlikely that they may 

 find application in some cases where IB or NAA failed to induce roots. 



Some physiological aspects of auxin action in cuttings. — Cuttings of 

 many plant species have responded to auxin treatment, yet many remain 

 that as yet have not produced roots. A number of woody species such 

 as Hevea, coffee, and mango will form roots when the cuttings are taken 

 from young parent plants but fail when they are taken from older trees. 

 The reasons for failure to root are not well understood at present. It 

 would seem that it is not due to lack of auxins but rather to lack of 

 proper cofactors. Auxins will act only when cofactor requirements are 

 satisfied, or in other words, when the tissue is in reactive condition. 

 There are many examples of this in auxin physiology. Auxins will cause 

 tumor formation only when the tissue has been prepared by Bacillus 

 tumefaciens (see 47). Auxin will cause cambial growth only if the tissue 

 has been made reactive to it; this can be done by treating dormant 

 branches with ethylenechlorohydrin (34). In cuttings too, auxin will 

 cause root formation only when the tissue has been put in a responsive 

 condition. What this involves has been analyzed in Hibiscus cuttings 

 (52) and will be briefly discussed. 



It is known by horticulturists that the presence of leaves on cuttings 

 greatly increases the chances of successful root formation (15). This 

 effect of leaves is demonstrated on coffee cuttings in Figure 5. It will 

 be seen that even though both the leafy and the defoliated cuttings 

 received the same auxin (IB) treatment, only the leafy cuttings re- 

 sponded with root formation. This effect is further elaborated in Figure 

 6 for Hibiscus cuttings. In these the terminal bud was removed in order 

 to avoid complications with the naturally produced auxin. Without 

 leaves and without auxin (IB) treatment no roots were formed, as 

 indicated by point A in Figure 6. Without auxin treatment an increase 

 of the number of leaves left on the cuttings is also ineffective for root 

 formation (curve AB). Defoliated cuttings treated at the base by dipping 



