Plant Qrowth'Suhstances 



substance was obtained when the first bath was at 25° C. 

 Addition to the bath of small amounts of one-thousandth or 

 one-hundredth normal^ solutions of indole-acetic acid mark- 

 edly increased the number of roots formed under the same 

 conditions. 



Forest Trees 



Some work with woody cuttings has already been cited. 



Only two other papers will be mentioned here. They are 

 given as illustrations of the type of work that has been done, 

 of the variability of the results obtained, and of the fact that 

 generalization is not possible. See also p. 39. 



Snow (1938) reported that (i) dormant cuttings of trembling 

 and large-toothed aspen (Populus tremuloides Michaux and P. 

 grandidenta Michaux) can be rooted to the extent of at least 

 65 per cent, (of cuttings treated) by dipping them into a 

 solution of 10 milligrams of indole-butyric acid per litre (one 

 part in 100,000) for about 27 hours; (2) maximum rooting is 

 obtained when dormant cuttings of these species are taken just 

 before the buds are beginning to burst in the spring; (3) 

 abundant callus is not indicative of potential root-formation ; 

 (4) new hybrids of white poplar treated similarly behaved 

 variously, some rooting to the extent of 70 per cent., others 

 not at all. 



Afanasiev (1939) tried the effect of indole-butyric acid on 

 the rooting of greenwood cuttings of deciduous trees, using 

 two species each of Betula, Acer, and Populus. No rooting was 

 obtained with hard maple and aspen, but in four other species 

 30-60 per cent, of the plants rooted in about 7- 11 weeks after 

 treatment with a 2 in 100,000 or weaker solution for 6 to 24 

 hours. 



Sub-tropical Species 



Since most of the work on the applications of synthetic 

 growth-substances has been done in temperate regions, many 

 workers in warmer regions will be interested in this section. 



^For explanation of the term normal, see p. 55. 



40 



