Plant Qrowth'Suhstances 



3-indole-acetic acid ... 7s. gd. per one-gram tube 



3-indole-butyric acid ... I OS. od. ,, ,, 



a- or p-naphthyl-acetic 



acid ... ... ... 6s. 8d. „ ,, 



Since a usual strength for a solution of these substances is 

 5 parts, more or less, of the pure substance in 100,000 parts 

 of water, one gram will make 20,000 grams (20 litres) or 

 about sixteen quarts of usable solutions, the cost of a quart 

 thus being pennies only. Small users can buy still smaller 

 quantities: at least some of the substances are sold in tubes 

 containing one-tenth of a gram, and costing about a shilling 

 each. 



The Choice of Substance 



There is not much difference between the efficiency of 

 indole-acetic, indole-butyric, and a-naphthyl-acetic acids. 

 No one of them is "best" in all circumstances or for all plants, 

 and no definite figures in support of this or that substance 

 can be given. Dr. M. A. H. Tincker in his 1938 paper, of 

 which the reference is given on p. 36, reported that both 

 indole-acetic acid and a-naphthyl-acetic acid brought about 

 the formation of roots on about 55 per cent, of the species he 

 tried, and on about 40 per cent, of those species known to be 

 difficult to root in the ordinary way. These figures will give 

 you an idea of your chance of success in bringing about good 

 rooting of cuttings : with any one species the synthetic growth- 

 substances are about as likely as not to be effective. Even 

 when they bring about rooting of a given species, it may 

 happen that not every cutting of a batch will root. The 

 synthetic growth-substances will not work miracles. What 

 they do is to hasten root formation on some species (after 

 planting out into rooting compost) or to increase the length or 

 number of roots on other species. 



Published work with indole-butyric acid has not taken 

 the form that would enable calculations to be made com- 

 parable with the above, but it seems to be about as effective as 

 the other two synthetics. There is, however, a growing im- 



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