PLANT HORMONES 215 



gram of talcum powder in which the basal end of the cutting is dipped and 

 then planted in the rooting medium. Enough of the mixture clings to the 

 basal end of the stem to induce roots the same as the other treatments. 



All three of the methods described have their supporters among the ama- 

 teurs and commercial growers. It is generally recommended that interested 

 growers familiarize themselves with all the methods and then select the 

 one which is most satisfactory. Also, for the average grower it is better to 

 use a recommended commercial preparation than to attempt to measure 

 and prepare the substances in small quantities. Details for propagating 

 plants are given in several Boyce Thompson Institute papers.^^- ^^"^^ 



i3-Indolebutyric acid, which accelerates callus formation, has been found 

 effective also for accelerating union between the scion and the stock where 

 the grafting method is used. The basal end of the scion is dipped into a 

 water solution containing 50 to 80 mg of /3-indolebutyric acid per liter of 

 water and then grafted as usual. 



Lily scales which are used for growing bulbils may be treated by any of 

 the methods described for cuttings and planted in the usual way. jS-Indole- 

 butyric acid accelerates rooting of the scale, thus keeping it in good condi- 

 tion until the bulbils are formed. Though various claims have been made, 

 it has not been proved that any of the known growth substances to date 

 initiate shoot buds. 



Saintpaulia leaves used for propagation purposes can be induced to form 

 many roots by treatment with well-known growth substances. In no case, 

 however, has acceleration of buds been proved. Generally, bud initiation 

 is inhibited by the treatment. 



Preharvest apple drop. The practical use of growth substances has been 

 greatly extended by the work of Gardner et al}^ in connection with abscis- 

 sion layers and premature falling of fruit. Of all the substances listed in 

 Tables 23 and 24, it appears that a-naphthaleneacetic acid is the most 

 effective substance for preventing preharvest apple drop. The water solu- 

 tion containing 10 to 50 mg of a-naphthaleneacetic acid per liter of water 

 is sprayed on the entire tree at the time the apples begin to fall prematurely. 

 The chemical prevents the separation of the abscission layer, thus causing 

 the apples to stay on the tree. It has been reported that trees normally 

 losing 50 to 75 per cent of their apples before harvest time have been 

 reduced to 5 to 10 per cent drop. Many of the active substances listed in 

 this chapter are effective for preventing abscission of leaves, but so far 

 none has proved as effective for preharvest apple drop as a-naphthalene- 

 acetic acid. Not all varieties of apples respond alike. For example, the 

 Mcintosh variety is very resistant, the results being negative in practically 

 all reports. Williams variety is perhaps among the most sensitive, the treat- 

 ment preventing practically all the apples from dropping. The effective- 

 ness for other varieties fluctuates between these two extremes. Variations 

 are also reported for a given variety in different locations over the 

 country. 



