582 GROWTH HORMONES 



(1935) have shown that a number of substances will promote root formation 

 when applied to stems in lanolin paste or when injected as aqueous solutions 

 (Fig. 130). The most effective of these compounds in root formation were 

 alpha-napthalene acetic acid, and indole butyric acid. Neither of these com- 

 pounds is known to occur in plants, but both cause curvatures when tested on 

 oat coleoptiles (Avery, et ah, 1937). 



Several of these compounds, notably indole-3-acetic acid, alpha-naphthalene 

 acetic acid and indole butyric acid, are now being widely used in a practical 

 way for speeding up the rooting of commercially important plants. The 

 usual procedure is to immerse the basal end of the cuttings in an aqueous solu- 

 tion of the substance used (Hitchcock and Zimmerman, 1936). Such treat- 

 ments not only speed up rooting, but induce the production of an increased 

 number of roots, and cause emergence of roots to take place over a greater area 

 of the stem. Treatment for about 24 hours with a solution containing 4 to 

 20 mg. per 100 cc. of any of these compounds has been found to be effective 

 on a number of different species. 



Root formation is influenced by a complex of factors, all of which must 

 be considered in evaluating the effect of auxin. Carbohydrates and other foods 

 are necessary and evidence is gradually accumulating that certain other hor- 

 mone-like substances are required. Vitamin B^^ (thiamin), for example, is 

 apparently necessary for root formation (Robbins and Schmidt 1938; Went, 

 et al. 1938). Even when auxin is present in adequate quantities production 

 of roots will not occur if one of these other factors is limiting. 



Other Effects of Auxins. — The physiological phenomena described in 

 this chapter are not the only ones known to be influenced by auxins. Other 

 important physiological effects of these substances are described in Chapters 

 XXXIV, XXXVI, and XXXVII. 



Other Plant Hormones. — Despite their manifold effects it seems clear 

 that the auxins are only one of a number of types of hormones which occur in 

 plants. For example, Haberlandt (1921) showed that if a freshly cut plant 

 tissue is immediately rinsed with water very few cell divisions occur in the 

 cells adjacent to the wound. However if the wounded area is smeared with 

 finely ground tissue of the same species considerable cell division occurs. This 

 result led him to postulate the presence of substances which he called "wound 

 hormones" in injured tissues which are required if cell division is to take place 

 in the cells bordering a wound. Bonner and English (1938) have succeeded 

 in extracting from dried bean pods and in partially purifying a compound 

 which acts like a "wound hormone." They propose the name trau?natin for 

 this substance and have also devised a quantitative physiological method of 

 testing for its presence. 



