124 GROWTH HORMONES IN PLANTS 



and carbon monoxide gases (Crocker, Hitchcock, and Zimmer- 

 man, 1935) and by a-naphthalene acetic acid, ;S-naphthalene 

 acetic acid, acenaphthyl-5-acetic acid, fluorene acetic acid, 

 anthracene acetic acid, and a-naphthyl acetonitrile (Zimmerman 

 and Wilcoxon, 1935) is of great interest. Bauguess (1935) has 

 reported similar epinastic effects with other organic acids. 

 Further investigations are needed to discover the mechanisms 

 that lead to these manifestations. 



Root Formation. — The influence of many kinds of substances 

 upon the initiation and growth of roots has been studied by 

 numerous workers, but only in comparatively recent years has it 

 become apparent that the old hypothesis of root-forming sub- 

 stances, proposed by Sachs (1882a), may possibly have some basis 

 in fact (see also Morgan, 1903). Loeb (1916, 1917, 1924) per- 

 formed some very instructive experiments which led to the obser- 

 vation that a stem segment of Bryophyllum forms roots and 

 bends geotropically more readily when a vigorous leaf remains 

 attached to the stem (Fig. 1, Loeb). As a result of his observa- 

 tions, he wrote: ''All these facts suggest a close association if not 

 identity between the root-forming substances and the substances 

 (or hormones) causing geotropic curvatures." The significance 

 of this statement was not realized until the more recent develop- 

 ments in the field of plant hormones. 



The work of van der Lek (1925, 1934), showing that the 

 presence of leaves or buds promotes the formation of roots at the 

 morphological base of a cutting (Fig. 39), led to further investiga- 

 tions of the possible role of hormones in the initiation of roots. 

 Following this line of attack, F. W. Went (1929) found that a non- 

 specific, heat-resisting substance could be extracted from leaves 

 and germinating barley which, when applied to cuttings, promoted 

 the development of new roots (Fig. 39). F. A. F. C. Went (1930) 

 investigated the root-forming substance in Bryophyllum calycinum. 

 The function of root-forming substances was studied further in 

 Impatiens and Acalypha by Bouillenne and Went (1933) (see 

 Bouillenne, 1933). Laibach, Miiller, and Schafer (1934) demon- 

 strated the formation of roots by urine extracts applied to inter- 

 nodes of Tradescantia, Helianthus, and Ligustrum. Laibach 

 (1935) and Fischnich (1935) obtained similar results with 

 3-indole acetic acid on Coleus (Fig. 39). Went (1934a) named 

 the substance that stimulates root formation rhizocaline and 



