OCCURRENCE AND FORMATION OF GROWTH SUBSTANCES 59 



Determinations of the diffusion coefficient of this "regenerated" 

 growth-promoting substance yielded a mean value of D22 = 0.434 

 as compared with the theoretical value of 0.416 for auxin a, which 

 indicates that "regenerated" growth substance is probably 

 identical with ordinary auxin. 



The investigations of Kogl, Haagen Smit, and Erxleben (1934, 

 Mitt. XII) have led to the identification of auxin as the chemical 

 growth activator in Avena. Further details of this work may be 

 found in Chap. III. That the growth substance found in the 

 Avena seedling is auxin has been substantiated further by Heyn 

 (1935) who found the coefficient of diffusion of the extracted 

 substance approximately the same as that of ordinary auxin. 



Foliage Leaves. — Van der Weij (1933c) demonstrated the 

 presence of growth substance in young leaves of Elaeagnus 

 angustif alius, Thimann and Skoog (1934) in Vicia Faha, and 

 Koning (1933) in Ipomoea. With the Avena technique, Avery 

 (1935) obtained quantitative information in regard to the occur- 

 rence of a growth substance (probably auxin) in the leaves of 

 Nicotiana. It was found to be plentiful in the young leaves 

 and less abundant in the older ones. 



Growth Substance in Hypocotyls, Shoots, and Flower Stalks. — 

 Although the investigations deahng with growth hormones in 

 plant shoots are still few, it is clear, nevertheless, that growth 

 substances are widely distributed in both mono- and dicoty- 

 ledonous stems. 



That the growth of flower stalks is decreased greatly by removal 

 of the inflorescence has been shown by Soding (1926) for Car- 

 damine pratensis, Cephalaria tatarica, Chrysanthemum leucan- 

 themum, Heliopsis laevis, and Helenium autumnale. A similar 

 observation has been reported by Uyldert (1928) for Bellis. 

 Some genera, for example, Symphoricarpos and Rheum, show a 

 growth substance to be present, although they do not react when 

 the Avena coleoptile is employed as the test object (Soding, 

 1935b). Oosterhuis (1931) showed that stem growth in Aspara- 

 gus plumosus and A. Sprengeri is regulated by axillary and 

 terminal buds. These results indicate that growth substances 

 are active in these plants, a point made clearer by the fact that 

 Uyldert (1928) and Nielsen (1930a) showed that the flower stalk 

 of Bellis reacts to the application of growth substance. No 

 definite demonstration of the presence of growth substance 



