GROWTH SUBSTANCES FOR OTHER PHENOMENA 123 



workers (Snow and LeFanu, 19356; Snow, 1935a, 6) used purified 

 auxin a and 3-indole acetic acid in concentrations of 1 or 2 p. p.m. 

 When aqueous solutions of these substances were applied to the 

 upper ends of decapitated Helianthus seedlings, cambial division 

 was stimulated. 



The demonstrated presence of growth substances in sprouting 

 buds and young leaves, and the effectiveness of these substances 

 in promoting cambial activity, lead to the conclusion that the 

 earlier suggestions of a hormone stimulus passing from the leaves 

 to the cambium in the stem is probably true. The precise way 

 in which the auxins bring about increases in both cell size and 

 number in different plants under different circumstances is not 

 yet clear. 



Nastic Movements. — The great volume of evidence concerning 

 the role of growth substance in tropic curvatures suggests strongly 

 that some similar mechanism may be involved also in nastic 

 responses. The differential growth of bilaterally symmetrical 

 organs, in response to stimulation by light, temperature, electric- 

 ity, touch, gases, etc., has been studied in detail (Hennings, 1930; 

 Zimmermann, 1931, 1932; Zeltner, 1932; Schmitz, 1934; etc.), 

 but only recently has the growth-substance explanation actually 

 been tried out experimentally in this connection. Avery (1935) 

 showed that epinasty or hyponasty could be produced reachly in 

 Nicotiana by applying a small amount of growth substance in 

 lanolin to the adaxial or abaxial surface of the petiole (Fig. 37C). 

 The well-known movements of Mimosa have been found to be 

 remarkably influenced also by the application of 3-indole acetic 

 acid to the puKdni (Fig. 37 D) (Burkholder and Pratt, 1936). 

 Nastic movements of Coleus leaves following treatment with 

 3-indole acetic acid (0.5 per cent in paste or solution) have been 

 studied in detail by Fischnich (1935). The amount and duration 

 of response varied directly with the concentration of the applied 

 substance. Hitchcock (1935a, h) has demonstrated recently 

 that a number of different substances, when applied to leaves of 

 tobacco and tomato, are capable of bringing about epinastic 

 movements. The decreasing order of effectiveness for a series 

 of compounds producing epinasty was as follows: naphthalene 

 acetic and indole acetic acids, indole butjo-ic and indole propionic 

 acids, phenylat-etic, phenylpropionic, and phenylacrylic acids. 

 The production of leaf epinasty by ethylene, acetylene, propylene, 



