ROOT FORMATION 191 



ment is longitudinal and not transverse, and in physiological 

 concentrations is always polar. It is almost completely 

 stopped by chilling to below 5° C. a 5 cm. section of the 

 cutting. 



The promotion of root formation by auxins has been 

 studied on a great number of different plants; Laibach, 

 Muller, and Schiifer (1934) and Miiller (1935) found that 

 Tradescantia internodes and Helianthus hypocotyls were 

 induced to form roots by urine or orchid pollen applied in 

 the form of lanohne paste; the effect was doubtless due to 

 the auxin. Laibach (1935), Fischnich (1935), and Laibach 

 and Fischnich (1935) subsequently obtained roots on intact 

 plants of Coleus, Vicia Faba, and Solanum lycopersicum by 

 appHcation of indole-acetic acid in lanoline. Hitchcock 

 (1935, 1935a) and Zimmerman and Wilcoxon (1935), also 

 working with intact plants, induced root formation on 

 stems by local application of lanoline pastes containing 

 indole-acetic acid and various other related substances. 

 Crocker, Hitchcock, and Zimmerman (1935) then compared 

 the long-known activity of ethylene, carbon monoxide, and 

 other gases in promoting root formation with the action of 

 the auxins. They concluded that ethylene may itself act 

 as a hormone (c/. X M). However, the effect of ethylene 

 on growth by elongation is to inhibit and not to promote it. 

 Further, Michener (1935) has shown that in Pisum cut- 

 tings, which root vigorously in response to auxin, no roots 

 are produced by ethylene, nor does ethylene increase the 

 number of roots produced by a given auxin treatment. In 

 Salix cuttings, ethylene alone has a small effect in increasing 

 the number of roots, but if they are treated with auxin, its 

 effect is greatly increased by ethylene treatment. Thus 

 ethylene only seems to be effective in the presence of auxin. 

 Since the experiments of Crocker, Hitchcock, and Zimmer- 

 man were carried out on green plants in the light, rich in 

 auxin, it is highly probable that the action of ethylene 

 which they observed was through its effect upon the auxin 

 already in the plant. 



