GROWTH SUBSTANCES FOR OTHER PHENOMENA 127 



The latter workers have shown that in horizontally placed stems 

 there is a displacement of root-forming substances from the upper 

 to the lower side just as is the case for the cell-elongation sub- 

 stances. Initiation of roots is brought about in a region where 

 longitudinal transport of the substance is prevented, e.g., by a 

 wound. The synthesis of root-forming substances in leaves 

 through the action of orange-red light and their polar transport 



Fig. 41. — Rooting effects obtained by applying synthetic substances to plants. 

 A, Nicotiana. An overhanging slit portion of the stem is immersed in 0.05 per 

 cent indole butyric acid. The acid travels upward in the transpiration stream 

 and causes adventitious roots to be produced along the stem. B, Lycopersicon, 

 adventitious roots on stem 14 days after treatment with 2.0 per cent 3-indole 

 acetic acid in lanolin. {From Zimmerman and Wilcoxon, 1935.) 



have been pointed out recently by Went (19356). Attention 

 should be called to the fact that the same substances that 

 cause formation of roots also inhibit their growth in length. 



That the auxins are not unique in their ability to produce new 

 roots has been shown by a series of contributions from the Boyce 

 Thompson Institute. The initiation and stimulation of adven- 

 titious roots by treatment with appropriate doses of ethylene, 

 acetylene, propylene, and carbon monoxide gases have been 

 demonstrated in some 15 species and varieties of plants (Fig. 39) 

 (Zimmerman and Hitchcock, 1933; Zimmerman, Crocker, and 



