Morphogenesis 211 



1936, 350, 1938, 351, 352, 1939, 353 ; Gardner and 

 Marth, 1937, 348), to initiate the activity of cam- 

 bium (Snow, 1933, 373, 1935, 374-, Avery, Burk- 

 holder, and Creighton, 1937, 341), to regulate the 

 specific branching habit of plants (Lehmann, 1936, 

 360; Delisle, 1937, 347; Goodwin, 1937, 349), to 

 mediate between genes and growing tissues in con- 

 trolling the procumbent vs. erect habit (van Over- 

 beek, 1936, 363, 1938, 365), in producing dwarf ness 

 (van Overbeek, 1935, 362, 1938, 364), to take part 

 in the production of bacterial root nodules (Thi- 

 mann, 1936, 375), and many other responses. 

 (Compare Appleman, 1918, 340; Went, 1928, 379, 

 1932, 381). All these responses are attributed to 

 the same substance. If this view is correct, then 

 the differences in response must depend on corre- 

 lative factors resident in, or in the neighborhood 

 of, the responding cells. A growing, undifferen- 

 tiated mass of cells, such as a callus culture, or a 

 mass characterized by a definite but limited degree 

 of differentiation, such as an excised root, should 

 not only possess fewer of these internal variables 

 but should permit a more rigorous control of the 

 external variables as well. If auxin is truly a 

 specific root-forming hormone (Went, 1929, 380) f 

 it should be possible to set up conditions under 

 which its local application to such a callus culture 

 would regularly induce the local formation of 



