212 Plant Tissue Culture 



roots. If auxin is truly a hormone of tropic re- 

 sponse (Boy sen- Jensen, 1910, 344), its unilateral 

 application to growing excised roots should regu- 

 larly induce unilateral curvature. Moreover, 

 there have recently been described a number of 

 other specific "hormones," a flower-forming hor- 

 mone (Melchers, 1937, 361; Hamner and Bonner, 

 1938, 356), a stem-forming hormone, "cauloca- 

 line" (Went, 1938, 382), and a leaf -forming hor- 

 mone, "phyllocaline" (Went, 1938, 382), in addi- 

 tion to the root-forming hormone, "rhizocaline," 

 which Bouillenne (1938, 206) and Cooper (1935, 

 345, 1936, 346) believe to be different from the 

 auxin of Went (1929, 380) and Kogl, Haagen- 

 Smit, and Erxleben (1933, 357). If these sub- 

 stances are truly specific for the processes from 

 which they take their names, and if, as we have 

 suggested, the cells of tissue cultures are essen- 

 tially totipotent, it should be possible to set up 

 cultural conditions for such tissues such that ap- 

 plication of a flower-forming hormone would regu- 

 larly induce the formation of flowers, application 

 of rhizocaline would regularly induce the produc- 

 tion of roots, etc. The tissue culture technique 

 seems to offer possibilities for the analysis of 

 these relations, under conditions relatively free of 

 uncontrollable variables. 



On the other hand, there is already available 



