Morphogenesis 217 



some evidence that the differentiation of specific 

 organs, while it may be influenced by specific sub- 

 stances, may also be controlled to some extent at 

 least by concentration gradients of non-specific 

 substances, hence by polar (physical) as well as 

 chemical relationships. The phenomena of polar- 

 ity mentioned at the very beginning of this book in 

 connection with the work of Vochting, Goebel, and 

 others thus offer another field in which the tissue 

 culture technique provides a means of approach 

 not heretofore available. Priestley (1928, 314, 

 1930, 315, Pearsall and Priestley, 1923, 313) 

 attributes the formation of cambium in radial 

 plant bodies to a radially distributed hydrogen-ion 

 gradient. Lund (1931, 247) and Rosene and Lund 

 (1935, 319) have shown that there is a correspond- 

 ing electropotential gradient which can be modi- 

 fied by imposing artificial oxygen-hydrogen gradi- 

 ents. White (1939, 333) has shown that an oxy- 

 gen gradient may be correlated with the response 

 by which undifferentiated callus cultures (Figs. 

 17, 18, 61, 65) are induced to form stems and 

 leaves without the intercession of a caulocaline 

 (Figs. 68, 71). White (1938, 267) and Rosene 

 (1937, 259, 1941, 260) have shown that there is a 

 hydrostatic flow correlated with the electrostatic 

 gradient along excised and intact roots. But it is 

 as yet quite unknown which of all these gradients 



