218 Plant Tissue Culture 



— of pH, of redox-potential, of electrostatic poten- 

 tial, of hydrostatic potential, and of tendency to 

 differentiate — is primary and which secondary. 

 It should be possible in tissue culture to artificially 

 impose or modify any or all of these gradients, 

 except the last, along controlled spatial axes and 

 thus to determine with which other gradients they 

 bear a causal relationship. Brachet has applied 

 this method in animal embryology to the study of 

 effects of imposed oxygen gradients (Brachet, 

 1937, 387; Brachet and Shapiro, 1938, 388), and 

 Huxley (1926, 417, 1927, 418), Gilchrist (1928, 

 412), and Vogt (1932, 434) have applied it to the 

 study of temperature gradients. Fife and Framp- 

 ton (1936, 241) have applied it in plant pathology 

 to the study of the causal factors involved in the 

 orientation of a parasitic insect's proboscis in the 

 tissues of the host, with brilliant results. With 

 plant tissue cultures it is a virgin field except for 

 White's preliminary observations on oxygen gra- 

 dients and differentiation (Figs. 68, 71) (1939, 333, 

 1942, 334) and, in a sense, those of Nobecourt (1939, 

 448) and of Gautheret (1940, 286-288) on bud 

 formation. Yet these methods of approach 

 should prove effective with excised organs (roots) 

 and undifferentiated masses (callus) and perhaps 

 even more with young developing embryos, espe- 

 cially if these can be grown from the fertilized egg. 



