194 Plant Tissue Culture 



growing root cultures in a nutrient containing 5 

 ppm. of tyrosine, a concentration which permits 

 normal growth (Fig. 60) (White, unpublished). 

 The behavior of localized root cells towards indole 

 acetic acid with the formation of scattered necrotic 

 lesions produced by the bursting of individual cells 

 (plasmoptysis) has likewise been studied (Fig. 

 51) (White, unpublished). The tendency towards 

 differentiation of scalariform cells in the interior 

 of callus cultures (White, 1939, 78) may be looked 

 upon as an example of cellular response to a par- 

 ticular environmental complex, in this case one of 

 low available oxygen content (Figs. 62, 63, 65, 66). 

 Cyclosis can be observed very satisfactorily in the 

 large " callus-blasen " to be found on the surfaces 

 of callus cultures (Fig. 61) (White, 1932, 89), 

 which should facilitate an accurate study of the 

 effects of environmental variables on this process. 

 These serve merely to point out some few places 

 at which physiological processes have been or can 

 be approached by means of tissue cultures. 



Pathology 



In the field of phytopathology, the most obvious 

 of all applications of a new technique for the cul- 

 tivation of phanerogamic tissues is in the main- 

 tenance of such tissues as substrata for either 

 obligate or facultative pathogens. The method as 



