220 Plant Tissue Culture 



tions of the type suggested by Sachs (1880, 370, 

 1882, 370, 1893, 320, 371), could be brought within 

 the scope of experimental analysis (see Went, 

 1941,555). 



Another field in which this technique might pre- 

 sumably find valuable applications is the study of 

 the physiological aspects of heterosis and incom- 

 patibility. Bobbins has already studied the 

 growth behavior of excised roots from certain 

 hybrid tomatoes, compared with that of roots 

 from the two inbred parents (1941, 318). In cases 

 where the hybrid is ' ' physiologically sterile, ' ' due 

 to unsatisfactory endosperm development, as in 

 certain peaches (Tukey, 1933, 327; White, 1928, 

 332), the tissue culture method has proved its 

 value in permitting the propagation of embryos 

 which would otherwise perish. Where adventi- 

 tious embryos arise aposporously from the nucel- 

 lar wall, as in many Citrus fruits, Taraxacum, 

 certain Daturas, etc. (Blakeslee and van Over- 

 beek), the technique should prove valuable, not 

 only in propagating these embryos in refractory 

 cases, but also in determining the causes of poly- 

 embryony and aposporous embryogony. More- 

 over, where "incompatibility" is suspected of 

 being physiological rather than anatomical, the 

 cultivation of tissues of the two incompatible mem- 

 bers together in vitro might well throw light on 



