Morphogenesis 



223 



Fig. 68. A culture of Nicotiana callus maintained for 8 weeks 

 in an undifferentiated state by repeated transfer on a serni-solid 

 nutrient and then placed for 2 weeks in a liquid nutrient. Reduc- 

 tion in available oxygen supply under the latter conditions has 

 resulted in the formation of differentiated growing points and 

 leaves, x 7.5. (From White, P. R. 1939. Bull. Torrey Bot. Club 

 66: 509, fig. 2, 333.) 



