General Physiology of Plants 



193 



Cellular reactions. While the use of hanging- 

 drop cultures adapted to continuous observation 

 of single cells has so far been little developed as 

 applied to plant tissue cultures, there seems to be 

 no a priori reason why questions of the reactions 

 of individual cells should not be followed by means 

 of such a technique. This has, indeed, been done 

 in a number of scattered instances. The gradual 

 resorption of starch from the plastids in hanging- 

 drop cultures of Stellaria media has been studied 

 in this way (Fig. 54) (White, 1933, 116). The 

 localization of tyrosinase in the plastids of root- 

 hairs and cortical cells has been demonstrated by 



Fig. 54. A typical developmental cycle of individual cells in a 

 culture of the stem tip of Stellaria media maintained for two weeks 

 in a hanging drop of nutrient, showing the progressive increase and 

 then decrease in size of the plastids as the cell grows, reaches 

 maturity, passes into senility, and finally dies. (From White, P. R. 

 1933. Protoplasma 9 : 110, fig. 19, 116.) 



