PLANT CELL GROWTH AND NUTRITION 



481 



Figure 12. Appearance under the electron microscope of the wall of cul- 

 tured carrot cells. (From a photograph by Muhlethaler of material supplied 

 by the author.) 



plants, can form only under the influence of the leucoplast as a living 

 inclusion in the cell. 



Redistribution of solutes to centers of growth 



If salt accumulation in cells follows the lead of growth, it should 

 follow that the redistribution of solutes within the plant body should 

 be related to centers of growth. A powerful "sink" where cells are grow- 

 ing will draw solutes over long distances from a "source" at which cells 

 have completed their development and can only release their solutes. 

 Moreover, the movement would not only be determined in its direction 

 by these growth events; it must inevitably be responsive to the ability to 

 accumulate at the "sink" and to release solutes at the "source." Certain 

 centers of growth and salt accumulation in angiosperms have been 

 recognized, and solutes may be redistributed to these after their initial 

 absorption and accumulation by roots {cf. Steward, 1954). 



