iqiq] 



THOMPSON— COMPANION CELLS 



457 



tion of affairs is illustrated in fig. 6. At the top of the figure, 

 although the bast is old and beginning to collapse, the companion 

 cells are still in continuous rows. This condition is frequently 

 found in roots, young stems, and reproductive axes. It evidently 

 represents a retention of a primitive condition. 



Fig. 6 



Fig. 7 



Figs. 6-7. — Fig. 6, cambium and bast of root of Gnettim scandens showing 

 uninterrupted rows of companion cells; Xiooo; fig. 7, wood, cambium, and bast of 

 young stem of Gnetiim moluccense showing absence of companion cells and wood 

 parenchyma; Xiooo. 



Again, in certain regions of some species companion cells are 

 entirely absent, the bast then consisting entirely of sieve tubes 

 and rays. This is illustrated in fig. 7, from the young stem of G. 

 moluccense. When this is the case, wood parenchyma is also lack- 

 ing, as might be expected from the relationship which has been 

 shown to exist between companion cells and wood parenchyma. 

 The absence of companion cells has been noted in seedlings of 

 several species and in reproductive axes, as well as in roots and 

 young stems. 



