Meristems 



77 



condition in Zea. Both these workers believe that Brumfield's results can 

 better be explained on the assumption of such a small meristematic cen- 

 ter than of a group of apical cells, thus far unobserved. Clowes ( 1950 ) 

 found meristematic layers in the root tip of Fagus that seemed discrete 

 enough to be called histogens. 



Williams (1947) observed in many roots of vascular plants a rather 

 simple pattern of development. The epidermis, hypodermis, and endo- 

 dermis could all be traced back to a small group of cells at the very tip of 

 the plerome. The endodermal row, coming from this, gives rise by re- 



Fig. 4-17. Phleum root tip. Graph showing rate of root elongation (A), average 

 length of epidermal cells (B), and new transverse cell walls (C), at various distances 

 from the root tip. ( From Goodwin and Stepka. ) 



peated tangential divisions to the cortex. This accounts for the fact that 

 the cortex, particularly in its inner layers, is often made up of radial rows 

 of cells (Fig. 4-16). A second small group of cells, just below the plerome 

 tip, produces all the stelar tissues. The progress of cell division, par- 

 ticularly in the surface layer and the cortex, was followed by Wagner 

 ( 1937 ) by means of tracing cell groups, or "complexes," each of which 

 had descended from a single meristematic cell. Sinnott and Bloch (1939) 

 studied cell division in living root tips of small-seeded grasses by camera- 

 lucida drawings. Brumfield (1942) continued this work by the use of 



