CYTOGENETICS AND EVOLUTION OF THE GRASS FAMILY 



175 



In addition to the eight characteristics illustrated, four others are worthy 

 of mention, since they show a connection between evolutionary divergence in 

 morphology and in physiology. The first is the way in which root-hairs de- 



FiG. 2. Diagram showing the evolutionary interrelationships of the principal sub- 

 families and tribes of the Gramineae. The irregular outlines represent approximately 

 the relative size and diversity of the groups named in them, and the distance of a 

 group from the star in the center is a rough indication of its degree of evolutionary 

 specialization. The numbers represent single genera or small clusters of genera which 

 are not easily placed in any of the major groupings, as follows: 1, Streptochaeta; 

 2, Pariana; 3, Pharus; 4, Centotheceae ; 5, Anindinelleae ; 6, Uniola, Brylkinia; 

 7, Distichlis, Aeluropns, Vaseyochloa, Ectosperma; 8, Orcuttia; 9, Neostapfia; 10, 

 Aristida; 11, Melica, Glyceria, Schizachne ; 12, Nardtis; 13, Monerma; 14, Scrib- 

 neria. 



velop from epidermal cells (Reeder and Von Maltzahn, 1953; Row and 

 Reeder, 1957). In the Panicoideae as well as in the Chloridoid-Eragrostoid 

 group, the epidermal cells in the region of root-hair development are all 

 alike in size, and any one of them can give rise to a root-hair. Furthermore, 



