variation, and frequently show the cytological staining reaction charac- 

 teristic of heterochromatin. There are, however, many cases in which a 

 distinction on the basis of staining reaction is impossible. In Tradescautia 

 and Trillium, for example, supernumeraries appear to be largely euchro- 

 matic and in maize they contain both kinds of chromatin (Figure 4-15). 

 Since they tend to persist in a population with a rather constant average 

 frequency per individual, it has been suggested that they perform some 

 rather subtle function which is of value to the species as a whole. In 

 many cases, their behavior during both mitotic and meiotic divisions is 

 erratic. Such behavior not infrequently leads to random elimination and 

 fragmentation. In general, their origin is not known, although it is pre- 

 sumed that they represent kinetochore-containing heterochromatic seg- 

 ments from normal chromosomes. 



(b) 



Figure 4-14. (Contin.) 



Schematic Representation of the Differentially Staining Regions Typically 

 Found in Trillium grandiflorum (left) and T. flexipes (right) After Pro- 

 longed Cold Treatment. (From Giles, R. A. and Wilson, G. B., 1956. "A 

 Cytological and Morphological Study of Two Populations of Trillium grandi- 

 florum (Michx.) Salisb.," Cytologic, 21, Fig. 5, p. 381, and Fig. 4, p. 380.) 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 93 



