Measurements of distances between telocentric chromosomes, either homologous or 
representing the opposite arms of a metacentric chromosome (complementary 
telocentrics), were made at metaphase in root tip cells of common wheat carrying 
two homologous pairs of complementary telocentrics of chromosome 1 B or 6 B 
(double ditelosomic 1 B or 6 B). The aim was to elucidate the relative locations 
of the telocentric chromosomes within the cell. The data obtained strongly 
suggest that all four telocentrics of chromosome 1 B or 6 B are spacially and 
simultaneously co-associated. In plants carrying two complementary (6 B (S) and 
6 B (L)) and a non-related (5 B (L)) telocentric, only the complementary 
chromosomes were found to be somatically associated. It is thought, therefore, 
that the somatic association of chromosomes may involve more than two 
chromosomes in the same association and, since complementary telocentrics are as 
much associated as homologous, that the homology between centromeres (probably 
the only homologous region that exists between complementary telocentrics) is a 
very important condition for somatic association of chromosomes. The spacial 
arrangement of chromosomes was studied at anaphase and prophase and the polar 
orientation of chromosomes at prophase was found to resemble anaphase 
orientation. This was taken as good evidence for the maintenance of the 
chromosome arrangement - the Rabl orientation - and of the peripheral location 
of the centromere and its association with the nuclear membrane. Within this 
general arrangement homologous telocentric chromosomes were frequently seen to 
have their centromeres associated or directed towards each other. The role of 
the centromere in somatic association as a spindle fibre attachment and 
chromosome binder is discussed. It is suggested that for non-homologous 
chromosomes to become associated in root tips, the only requirement needed 
should be the homology of centromeres such as exists between complementary 
telocentrics, or, as a possible alternative, common repeated sequences of DNA 
molecules around the centromere region.