446 



Morpho genetic Factors 



Chromosome length (relative units) 



Fig. 19-17. Correlation between average cell volume (of dermatogen cells in root tip) 

 and average total chromosome length (chromatin mass) in 13 species of Crepis. The 

 length of bar for each species indicates the probable error for that determination. 

 ( Redrawn from Navashin. ) 



of chromosomes was concerned with cell size. This presumably was de- 

 termined by the genes the chromosomes carried (Table 19-1). 



Table 19-1. Average Cross-sectional Area (in Square Microns) of Primary 



Dermatogen Cells in Root of Diploid Crepis tectorum and in 



Four of Its Trisomies * 



Diploid 260.0 ± 5.3 



Trisomic A 235.5 ± 3.6 



Trisomic B 282.6 ± 4.0 



Trisomic C 271.0 ± 2.4 



Trisomic D 228.5 ± 5.5 



° From Schkwarnikow (1934). 



A somewhat similar result was found by Sinnott, Houghtaling, and 

 Blakeslee (1934) in the primary chromosome mutants of Datura stra- 

 monium. This species has 12 pairs of chromosomes, and in each mutant 

 race there are three representatives of one of these chromosomes. The 12 

 trisomic races differed very considerably from one another, both ex- 



