1925] Mann: Chromosome Number and Individuality in the Genus Crepis 301 



in the short root of the cotyledon stage, because there is a longer growing 

 area in which the cytoplasm is less dense than it is at the tip, so that 

 the chromosomes spread out more freely and the picture is less obscured 

 by cytoplasmic inclusions. 



Table 3 is a compilation of measurement data for somatic metaphase 

 figures in nineteen species of Crepis. In each case, except japonica 

 and sieberi, ten somatic polar metaphases were drawn with a camera 

 lucida. The magnification of the drawings is 4000 diameters. A 

 moistened thread was placed along the center of the drawing of each 

 chromosome, and then straightened and measured in millimeters. The 

 figures were then placed in columns, the two largest in the first, and so 

 on down to the two smallest. A sample of these records for a five- 

 pair species, alpina, is given below in table 2. 



TABLE 2 

 Actual Measurements of Drawings 



Differences from Average 



It is evident that even measurement by the rather crude method 

 described above gives a fairly definite clue to the individuality of the 

 species. It will also be noted that when the larger figure of each set 

 is compared with the average for the chromosome, obtained by dividing 

 the sum of the ten larger of the twenty chromosomes of one type by ten, 

 the deviations for any one metaphase set are generally in the same 

 direction (+ or — ). (See column headed "Differences from the 

 average.") This deviation indicates that the error of measurement 

 was not sufficient to conceal the fact that the chromosome lengths of a 

 species maintain certain size relations at least throughout the later 

 periods of shortening. It also shows that it is fair to use an average 



