8o MITOSIS : THE VARIATION OF THE CHROMOSOMES 



intersterile or yield sterile hybrids, a fact which the systematist may 

 or may not take into consideration. 



(ii) General. We now see the whole capacity for variation in the 





^ 



¥^ ^^ M 



C G D 



Fig. 20. — Variation found in the chromosome complement in species 

 of a natural group, the Tradescantieae. A. Rhceo discolor, 

 2M = 12. B. Cyanotis zenonii, 2n = i6 (by fragmentation). 

 C. Spironema fragrans, 2n =12 (by loss of parts). D. Zehrina 

 pendula, 2n = 24 (by polyploidy). E. Coleotrype natalensis, 

 2n = 36. F. Commelina ccelestis, 2n = 90 (by polyploidy and 

 fragmentation). G. Cyanotis somaliensis, 2n = 28 (by 

 fragmentation). F and G show reduced chromosome width. 

 G shows reduced bulk, x 1,800 (from D.,1929 c, and unpublished ; 

 cf. Matsuura and Suto, I935)- 



chromosome complement as it is shown to us at mitosis. The 

 chromosomes are subject to two kinds of change : change governed 

 directly by the activities of the part affected, and change governed 

 by the reaction of the genotype as a whole, which usually has a 



