128 



INTRODUCTION TO CYTOLOGY 



The diagrammatic representation of a karyotype, as in Fig. 70, is called 

 an idiogram (S. Nawaschin, 1921). One of the most important lines of 

 investigation in modern cytology is that in which the nuclear consti- 

 tutions of related organisms are studied through comparisons of their 

 idiograms. Such studies afford invaluable evidence supplementing that 

 of the geneticist, taxonomist, and student of phylogeny. 



In Crepis, which has come to be one of the most important genera for 

 investigations in this field, -^ there are many species with only three, four, 

 or five pairs of chromosomes. These appear to be of about five dis- 

 tinguishable kinds as regards size and shape, each species having a 

 characteristic complement composed of from three to five of these chromo- 

 some types (Fig. 69). It is observed that a chromosome of a given type 





M./ongf 



A B C) C^ C3 ol, 0I2 £^3 <^4 



M.fen 



%%%% 



A B C| C2 Cj d| dg ^z "4 



M. monsir. 



Illilfiii 



A B C| C2 C3 ol| d2 f^i 0)4 



Fig. 70. — Somatic chromosome complements of three species of Muscari: longipes, tenui- 

 florum, and monstrosum. {After Delaunay, 1926.) 



may vary considerably in length in the complements of the different 

 species. The fact that the order of the species, when arranged in a series 

 on the basis of a progressive lengthening or shortening of a given chro- 

 mosome of the set, may differ from the order in a series similarly arranged 

 for a different chromosome is taken to mean that the chromosome set is 

 not a simple unit, but is rather a harmonious system of autonomous 

 entities which are in some measure independent of one another in their 

 evolutionary alteration. 



In Muscari, Delaunay (1926) finds that the idiograms in three species 

 differ chiefly in the absolute and relative length of the members (Fig. 70) 

 and believes that in many such cases a gradual shortening of the chromo- 

 somes may have occurred during the evolution of the genus. Studies 

 on the karyotypes of several groups, notably the Helleborese,^^ have led 



23 See especially M. Nawaschin (1925a, 1926), Hollingshead and Babcock (1930), 

 and Babcock and Nawaschin (1930). 



^^Lewitsky (1931a); also Senjaninova-Korczagina (1930, 1931) on ^gilops and 

 Vicia, Avdulow (1931) on Agropyrtim, and Lewitsky and Tron (1930) on Bellevalia 

 and Muscari. 



