CYTOLOGY AND TAXONOMY 



245 



devised for iininediate i)urpo.ses, reineinl)ering that they, like other 

 concepts based on growing evidence, are always subject to further 

 improvement. It is beyond the scope of this book to discuss experi- 

 mental taxonomy in detail. Our purpose is to show the bearing of 

 cytology upon biological investigation in a related field and to cite certain 

 instructive examples of the results obtained. 



The taxonomy of Crepis, a genus of composites, has been intensively 

 investigated for many years with the aid of cytogenetics and geographical 

 studies. The literature on the subject is one of considerable size, but 

 the results of the investigations may be indicated by statements selected 



EUCREPIS-PHYLOGENY AND CHROMOSOME NUMBER 



HIERACIUM 18 



^ATRISARBA 8S? 

 MONTICOLA 55? LeAReiGERA 89? 

 SCOPULORUM 447 I L ACUMINATA 33.44.55? 

 OCCIDENTALIS 2J.44\\ LcRACILIS J2.55? 



\\ 

 AMPWIOIPLOIO HYBRIDS 22 



PTEROTHECOIDES 8, ; LEONTODONTOIOES 10 



PULCHRA 8, 

 CRAAJATENSIS S 

 PALAESTINA 8. 

 MuaifLORA 8 

 DIOSCORIDIS 8. 

 TUBAEFORMIS 8. 

 ARCOLICA 8. 



ANOCRSONII 22 

 CLAUCA 22 

 RUNCINATA 22 



.SUrrRENIANA S 

 NEOIECTA 6 8 

 PARVIELORA 8 

 CAPILLARIS 6 

 TECTORUM 8 

 NICAEENSIS 8 



LACTUCA 18.16 



PRENANTHCS 18 



-INTERSPECIFIC HYBRIDIZATION AND 

 AMPHIOIPLOIDY (5+4X2=18) 



5-PAIRED AND 4-PAIRED SPECIES 



-ORIGIN OF 8-CHROMOSOME SPECIES 



10 -CHROMOSOME PROGENIAL STOCK 



PRE-EUCREPIS 



Fig. 174. — Right, diagram illustrating hypothetical origin of the genus Crepis and 

 its relatives. Left, diagram illustrating the phylogeny of the subgenus Eucrepis. (After 

 E. B. Bnhcock.) 



fi-om recent summaries. The genus comprises more than 200 species 

 distributed widely in the Northern Hemisphere and Africa. These 

 evidently constitute a natural group with a common origin and center of 

 distribution. The subgenera Catoyiia, Eucrepis, and Barkhausia are 

 characterized in this order by increasing morphological specialization 

 with decreasing size and length of life cycle. The genus includes a 

 remarkable range of morphological types, and with such e\ddence of 

 progressive evolution there is associated an orderly and progressive 

 modification in chromosome number and morphology. 



The subtribe of which Crepis is a member seems to have had a common 

 origin in a stock with 10 somatic chromosomes (Fig. 174). The general 

 trend of modification within the Crepis karyotype has been from nearly 



