CYTOLOGY AND TAXONOMY 247 



special reference to the influence of climate and ecological habitat. 

 Perennial species including many climatic races in the Pacific Coast 

 region have been placed in gardens at three localities with very different 

 climates: near sea level on the San Francisco peninsula, at 4600 feet 

 elevation near Yosemite National Park, and at 10,000 feet elevation on 

 the Sierra Nevada. In this way it is possible to observe the effects of 

 different climates upon genetically uniform material as well as the 

 liehavior of races from different regions when brought into the same 

 environment. The ecological reactions and chromosome numbers of the 

 various races of a species or species group are compared with the purpose 

 of determining whether the differentiation of intraspecific races has a 

 visible cytological basis. In pursuing such studies the investigators 

 have found useful the terminology indicated in Fig. 175. They are, 

 however, quick to admit that one cannot formulate definitions covering 

 all differences between taxonomic units, since species are in all stages of 

 evolution. 



Some of the results of these studies are as follows. Three complexes 

 of climatic races (in Sisyrinchium heUum, Potentilla glandulosa, and 

 Penstemon procerus) have become differentiated without change in 

 chromosome number. In two other complexes, involving several other 

 species of Potentilla, the chromosomes vary in number and degree of 

 irregularity in behavior, and the plants are evidently apomictic, yet 

 climatic races have been successfully developed as in P. glandulosa. 

 In six complexes (in Zaiischneria, Viola, Aster, Artemisia, Achillea, and 

 Horkelia) there are differences in chromosome number that prevent free 

 interbreeding and are usually correlated with differences in morphology. 

 Such chromosomal groups can be recognized as taxonomic species, and 

 they usually occupy different climatic regions. 



These data and many others have led to the conclusions stated in the 

 following quotation. 



This survey and the one conducted by Turesson in Euroj^e indicate that the 

 genetic-physiologic (Hfferentiation of a plant group is correlated with the climatic 

 zones it occupies. This follows from the fact that the same kinds of environ- 

 ments are occupied by races that have similar patterns of reaction, even though 

 they belong to unrelated genera or families. This is found to hold irrespective 

 of whether or not the regional forms differ in chromosome number. 



The usual pattern of differentiation is purely genetic, with relatively few 

 major steps involved; but superimposed upon this one often finds a cytological 

 differentiation, with one or two changes in chromosome number across the 

 California transect. However, the effects of increases in chromosome number 

 must have been far overshadowed by the selective influence of the environment 

 in determining the appearance and reactions of plants. From these consider- 

 ations it appears that it is the genes in the chromosomes, and not the number of 

 chromosomes, which determine the climatic adaptation. 



