244 FUNDAMENTALS OF CYTOLOGY 



lime-rich soils. In Scandinavia the polyploid types among the Ericaceae 

 are usually more widely distributed in northern and severe habitats. 

 In certain other cases it has been found that polyploids predominate in 

 desert habitats. In other genera and floras, however, it has been shown 

 that there is no correlation between polyploidy and extremity of habitat; 

 indeed, autopolyploids may be less frost resistant, in part because of 

 their higher water content. Studies on Viola, Achillea, Potentilla, 

 Artemisia, and other genera growing in the Pacific slope region of the 

 United States have shown that although chromosomal differences are 

 commonly associated with different ecological requirements and thus 

 affect distribution, there is no rule as to the kind of region in which a 

 given chromosomal type is found. Among perennial forage grasses 

 growing in California, however, the drier and hotter regions contain a 

 distinctly higher proportion of polyploids, probably of hybrid origin. 



Experimental Taxonomy. — The kinds of facts cited in the preceding 

 section should make it evident that cytotaxonomj^ is a part of a larger 

 field of investigation. This more comprehensive field, known as experi- 

 mental taxonomy, not only adds cytological characters to those ordinarily 

 cited in manuals, but appeals to all other sources that might yield 

 information bearing upon its problems. The criticism from the geneticist 

 that many species listed in the manuals may be nothing more than 

 intraspecific Mendehan forms is being met by modern experimental 

 taxonomists in their attempts to test their provisional hypotheses by 

 suitable breeding experiments. Similarly, the criticism from the physiol- 

 ogist and ecologist that certain supposed species may be merely local 

 variants associated with a special habitat is being answered by the use 

 of data froin the field of experimental ecolog3^ Plants of the kinds in 

 question are grown in different soils and in different climatic situations 

 in order to distinguish more surely between physiological variations and 

 the characters trul}^ indicative of relationship. This often requires 

 observation extending over a period of years. Furthermore, observation 

 must also be extended in space, for a knowledge of the geographical 

 distribution of species and other taxonomic units is often essential to an 

 understanding of the relative age and advancement of related kinds of 

 organisms. Taxonomists have long been aware of this and have made 

 use of collections from widely separated localities in formulating their 

 conclusions. Now that cytological data are being sought in a similar 

 manner, cytologists share with taxonomists the benefits of this broader 

 observational foundation. 



It is obvious that conclusions based on such a variety of data can be 

 reached only very slowly, but that when attained they should be far 

 more dependable than concepts reached by shorter routes. In the mean- 

 time we shall have to make use of provisional schemes of relationship 



