TAXONOMY OF THE HIGHER PLANTS 1 99 



CYTOGENETICS IN RELATION TO TAXONOMY 



Much of the work in taxonomy is founded on the concept that the pheno- 

 type, if sufficiently and accurately studied, can be relied upon to reflect the 

 nature of the genotype. This requires the elimination from consideration of 

 the effects of the environment upon the phenotype, as suggested above. Basi- 

 cally, what we are saying is that the phenotype is the product of the genotype, 

 except for the modifications produced by the environment. This is the same 

 postulate, underlying the genetical principles of Mendel, that has proved to 

 be so fruitful for the field of genetics. Today, we include microscopic char- 

 acteristics as well as ultimate physiological reactions and characteristics of 

 the individual as features of the phenotype. A half century ago, the phenotype 

 was thought of pretty much in macroscopic terms with no thought of bio- 

 chemical or similar characters as a part of it. 



The establishment of the chromosomes, as bodies upon which discrete units 

 of heredity, the genes, are carried from one generation to the next, was a sig- 

 nificant step for taxonomy as well as genetics. It is only natural that out of 

 this basic discovery of the importance of the chromosomes should come a 

 tremendous emphasis upon the karyotype for its own sake and for its role in 

 relation to heredity, development, and evolution. The question for taxonomy, 

 aside from those related to heredity and evolution, has been: What added 

 information will the chromosomes provide? How significant are differences in 

 chromosome numbers in assessing the characteristics of different taxa? What 

 can be done with chromosome morphology from the taxonomical point of 

 view? These and other questions have been repeatedly asked in connection 

 with different plant groups, and we are now beginning to get at an evaluation 

 of chromosome studies in so far as taxonomy is concerned. 



Ordinarily, the same chromosome number prevails throughout a species. 

 However, there are exceptions, and it is unsafe to assume on slender evidence 

 that no chromosome-number differences exist within a species. It has often 

 been maintained that either aneuploid or polyploid chromosome difference 

 between two plants is sufficient evidence to establish them as separate species. 

 Now, we know that diploid and polyploid plants may and do occur within 

 the same species rather frequently. Aneuploid plants may also exist within 

 the same species, but cases of this kind are apparently less frequent. Certainly, 

 it should not be taken for granted that a single, accurate chromosome count 

 establishes the count irrefutably for a given species, nor should an investigator 

 be too surprised to discover that a given species has more than one chromo- 

 some number, particularly where a polyploid relationship is revealed. We do 

 not know at present how much the preconception that different chromosome 

 numbers are not allowable within a species has affected chromosome-number 

 reports, especially in cases where the chromosomes are excessively small. 



