Taxonomical Evaluation of Polyploids

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Abstract
Although geobotany is the most valuable of the auxiliary sciences to taxonomy, the disciplines of cytology, genetics, physiology, etc. are a very good support for systematists studying critical genera. The present paper deals mainly with the taxonomical value of differences in chromosome number. Only about 7 per cent.—about 17.000 out of 250.000—of the species of higher plants already described have been cytologically investigated. The highest frequency of species, or more than 90 per cent, of the floras, has ben cytologically studied in the floras of northwestern Europe, while the percentage of cytologically known species of the Tropics is very low. Intraspecific polyploidy has been reported from less than 7 per cent, of the species of northern Europe, if the species concept is taken in its widest sense. More than 90 per cent, of these polyplotypes have previously been described as distinct species by classical taxonomists. Morphologically completely indistinguishable polyplotypes are unknown, at least in northwestern Europe. Biologically isolated types of the lowest rank should be classified as separate species. They are intersterile or show a high degree of incompatibility. Subspecies and varieties are interfertile, though geographically and morphologically distinct. Taxonomical species are identical to the ecospecies of the genecologists. Types differing in chromosome numbers are abrupt-ecospecies, others being gradual-ecospecies. As to the classification of polyplotypes, the most convenient principle should be that enunciated by NANNFELDT (1938): «As soon as chromosome races (polyplotypes) are morphologically distinct and thus recognizable to the taxonomist, they had better to be recognized as species, even if the morphological characters are small ». This principle seems to result in that all types differing in the number of chromosomes should be classified as distinct species. This does, however, not refer to asexually reproducing triploid, pentaploid, etc. hybrids. Thirty odd examples are drawn in order to demonstrate the statement that most polyplotypes have been described as species by classical taxonomists.

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