Chromosome number is the karyotype feature most commonly used in cytotaxonomical analyses. The chromosome number can be a plesiomorphic characteristic of a large clade or a recurrent trait which arose independently in two or more clades. Some concepts regarding chromosome number variation, such as base number, aneuploidy, paleopolyploidy, and neopolyploidy have been used by different authors in quite different ways. Therefore, its use in cytotaxonomy and karyotype evolution deserves much attention. In this paper, these terms are reappraised and their meaning and implication for plant cytotaxonomy are discussed.

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