TAXONOMIC PRINCIPLES 23 



cases utilizing such information in conjunction with exomorphic fea- 

 tures for phyletic evaluation. As is obvious from this diagram (Fig. 

 2-6), putative diploid species must have preceded the derived poly- 

 ploids, but again the relative time of such divergences is not shown in 

 the diagram. 



Three-dimensional phylogenetic diagrams 



For higher plant groups where fossil data are mostly lacking, 

 three-dimensional schemes are usually purely speculative. Nonethe- 

 less, some monographers have ventured to reconstruct the phyletic 

 past using geomorphological, phytogeographical, ecological and other 

 lines of subtle evidence. If, for example, a North American genus with 

 five species is critically studied and it develops that two of the species 

 occupy mesic habitats which are believed to be floristically old (such 

 as extant remnants of the Arctotertiary flora; Chaney, 1938), while 

 the remaining species occur in grassland and desert habitats (which, 

 on paleobotanical grounds, are believed to be more recently derived 

 vegetational types; Axelrod, 1950, and others), then this information 

 can be used to give relative time dimensions to any appropriate 

 phylogenetic diagram. Phylogenetic schemes constructed from such 

 data are often severely criticized, but, as indicated elsewhere, as a 

 framework for future investigation they are often of definite value. 



Time-dimensional phyletic diagrams have been proposed for 

 the evolution of organic matter and organisms for the planet Earth 

 (Fig. 2-7), for the relationships between and within several families 

 (Fig. 2-3), for species within a genus (Fig. 2-2), and so on. 



Classification of vascular plants 



Because of the complex morphological variation of the vas- 

 cular plants, this group has been the most extensively and successfully 

 studied from a phylogenetic standpoint. This is particularly true of 

 the flowering plants, and a number of systems of classifications, usually 

 to the level of family, have been proposed for this group (Lawrence, 

 1951, for review; Cronquist, 1957, 1960; Benson, 1957; Hutchinson, 

 1959; Takhtajian, 1959; and others). However, only a few phylogenetic 

 systems have gained wide acceptance or attention, the more important 

 being the systems of Engler, Bessey, and Hutchinson. Certain aspects 

 of these three systems are discussed briefly below, mainly to acquaint 

 the nontaxonomist with their nature and objectives. 



