THE CLASSIFICATION OF PLANTS 2127 



If discontinuity is not complete there may occur Clines, that is to say, 

 spatial series of intermediates forming gradients between two groups. The 

 extreme forms may be intersterile and yet intermediate forms be capable of 

 interbreeding. The taxonomic status of such intermediates has never 

 been settled. 



An attempt was made in 1929 by Turesson to arrange his concepts 

 of the natural units into a hierarchy with the following distinctive 

 names. 



The major natural assemblage is known as the Coenospecies, which 

 corresponds to the morphological macrospecies and may sometimes, perhaps 

 often, be the same thing. It may be defined as an assemblage of individuals 

 which is genetically isolated from related assemblages and incapable of 

 exchanging genes with them, that is to say incapable of fertile interbreed- 

 ing. How this genetical isolation arises is another question and does not 

 concern us here; there may be many ways in which it originates. The coeno- 

 species is then the maximum natural species as the macrospecies is the 

 maximum museum species. Not all macrospecies correspond to coenospecies, 

 as is shown by the frequent occurrence of " interspecific " hybrids between 

 the former. These cases probably fall into the following category. 



The coenospecies may be divided into Ecospecies, groups which have 

 the same chromosome complement but are separated by ecological or 

 geographical, and usually also morphological, difi^erences. Between 

 themselves these groups are capable of a limited degree of hybridization, 

 but under natural conditions their ecological or geographical difi^erences 

 usually keep them apart. When ecospecies are morphologically distinct 

 they may be equivalent to museum species or sub-species. 



In this connection the genus Vaccinium is interesting. There are a 

 number of sub-genera, and within each sub-genus there is apparently some 

 hybridization between those species which have the same chromosome 

 number, but not between the diploids and the polyploids. The main 

 evolution of the genus has taken place at the diploid level and these inter- 

 breeding diploid species are morphologically distinct, so distinct in fact that 

 they are unhesitatingly called macrospecies by systematists. Viewed from 

 the standpoint of natural species, however, they would only be ranked as 

 ecospecies, because the only barrier to their fusion with one another is that 

 they are ecologically segregated. In Vaccinium the coenospecies would 

 correspond to the sub-genus. In other genera a single morphological 

 species may become genetically isolated by some nuclear peculiarity and 

 thus constitute a coenospecies. At the other extreme there are genera 

 among the Gramineae and Orchidaceae in which all the species are capable 

 of being crossed and even bi-generic hybrids are not uncommon. Where 

 the limits of the coenospecies may be in such groups is not clear. 



Within each ecospecies may also be found Ecotypes, which are geneti- 

 cally distinct groups within the ecospecies, generally showing special 

 ecological preferences for situations to which they may be specially well 

 adapted. These preferences tend to isolate them to some extent, but they 



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