2128 A TEXTBOOK OF THEORETICAL BOTANY 



are capable, when circumstances permit, of free hybridization with other 

 ecotypes of the same ecospecies. 



Conditions of cultivation or a change of climate, for example, may 

 create novel uniformity of habitat which permits hybridization to take place 

 among ecotypes formerly separated by ecological barriers. This has been 

 referred to as " hybridization of the habitat " and it sometimes results in the 

 production of hybrid swarms or the emergence of new, aggressive types by 

 transfer of genes, which may prove to be " weeds in embryo". 



Turesson began with the ecotype, which is the only one of the three 

 categories recognizable as an entity in the field. It is supposed to arise in the 

 course of migration of a species-population, by the selective elimination of 

 unfitted types under environmental influences. It is therefore an ecologi- 

 cally " adapted " group. It differs from an ecad (in Clements' sense) in 

 that the latter arises through individual plasticity and has no genetical 

 basis, while the ecotype exists because it has a genetical constitution 

 suitable for a particular habitat. The ecad is changeable and unstable but 

 the ecotype maintains its character even under cultivation. Turesson's 

 definition runs as follows: " An ecotype is the product arising as the result 

 of the genotypical response of a species to a particular habitat." He con- 

 sidered it to be akin to the taxonomic variety and some varieties are in fact 

 ecotypes though others may be topotypes, that is local populations 

 genetically differentiated from others but not related to ecological 

 differences. 



The Cytospecies, on the other hand, is an assemblage within the 

 coenospecies which has its own distinct chromosome number and is 

 ecologically or geographically isolated. Cytospecies are only capable of 

 limited hybridization, except in the case of paired species where one of the 

 pair is a polyploid of the other, when they may hybridize freely: e.g., Viola 

 riviniana (n = 2o), and Viola reichenbachiana (n = io). 



Many of the morphological species fall into the categories of Ecospecies 

 or Cytospecies, but in many other cases Cytospecies may not be morpho- 

 logically distinct. 



The Cytotype is a polyploid component of a cytospecies. 

 These terms are descriptive only and must not be understood as rigidly 

 defined categories which are mutually exclusive. For example, the cytotype 

 may arise either by spontaneous chromosome doubling (autoploidy) or by a 

 cross between two ecospecies accompanied by chromosome doubling 

 (amphiploidy) which transforms what would otherwise be a sterile cross 

 into a fully fertile and stable type, a potential new coenospecies. Further, 

 the two cytotypes of Valeriana officinalis, tetraploid and octoploid respec- 

 tively, show different ecological preferences and are apparently inter- 

 sterile. They would appear therefore to be true coenospecies, though they 

 are only distinguishable morphologically by the size of their pollen grains. 



A study of the numerous infra-specific groupings which experimentalists 

 have proposed, shows that the species-unit has been as thoroughly pul- 

 verized as the atom. Taxonomic species are undoubtedly of very diverse 



