| Summary: | The totality of living organisms which comprises the plant kingdom presents an extreme diversity of form. In broad conception it may be said that the analysis and understanding of this diversity embraces the whole field of botanical study. Traditionally however plant classification has been the task of taxonomists using morphological data with ancillary considerations of geographical distribution and hybrid sterility. The primary concern of orthodox taxonomy is to supply recognizable reference points for the use of general biologists, although their classifications may be later the subject of theoretical, e.g. evolutionary, interpretation. In recent years "experimental taxonomists" have developed a different method of approach to the problems of classification. This approach is based equally on the understanding of variation in the light of modern cytogenetic study and on intensive ecological and cultivation work with the living plants. The aim of this work is not limited by the practical considerations of orthodox taxonomy. Thus Turesson (1922, 23, 25) showed that widely distributed species become differentiated into local hereditary types, naturally selected by the action of local environmental factors on the gene complex of the plant population. The local populations become morphologically distinct, since some genes are selected, others being suppressed, by different combinations of habitat factors. The local populations may remain capable of interbreeding freely (ecotypes), though, with greater differentiation, interpopulation hybrids show increasing sterility. Such populations are termed "ecospecies ". This type of analysis has been confirmed by numerous other workers. Broadly speaking the categories of experimental taxonomy as at present visualised may be conveniently grouped under two heads: (1) Infra -specific intra- fertile populations between which free gene exchange is potentially possible, though not often realised in nature, and (2) Specific populations between which gene exchange is partially or totally prevented even when opportunit is presented. The problem presented to the experimental taxonomists has two aspects; first to assess the degree of homology between the hereditary constitutions of different populations,and second to determine the gene exchange potential amongst them. The present investigation deals with certain cytological problems which have been encountered during the course of an experimental taxonomic study of Plantago maritima L. and its allies. It has already been reported, Gregor (1939), that the N. American Greenlandic and N. European sea plantain populations are diploid (n = 6); those inhabiting the region of the Alps are of two kinds (a) diploid and (b) tetraploid (n = 12). Proof was obtained that the diploids and tetraploids can be crossed, Gregor (1939), but at that time it was not known whether gene transfer from one population to the other was actually possible. Consequently the two groups were provisionally regarded as belonging to separate coenospecies (i.e. to categories incapable of exchanging genes), but with the qualification that, "if it should be found that the diploid and tetraploid groups are capable of exchanging genes then they would belong to different ecospecies of the same coenospecies ". It has therefore been the primary object of the present investigation to determine whether or not gene exchange between diploids and tetraploids has any foundation in fact and at the same time to assess the degree of relationship in and between all diploid and tetraploid populations. That restricted gene exchange between populations is not necessarily expressed as visible morphological differentiation can be readily appreciated from the fact that a specimen supplied by the Royal Botanic Garden, Edinburgh, as Plantago alpina L. proved to be tetraploid, while other specimens collected from a reputed habitat of the same species have been found to be diploid. It might be argued that the morphological criterion of chromosome number would, alone, effectively solve this problem, but such a delimitatio would, by itself, be quite inadequate as a means of recording tie evolutionary independence of diploid and tetraploid populations. In such cases, therefore, taxonomic emphasis should be transferred from the morphological characteristics to the potential evolutionary significance of the populations concerned. A classification based on information of this kind would have a far greater theoretical significance, though obviously less practical utility, than one based on morphological criteria alone.
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