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GENUS ATRIPLEX 



HISTORY AND GENERIC LIMITS. 



Up to recent times the history of the genus Atriplex has been one of taxonomic segre- 

 gation. According to many pre-Linnaean botanists, it included practically all of what is 

 known as the family Chenopodiaceae, and apparently even a few species of Polygonaceae 

 found their way into the genus. Linnaeus drew the generic lines much more closely, 

 following in this respect a growing tendency among European taxonomists. Thus there 

 came to be established by the middle of the eighteenth century such well-known genera 

 as Chenopodium, Beta, Spinachia, Blitum, and Salsola. 



After the time of Linnaeus there was no further division of the genus until 1791, 

 when Gaertner described Obione (Gaertner, Fruct. 2:198), with a single species, 0. 

 muricata { = Atriplex siberica Linnaeus). This genus was accepted by Moquin-Tandon 

 in his monograph of the Chenopodiaceae (Chenopodearum monographica enumeratio 

 1840), and to it have been referred a majority of the American species now included 

 under Atriplex. The single constant character of Obione is the inverted embryo, the 

 radicle of which thus comes to assume a superior position in relation to the plumule. 

 The importance of this character of the embryo is fundamental. By reference to the 

 chart of relationships (fig. 29, p. 238) it will be seen that it is used in the present treat- 

 ment as indicating the phylogenetic separation of all of the American Atriplexes into 

 two principal stocks, or subgenera. However, its acceptance as a generic criterion 

 sunders groups of close affinity, with the result that the relationships between them are 

 no longer expressed. Furthermore, there is no positive evidence that the result is a 

 natural classification. For example, while it is probable that A. rosea and A. argentea, 

 the former a true Atriplex, the latter an Obione, have been derived from widely separate 

 stocks, yet they are so closely alike in many features that their distribution into distinct 

 genera is likely to be misleading. It is possible that these are no more widely separated 

 from each other phylogenetically than either of them is from A. hortensis or from A. 

 patula. As far as the American species are concerned, this character serves for the 

 primary division of the genus better than any other, and therefore it is used as a basis 

 for the recognition of two subgenera. In one introduced species, namely, A. semi- 

 baccata, the radicle lies to one side of the embryo. Such lateral radicles, which occur 

 in a number of other foreign species as well, may be taken as representing an intermediate 

 stage, the presence of which greatly weakens the status of Obione as a genus. It is 

 retained by a few European botanists, e. g., Ascherson and Graebner (Syn. Mitteleur. 

 Fl. 5:109, 1913), but it was not given generic standing by Bentham and Hooker (Gen. 

 PI. 3:54, 1880) nor by Volkens (in Engler and Prantl, Naturl. Pfianzenfam. 31-64, 

 1893), nor by Moss and Willmott (in Moss, Cambridge Brit. Fl. 2: 168, 1914). In America 

 Obione was accepted as a genus by Torrey but not by Nuttall, Gray, nor any of the more 

 recent writers. Watson treated it as a genus in his Report on the Botany of the King 

 Exploration of the Fortieth Parallel (1871), but when he came to his revision of the 

 North American Chenopodiaceae (Proc. Am. Acad. 9:82-126, 1874) he reduced Obione 

 into Atriplex. 



The next generic proposal was Pterochiton Torrey and Fremont (in Fremont, Rep. Rocky 

 Mts. Ore. Calif. 318, 1845). This was based upon a form now referred to A. canescens. 

 Its most distinguishing feature is the development of 4 pronounced wings to the fruiting 

 bracts. Aside from the fact that bract characters are of doubtful value as a basis for 

 genera, it is to be noted that in this case the wings are sometimes simulated by flattened 

 appendages in other groups. Furthermore, certain specimens to be mentioned under 

 A. canescens are either intermediate in this respect or are hybrids between some genuine 



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