A RECONSTITUTION OF THE GENUS MACHAERANTHERA

The preparation by one of the authors of a treatment of Aster for a forthcomning flora of California raises the perennial question of the proper limits of the genus. Ideally, the definition of any genus should be based on all its species, regardless of the part of the world in which they occur. Practieally, this is not yet possible in Aster. The genus in the broad senlse has so many species, occurs over so much of the world, is related or similar to so many, other genera, and has such vague morphological limits that its ultimate definition will probably be established only after a thorough study of most of the tribe. Meanwhile, an approach to the correct treatment can be malde by determining some of the elements which must surely be iiieluded in the genus, and by removing others which have their closest relatives elsewhere. There is a hard core of about a hundred species that certainly belong together as Aster proper. These are fibrous-rooted perennial herbs from a short or elongate rhizome, with simple, not at all spinulose leaves, several or many heads of moderate or rather small size, leafy or more often imbricate and merely green-tipped phyllaries, fertile ray-flowers with well developed eyanic or white ligules, numerous xanthic to eyanic disk-flowers with well-developed acute styleappendages, and several-nerved achenes with simple capillary pappus. The greatest concentration of species having all these features occurs in North America, especially the eastern United States, but such species are also found in South America, Europe, Asia, and northern Africa. Among the partly or wholly American segregates which seem to be closely allied to Aster proper are Amellastrum, Biotia, Doellingeria, Eucephalus, Heleastrum, lonactis, Oreastrum, and Sericocarpus. Each of these groups differs in one or more respects from Aster proper, but an attempt to segregate any one of them leads to problems which can be resolved only by segregating most or all of the others as well' The latter procedure merely provides the large genus Aster with a number of small satellite genera which differ from each other in trivial characters. All these segregates can be retained in Aster without doing violence either to natural relationships or to the morphologic coherence of the genus, and none of them tends to ally Aster to any other well-established genus. The status of Machaeranthera and Xylorhiza is another matter. Asa Gray retained Machaeranthera as a distinct genus until Benthamn submerged it in 1873 (Gen. P1. 21), and his only commeiit (Bot. Calif. 1876) in accepting Bentham's reduction was "For this flora at least it is best to receive it [Aster] in the extended form which it reassumes in Bentham and Hookler's Genera Plantarun." One of us (Crolnquist, Univ. Wash. Publ. Biol. 17. 1955) has already noted that Machaeranthera may be more closely allied to Haplopappus than to Aster. After consultation and further study we are convinced that both Machaeranthera and Xylorhiza are more nearly related to Hap,lopappvs than to Aster, and that their inclusion in Aster tends to destroy both the morphologic and the phylogenetic homogeneity of the genus. We propose that Machaeranthera and Xylorhiza be recognized as a single genus Machaeranthera, to be distinguished from the bulk of Aster by its taprooted rather than fibrous-rooted habit, and from Haplopappus by its eyanic or white rather than xanthic rays. The pappus-bristles of Machaeranthera are commonly very unequal in length and thiekness, just as in most species of Haplo-