THE SPECIES OF ZIZIPHUS INDIGENOUS TO UNITED STATES AND MEXICO

JOHNSTON, MARSHALL C. (U. Texas, Austin.) The species o Ziziphus indigenous to United States and Mexico. Amer. Jour. Bot. 50(10):1020-1027. Illus. 1963.-Taxonomic treatment is presented for the 7 species of Ziziphus P. Mill. (Rhamnaceae: Dicotyledoneae) recognized in United States and Mexico, with key, synonymies, descriptions, and distribution maps. The taxon Condaliopsis is restored to Ziziphus after having been treated as a subgenus of Condalia Cav. for more than 60 years on the basis of superficial vegetative characters. The name Z. sonorensis S. Wats. for the widespread Mexican species is replaced by the prior Z. amole (Sesse & Mogifio) comb. nov. The genus Sarcomphalus P. Br. is referred to Ziziphus, the type species becoming Z. sarcomphalus (L.) comb. nov. Some fossil remains attributed to this genus are mentioned, but phyletic speculations are advisedly postponed until the whole genus and related genera can be reviewed. ZIZIPHUS P. Mill., a cosmotropical genus of about 100 species of trees, shrubs and lianes, was last monographed by A. P. deCandolle (1825) and, needless to say, is badly in need of revision. A conspectus was published by Suessenguth (1953) but he was able to provide a key for fewer than half the species which have been proposed. On the basis of study of the inadequate material in American herbaria, I think the Old World species, including two-thirds the total number, comprise several phyletic series all of which are adequately distinct from the New World species. The latter can, therefore, be treated separately without fear that vital affinities will be missed. The Old World species are represented with us by the cultivated and/or escaped Z. jujuba P. Mill., which is the type species of the genus, and Z. mauritiana Lam., which often passes under the illegitimate homonym Z. jujuba (L.) Lam. These 2 species furnish the edible jujube fruits of local commercial importance, and they are illustrative of characters of several Mediterranean and Near Eastern species, and of some species of Paliurus P. Mill., characters which are not found in any New World species. The spines are paired, appear to be stipular in nature, and are often markedly dissimilar in size and direction of curvature at each node. The branches are like fronds in that the leaves and the branches in their axils are distichous, coming off in a single plane. In contrast, in many of the New World species, the structures which are commonly paired at the I Received for publication April 17, 1963. Some of this work has been supported by National Science Foundation grants NSF-G9234 and NSF-G14987 at the Plant Research Institute, The University of Texas. The considerable cost of shipment of borrowed specimens has been borne by the Herbarium of The University of Texas. I am indebted to those in charge of numerous herbaria for permitting me to examine the collections; the institutions of the few specimens here cited are indicated by the standard abbreviations of Lanjouw and Stafleu (1959). I follow the original spelling of the generic name by Philin Miller (AhridLed Gard. Diet. 175;4) nodes and which have, therefore, nearly always been called stipular spines (Johnston, 1962), are thorn-tipped lateral branches emerging from the lower nodes of the axillary bud (Malme, 1920). The stipules are minute and early deciduous, the thorns commonly stout and persistent. Such usually paired thorns are characteristic of the species of South America, a notable exception being Z. cinnamomeum Tr. & P1. of Venezuela, which pertains to a peculiar Antillean triumvirate including Z. chloroxylon (L.) Oliv. and Z. rhodoxylon Urb. The continental North American species include only olle in which the characteristic paired thorns are present, the one which has up to now been called Z. sonorensis S. Wats. (Z. amole of this paper). It is a widespread arid tropical species most closely related to the South American Z. mistol Griseb. and Z. oblongifolia S. Moore. The other Mexican species have thorn-tipped branchlets but these are rarely paired at the nodes. For more than 50 years these species, on the basis of the superficial characters of unpaired, thorntipped branchlets and small leaves, have been included in the remotely related genus Condalia Cav. as subgenus Condaliopsis Weberb. Reasons for excluding these species from Condalia were given in a revision of that genus (Johnston, 1962). I have been unable to find characters to exclude them from Ziziphus. One of them, the distinctive endemic Z. yucatanensis Standl., shows close relationship to the type species of the West Indian genus Sarcomphalus P. Br., which displays the thorn-tipped branchlets of Ziziphus (Urban, 1924) and has been separated by the supposed character of the extrorse placement and dehiscence of the anthers. In most Rhamnaceae the anthers are essentially latrorse or slightly introrse, and though they are more extrorse in Sarcomphalus (as in Ceanothus L. and some of the colletioid genera), the difference is one of degree. Grisebach (1864), who remarked that there was nothing peculiar about the anthers, maintained