JONES, -—— A REVISION OF THE GENUS ZEXMENIA. 145 
may well be called § Awehenocarpa, a name which indicates the achenial 
character common to all these species, namely, a.more or less marked 
constriction of the achene near its summit, forming a sort of neck, which 
separates the insertion of the pappus from the body of the achene. The 
awns in this section are generally fragile and pubescent. Squamellae 
are present in most of the species but are usually quite distinct. The 
body of the achene varies in the different species from the type found in 
§ Huzexmenia on the one hand to 4-angled in the ray-flowers and 3- 
angled in the disk-flowers, or somewhat rounded in both. In none of the 
species of § Auchenocarpa.do we find the rather dense inflorescence of 
many species of § Huzexmenia, the heads being commonly solitary on 
long leafy stems, or in few-headed cymes or more rarely somewhat 
corymbose. 
So far as known Zexmenia is strictly a New World genus and is con- 
fined to the tropical and subtropical regions of the mainland. Three 
species are found in the southwestern part of the United States and four 
in South America. The greater number, however, are confined to 
Mexico and Central America. 
This revision has been prepared at the Gray Herbarium, under the 
direction of Dr. B. L. Robinson, where it has been possible to study 
what is probably the largest collection of this particular group. The 
material examined, however, has also included that of the United States 
National Museum, the herbarium of Mr. John Donnell Smith, and of 
the Missouri Botanical Garden. Acknowledgment is due to Mr. F. V. 
Coville and Dr. J. N. Rose of the National Museum, Mr. Smith of 
Baltimore, and Professor Trelease of the Missouri Botanical Garden, for 
their obliging loan of these specimens. Through the kindness of Sir 
W. T. Thiselton Dyer and Mr. W. Botting Hemsley of the Royal 
Gardens at Kew, and Mr. Casimir de Candolle of Geneva in sending 
type-material and tracings, several species have been identified which 
would otherwise have remained indefinite. Special thanks are due to 
Dr. B. L. Robinson, Dr. J. M. Greenman, and Miss Mary A. Day for 
their assistance in the preparation of the paper, the revision of the manu- 
script, and the proof-reading. 
§ 1. EUZEXMENIA. Perennial herbs or shrubs, with simple or 
branching stems, generally pubescent: leaves all opposite, more or less 
serrate, short-petioled or sessile: peduncles solitary or the heads corym- 
bose or borne in umbelliform clusters; heads never more than 2 cm. long ; 
involucre subglobose, campanulate, or short-cylindric, the scales 1-5- 
seriate: ligules usually conspicuous, yellow (except in Z. zinniotdes) ; 
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