APRIL, 1904. PLANTS YUCATAN^ MILLSPAUGH & CHASE. 



87 



XANTHIUM L. Sp. PL, 987. 



Heads unisexual, the fertile with 2 apetalous pistillate flowers in 

 a closed bur-like, beaked involucre, only the style-branches exserted 

 through an aperture in the beak; the sterile of numerous staminate 

 flowers in an open polyphyllous involucre, the heads in a raceme. 

 Receptacle of sterile heads cylindrical, chaffy; scales partly enclosing 

 the flowers. Achenes dorsally compressed, oblong; pappus none. 



Coarse monoecious herbs with 

 inconspicuous heads. 



Xanthium strumarium L. Sp. 



PL, 987. 



A coarse annual, with angled, 

 hispid stem and branches and 

 large alternate, petioled, 3-lobed, 

 coarsely dentate, scabrous leaves. 

 Inflorescence of unisexual clus- 

 ters, the staminate capitate-clus- 

 tered at the ends of the striate, 

 hispid branches. Fertile heads 

 sessile, 2-4 together, in the axils, 

 of the leaves, subtended by 4-6 

 small, linear bractlets. Mature 

 involucre (fruit) 1.3-1.5 x 2.3-2.5 

 cm. including spines (.8 x 1.9 cm. exclusive of spine) ellipsoid, hispidu- 

 lous; beaks erect, slightly hooked, 6-7 mm. long; spines rather slender 

 4-5 mm. long. Achene slatey-fuscous, 4x13-15 mm., oblong-lanceo- 

 late, abruptly acuminate into a slender beak; in section convexo-con- 

 cave, the ventrum nearly plane, 3 ridged, dorsum faintly 5 nerved; 

 glabrous. 



Hab. "Herb, 2 feet high, uncommon near Merida, where it has 

 probably been introduced in foreign baled hay," Gaumer 1145 (Xan- 

 thium Canadense Field Col. Mus. Bot. 1 1397) Progreso 2512. 



These Yucatan specimens are immature; the figure is drawn from 

 Ricksecker 266, Island of St. Croix, which matches Dr. Gaumer's 

 specimens. 



AMBROSIA L. Sp. PL, 987. 



Heads unisexual, the fertile with a single apetalous, pistillate 

 flower in a closed bur-like involucre, only the style branches exserted; 

 the sterile of numerous staminate flowers in an open gamophyllous 

 involucre, the heads in a raceme above the fertile ones. Receptacle 

 of sterile heads flat, with filiform chaff among the outer flowers. 

 Achene turgid, subglobose; pappus none. Monoecious herbs with 

 racemose panicled, inconspicuous heads. 



Ambrosia hispida Pursh Fl. Am. Sept. SuppL, 743. 



Spreading from a suffrutescent base, the branches prostrate, some- 

 times rooting at the nodes, terete, hispid. Leaves opposite, petioled, 

 twice or thrice pinnatifid, thickish, strigose-hispid. Inflorescence a 



