STANDLEY TREES AND SHRUBS OF MEXICO 1407 



Pappus awns usually equal and wingless (with a 

 small wing in V. robinsonii, which has a sub- 

 equal herbaceous involucre). 



75. VERBESINA. 



Achenes wingless. 



Leaves opposite; heads whitish, discoid. 



66. SALMEA. 

 Leaves alternate; heads yellow, usually radiate. 

 Achenes not very strongly flattened, usually 

 not white-margined; plants strongly resin- 

 ous 65. FLOURENSIA. 



Achenes very strongly flattened, narrowly 

 white-margined, villous-ciliate; plants usu- 

 ally not resinous 68. ENCELIA. 



Pappus of awns and squamellae. 



Disk achenes closely enveloped and hidden by the 

 transversely rugose pales-51. RHYSOLEPIS. 

 Disk achenes not closely enveloped by the pales, 

 the latter not transversely rugose. 

 Leaves alternate; plants strongly resinous. 



65. FLOURENSIA. 

 Leaves opposite (alternate in Zexmenia brevifolia, 

 which has pistillate rays) ; plants rarely resin- 

 ous. 



Rays neutral 74. OYEDAEA. 



Rays pistillate or wanting. 



Achenes margined or winged, the wings not 

 decurrent on the awns. 



72. ZEXMENIA. 

 Achenes winged, the wings decurrent on the 



awns 73. OTOPAPPUS. 



Achenes thickened. 

 Rays pistillate. 



Pappus of the disk achenes wanting. 



Receptacle merely convex; outer phyllaries her- 

 baceous, the inner membranous. 



50. RUMFORDIA. 

 Receptacle conical; phyllaries all similar. 



55. ZALUZANIA. 

 Pappus of the disk achenes present. 



Pappus a 4-toothed crown; phyllaries and pales 



spinescent-tipped 56. BORRICHIA. 



Pappus a crown of squamellae and usually 2 

 awns; phyllaries and pales not spinescent- 

 tipped. 

 Achenes acutely margined or winged. 



72. ZEXMENIA. 

 Achenes not acutely margined or winged. 

 Leaves opposite; achenes plump, with 



rounded sides 57. WEDELIA. 



Leaves alternate; achenes subquadrangular. 

 58. WYETHIA. 

 Rays neutral or wanting. 



Heads large, 5 to 14 cm. wide, borne on fistulose 



peduncles 60. TITHONIA. 



57020—26 7 



