12 FIELD MUSEUM OF NATURAL HISTORY BOTANY, VOL. IV. 



ABBREVIATIONS USED FOR HERBARIA 



Hb. Calif. Herb. University of California 



Hb. Can. Herb. Canadian Geological Survey 



Hb. Chi. Herb. University of Chicago 



Hb. Field Herb. Field Museum of Natural History 



Hb. Mo. Herb. Missouri Botanical Garden 



Hb. N. Y. Herb. New York Botanical Garden 



Hb. U. S. United States National Herbarium 



Xanthium (Tourn.) Linn. Sp. PI. 987. 1753 

 [Tourn. Instit. tab. 252, 1700. Linn. Gen. PI. 424, 1754] 



Herbae annuae, ramosae, crassae, plerumque scabridae aut spinosae, 

 monoicae, sporadicae. Folia alterna, petiolis tenuibus petiolata, plus 

 minusve lobata aut rarissime inciso-divisa, plerumque punctis resinosis 

 parvis numerosis punctata. Capitula discoidea, homogama. Involucra 

 mascula summa, subglobosa, multiflora, bracteis liberis i-3-seriatis; 

 receptaculo cylindraceo, paleaceo; corollis tubulosis, clavatis, apice 

 5-dentatis; staminum filamentis monadelphis, antheris liberis, con- 

 niventibus, basi ecaudatis, appendicibus apicalibus incurvato-mucro- 

 natis; stylo simplici, tenue, ad apicem plus minusve dilatato; achaenio 

 rudimentario. Involucra feminea gamophylla, clausa aut rare sub- 

 clausa, cylindrica aut ovoidea aut etiam subglobosa, hamato-aculeata 

 aut rarissime simpliciter aculeata, ad apicem 2-rostrato (in Sect. II, 

 Acanthoxanthio, uno aut duobus rostris saepius absentibus); intus 

 2-loculare; corollis absentibus; styli ramis e rostris exsertis; acheniis 

 linearibus aut etiam ovatis, compressis, plus minusve crassis, ad apicem 

 tenuiter rostratis, levibus, singulis in loculis singulis inaequalibus 

 involucri maturatis et manentibus. 



CLAVIS GENERIS 



Folia attenuata utrinque; axillis spinis tripartitis munitis; aculeis ad 



apicem cygneo-hamatis i. X. spinosum 



Folia cordata, ovata aut subtriangulata; axillis inermibus; aculeis ad 



apicem rectis aut simpliciter hamosis: 



Fructus maturi plerumque maximi, aculeis et rostris adjectis 



2.8-4 cm. longi et 2-3 cm. lati: 

 Aculei plerumque arcuati et corniformes, subcrassi: 



Aculei saepius numerosi et conferti, teretes 2 1 . X. campestre 

 Aculei plerumque pauciores et remoti aut subremoti, infimi 

 ad faciem ventralem canaliculati 19. X. oviforme 



Aculei non perspicue arcuati nisi ad apicem 



hamosum, tenues 20. X. speciosum 



Fructus maturi minores: 

 Aculei pauci et remoti (20-50) : 



Fructus magni, corpore 1.51.7 cm. longo et 6-7 mm. lato; 

 aculeis 8-10 mm. longis 10. X. cenchroides 



