STANDLEY TREES AND SHRUBS OF MEXICO 1519 



2a. Melampodium cinereum ramosissimum (DC.) A. Gray, Syn. Fl l^: 

 2,39. 1884. 

 Melampodium ramosissimum DC. Prodr. 5: 518. 18.36. 

 Coahuila. Texas; type from "Texas." 

 Hood of the fruiting phyllaries mucronate; otherwise as in the typical form. 



3. Melampodium leucanthum Torr. & Gray, Fl. N. Amer. 2: 271. 1842. 

 Chihuahua. Kansas to Texas and Arizona; type from Texas. 



Similar to M. cinereum; usually about 30 cm. high; leaves often entire; heads 

 much larger; hood of the fruiting phyllaries muticous. 



4. Melampodium nelsonii Greenm. Proc. Amer. Acad. 41: 260. 1905. 

 Definitely known only from the type locality, Volcan de JoruUo, Michoacdn. 

 Suffruticulose, hirsute; leaves linear or linear-lanceolate, entire or pinnately 



3-lobed; heads 1 to 1.5 cm. wide; hood of the fruiting phyllaries sometimes v/ith 

 caudate apex. 



5. MelamLpodium heterophyllum Lag. Gen. & Sp. Nov. 33. 1816. 

 Tamaulipas to Oaxaca; type from Mexico. 



Herbaceous or suffruticulose, up to 40 cm. high; leaves linear to lanceolate, 

 entire or with long narrow lobes toward base; heads about 12 mm. wide. 



41. PARTHENIUM L. Sp. PI. 988. 1753. 

 References: J. M. Hillier, Guayule rubber {Parthenium argentatum, A. 

 Gray), Kew Bull. 1907: 285-294. 1907; F. E. Lloyd, Guayule {Parthenium 

 argentatum Gray), a rubber-plant of the Chihuahuan desert, Carnegie Inst. 

 Washington Publ. 139. 1911; C. Patoni, El guayule (Parthenium argentatum 

 A. Gray). Mexico, 1916. 



Shrubs or herbs; leaves alternate; heads small, radiate, white, in terminal 

 cymes or panicles; phyllaries usually 2-seriate, suborbicular, dry; rays short, 

 erect, fertile, the disk sterile; ray achenes obcompressed, their nerviform margins 

 adnate at base to the involute paleae of the two opposed outer disk flowers and 

 at maturity separating from the body of the achene nearly to apex, the whole 

 falling together; pappus 2 or 3 short awns or none. 



Leaves silvery-canescent on both sides 1. P. argentatum. 



Leaves not silvery-canescent, usually tomentose or velutinous beneath. 

 Leaves ovate, subentire to repand or crenate, not lobed. 



Leaves harshly hispidulous above with tuberculate-based hairs. 



Pappus wanting , 2. P. parviceps. 



Pappus present. 



Leaves densely cinereous-tomentulcse beneath 3. P. fruticosum. 



Leaves green or greenish and merely strigillose or puberulous beneath. 



4. P. schottii. 

 Leaves smooth or only slightly asperulous above. 



Pappus present 4. P. schottii. 



Pappus wanting. 



Leaves conspicuously crenate; involucre 2.5 to 3.5 mm. high. 



5. P. tomentosum. 

 Leaves rather obscurely repand or crenulate; involucre 1.5 to 2 mm. 



high 6. P. stramonium. 



Leaves triangular to obovate, hastately or lyrately lobed or pinnatifid; pappus 

 present. 

 Leaves triangular to obovate, lyrate-pinnatifid or lobed, usually gray and 



floccose-tomentose above 7. P. incanum. 



Leaves deltoid-ovate, coarsely lobed, green and somewhat harshly puljescent 



above 8. P. lozanianum. 



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