1016 MISC. PUBLICATION 42 3, TJ. S. DEPT. OF AGRICULTURE 



1. Cynara scolymus L., Sp. PL 827. 1753. 



Near Phoenix (Maricopa County), occasional along roadsides and 

 in waste places, May to August. Occasionally or frequently escaping 

 from cultivation in the western United States, most commonly in 

 California, where the artichoke is grown extensively ; native of Europe. 



119. SILYBUM. MlLKTHISTLE 



Coarse thistlelike herb; leaves large, lobed or pinnatifid, white- 

 veined and white-blotched; heads large (the disk 2.5 to 5 cm. wide), 

 purple; involucre subglobose, the phyllaries several-seriate, rigid, with 

 spreading, ovate or lanceolate, spinulose-margined and stiffly spine- 

 tipped, coriaceous-herbaceous appendages. 



1. Silybum marianum (L.) Gaertn., Fruct. et Sem. 2: 378. 1802. 



Car duns marianus L., Sp. PI. 823. 1753. 



Gila, Maricopa, and Pinal Counties, occasional at roadsides and in 

 waste ground, May to September. Occasional in much of the United 

 States and adjacent Canada, naturalized in California; native of 

 Europe. 



The plant has been used medicinally in Europe. A noxious field 

 weed, difficult to exterminate. 



120. CENTAUREA. Star-thistle, knapweed 



Herbs, with entire leaves or the lower ones pinnatifid ; heads medium 

 or large, pink, purple, or yellow, rarely white; involucre strongly 

 graduated, the phyllaries append aged with spines or prickles, or with 

 scarious or hyaline tips ; outer flowers sometimes enlarged and falsely 

 radiate ; achenes with an oblique attachment ; pappus various, setose, 

 paleaceous, or wanting. 



The introduced species are objectionable weeds in California, 

 especially in grainfields, because of the prickly involucres. The plants 

 are not yet sufficiently abundant in Arizona to be troublesome. 



Key to the species 



1. Phyllaries tipped with a stiff spreading spine or prickle, this with 2 or 3 pairs 



of smaller prickles at its base; leaves more or less decurrent; corollas 



yellow (2). 



2. Terminal spine of the phyllary yellowish, comparatively stout, 1.2 to 2 cm. 



long; plant thinly torn entose 1. C. solstitialis. 



2. Terminal spine or prickle of the phyllary normally purplish toward the base, 



rather slender, less than 1 cm. long; plant hispidulous. 



2. C. MELITENSIS. 



1. Phyllaries not spiny-tipped; leaves not decurrent; corollas rosy or purplish, 

 rarely white (3) . 



3. Phyllaries with thin, whitish, essentially entire, hyaline tips, those of the 



outer series obtuse and practically glabrous, those of the inner series 

 pointed and densely pilose; heads small, the involucre less than 1 cm. 

 thick; outer corollas not enlarged; leaves mainly linear, not clasp- 

 ing 3. C. picris. 



3. Phyllaries with a pectinate-fringed scarious terminal appendage; heads large, 



the involucre mostly 2 to 4 cm. thick; outer corollas much enlarged; 



leaves lanceolate or ovate, the larger ones more or less clasping (4) . 



4. Appendage of the phyllaries whitish (rarely brownish or purplish), with 



4 to 6 pairs of not conspicuously ciliolate lobes, its undivided portion 



lanceolate 4. C. Americana. 



4. Appendage deep brown, with 8 to 12 pairs of light-edged conspicuously 

 ciliolate lobes, its undivided portion broadly triangular or ovate. 



5. C. ROTHROCKII. 



