Spring Flora of Oklahoma 
205 
Leaves tomentose on both sides, or becoming glabrous 
above; western. 2. C. ochrocentrum. 
1. Cilrsium yirginianum (L.) Michx. Virginia Thistle. Bi¬ 
ennial. Stem slender, naked or scaly above, pubescent or somewhat 
tomentose, 2 0 -3%° high. Leaves oblong, oblong-lanceolate, or the 
lowest slightly spatulate, sessile, or somewhat clasping, spinulose- 
margined, entire, lobed or pinnatifid into triangular-lanceolate lobes, 
the lower pubescent or glabrate above, and densely white-tomentose 
beneath. Heads broad, about 1' high. Outer bracts tipped 
with weak, short bristles, the inner merely acuminate. Flowers pur¬ 
ple. 
In dry Woods and thickets. April-September. Common. 
2. Circium ochrocentrum A. Gray. Yellow-spined Thistle. Bi¬ 
ennial, persistently and densely white-tomentose throughout, or the 
upper surfaces of the leaves at length green and glabrous. Stem 
leafy, often 6° high. Leaves usually very deeply pinnatifid into tri¬ 
angular-lanceolate, serrate or entire segments, armed with numerous 
long, yellow prickles. Heads about 21' broad, l%'-2' high, solitary 
at the ends of the branches. Outer bracts tipped with stout, yellow 
prickles, the inner long-acuminate. Flowers purple (rarely white). 
On plains. May-September. Infrequent. 
XXVIII. CENTAUREA L. 
Perennial or annual herbs. Leaves alternate, entire, 
dentate, incised or pinnatifid. Heads large or middle-sized, 
of tubular, purple, violet, white, or rarely, yellow flowers. 
Involucre ovoid or globose, bracts imbricated in many 
series, appressed, fimbrillate, or dentate. Receptacle flat, 
densely bristly. Marginal flowers usually neutral and lar¬ 
ger than the central ones, which are perfect and fertile. 
Corolla-tube slender, the limb regular or oblique, 5-cleft or 
5-lobed, the segments sometimes appearing like rays. 
Achenes oblong or obovoid. Pappus of several series of 
bristles or scales. 
l 1. Centaurea arnerlcana Nutt. American Star Thistle. An¬ 
nual, roughish. Stem 2°-6° high. Leaves entire or denticulate, the 
lower and basal ones spatulate or oblong, the upper oblong-lanceo¬ 
late, mucronate. Heads solitary at the ends of the leafy branches, 
very showy, 2'-4' broad. The involucral bracts ovate or lanceolate 
with conspicuously pectinate appendages. Flowers pink or pur¬ 
ple. Achenes compressed. Pappus of copious, unequal bristles 
longer than the achene. 
Dry plains. May-August. Common in southwestern part of 
state. 
