38 
CIRSIUM, thistle 
Cirsium altissimum (L.) Sprengel, tall thistle, roadside thistle, 
p- 1622 
f. altissimum 
f. moorei Steyerm. [G: C. altissimum] (we have arbitrarily 
chosen to recognize the white-flowered form, although 
the validity of maintaining this morph has not been 
investigated in detail 
+C. arvense (L.) Scop., Canada thistle, field thistle, p. 1624 
var. horridum Wimmer & Grab. [S: C. arvense var. arvense] 
(Cronquist, 1955; Moore and Frankton, 1974) 
f. horridum 
C. carolinianum (Walter) Fern. & B. G. Schubert, p. 16 
C. discolor (Muhlenb. ex Willd.) Sprengel, field thistle, p. 1621 
f. albiflorum (Britton) House 
f. discolor 
C. muticum Michaux, Swamp thistle, p. 1624 
um 
f. mut 
*+C. texanum Buckley, Texas thistle (Miihlenbach, 1979) 
texanum 
r. nu 
C. undulatum (Nutt.) Sprengel, wavy-leaved thistle [S: incl. var. 
megacephalum (A. Gray) Fern.] (Ownbey, 1952), 
1621 
*C. vulgare (Savi) Ten., bull thistle, p. 1621 
CONYZA [s: ERIGERON] 
(Cronquist, 1943) 
Conyza canadensis (L.) Crongq., horse weed, Canada fleabane, hog 
weed [S: Erigeron canadensis L.} (Cronquist, 1943, 
1947), p. 1529 
var. pusilla (Nutt.) Crongq. [S: Erigeron pusillus Nutt.] 
(Cronquist, 1947) 
e ramosissima Cronq., dwarf fleabane, spreading fleabane [S: 
Oi divaricatus Michaux] (Cronquist, 1943), 
p. 
S: Steyermark &: Great Plains Flora M: Mohlenbrock 
