272 American Midland Naturalist Monograph No. 7 



or annual; woods, common throughout 111. July-Sept. 

 [L. villosa Jacq.] Woodland Lettuce ..L.floridana (L.) Gaertn. 

 6. Heads 1-2 cm in diameter, the involucres 16-20 mm high; 

 leaves linear-lanceolate, entire or dentate to lobed or pin- 

 natifid, those of the stem partly clasping; plants perennial, 

 adv. from western U.S.; an occasional migrant along rail- 

 roads in the northern counties. July-Aug. Blue Lettuce 



L. pulchella (Pursh) DC. 



5. Pappus brown; flowers cream or bluish; leaves pinnatifid or 

 merely sinuate; alluvial soil in woods, occasional. Aug.- 

 Sept. Tall Blue Lettuce L. biennis (Moench) Fern. 



67. Sonchus L. — Sow Thistle 



1. Plants perennial; heads 4-5 cm in diameter, the flowers bright 



yellow; involucre 1.5 cm high; achenes striate and papillose. 



2. Involucres and peduncles glandular- setose; fields and waste 



places in a few scattered localities in the northern half of 



the state; first coll. in Cook Co., by E. J. Hill in 1882; nat. 



from Eur. July-Sept S. arvensis L. 



2. Involucres and peduncles glabrous; fields and waste places, 



first reported for 111. in 1934; recently spread with remark- 

 able rapidity almost throughout the northern half of the 

 state, where it is now an abundant weed; nat. from Eur. 

 July-Sept. [S. arvensis var. glabrescens Guenth., Grab. & 



Wimm.] .S". uliginosus Bieb. 



1. Plants annual; heads 1-2.5 cm in diameter, pale yellow; involucre 

 1 cm high. 



3. Leaves runcinate-pinnatifid, scarcely prickly, the tenninal seg- 



ment commonly large and triangular; upper leaves clasping 

 by an acute, sagittate base; achenes longitudinally striate and 

 papillose; fields and waste places; nat. from Eur. June-Nov. 



Common Sow Thistle ..S. oleraceus L. 



3. Leaves toothed or more or less curled or lobed, harshly prickly, 

 the basal auricles rounded; achenes longitudinally ribbed, 

 otherwise smooth; fields and waste places; nat. from Eur. 

 May-Aug. Spiny Sow Thistle S. asper (L.) Hill 



68. Prenanthes L. — Rattlesnake-root 



1 . Involucre glabrous. 



2. Heads about 2 mm thick, 5- to 7-flowered; pappus straw- 

 colored ; plants not glaucous ; oak woods, locally in the eastern 

 and southern counties; absent from the western part of the 

 state. Aug. -Sept P. altissima L. 



2. Heads 3-5 mm thick, 8-12-flowered; pappus reddish-brown; 



plants glaucous; woods in the northern half of the state. 



Aug.-Sept P. alba L. 



\. Involucre pubescent. 



3. Stem usually simple; heads 12- to 16-flowered. 



