248 FLOWERS OF FIELD, HILL, AND SWAMP 



which is the despair of every farmer once it makes its appearance 

 in his pasture. 



43. Canada Thistle 



C. arvSnsis, i to 2 feet high, with many small, purplish 

 heads of flowers, so troublesome a weed in New England 

 fields, has large, creeping roots, which can only be eradicated 

 by frequent, deep ploughing. 



44, Pasture Thistle 



C. piimilus is about the same height, with i, 2, or 3 rather 

 small heads of rose-colored flowers, i to 2 feet high. 



45 



The tallest of the thistles is C.altissimus, sometimes 10 feet 

 high, with rather small heads of rose-colored flowers. 



46 



C. horrldulus is a thistle whose flowers are pale yellow, 

 sometimes purple, very prickly. The small spines are yellow- 

 ish. These grow in dry or sandy soil as weeds more or 

 less troublesome in fields or pastures, i to 3 feet high. 



47. Venus's Looking-glass 



Specuthria perfoliata. — Family, Campanula. Color, blue, 

 with purplish tint. Leaves, round, clasping the stem with a 

 heart-shaped base. Time, June to August. 



Calyx, 3 to s-lobed. Corolla, 5-lobed. The wheel-shaped, 

 blue flowers of this plant are found singly or in twos or threes 

 in the upper leaf-axils. The stem, 3 to 20 inches high, is sim- 

 ple, weak, with milky juice. The fruit is a 3-sided capsule. 

 The lower flowers on the stem, 2 or 3 together, half -en- 

 closed by the shell - like leaf, are small, cleistogamous, fer- 

 tilized in the bud, and never expand into flower. The upper, 



