Xanthium.] XLII. COMPOSIT A. 935 
X. XANTHIUM. BURWEED. 
Coarse annuals, with alternate leaves, and unisexual, axillary or 
terminal heads of green flowers. Involucre of the males of several 
bracts in a single row, enclosing many tubular florets, separated by the 
scales of the receptacle. Anthers free. Female florets 2 together, 
combined with the involucre into an ovoid or oblong, prickly burr, 
terminating in 2 beaks, from which the stigmas shortly protrude. 
A genus of two or perhaps three species, from the Mediterranean 
region to the Levant, but spread as weeds of cultivation over a great 
part of the globe. Its immediate connection with the remainder of © 
Composites can only be traced through several exotic genera forming 
the small subtribe of Ambrosice, the general habit and unisexual flowers 
showing at first sight some analogy to Urticee and some other Monoch- 
lamydee. 
1, X. Strumarium, Linn. (fig. 520). Burweed.—A coarse, erect 
annual, 1 to 2 feet high. Leaves on long stalks, rather large, broadly 
heart-shaped, coarsely toothed or angular, rough on both sides. Flower- 
heads in axillary or terminal clusters, on short racemes ; the upper ones 
male ; the lower female heads forming, when in fruit, ovoid burrs, about 
6 to 8 lines long, covered with hooked prickles ; the stout, short, conical 
beaks erect or turned inwards. 
In cultivated and waste places throughout central and southern 
Europe and central Asia, extending, as a weed of cultivation, north- 
wards to the Baltic, as well as into many other parts of the globe. 
Occasionally found in some of the southern counties of England and 
-Ireland, but is not a British plant. Fl. summer. [Two species occur 
occasionally in Britain, X. Strumarium, L., with cordate leaves, and 
usually hooked beaks to the fruiting involucres ; and X. spinosum, with 
cuneate bases of the leaves and single straight beaks. ] 
XI. BIDENS. BIDENS. 
Glabrous herbs, with opposite leaves, and hemispherical heads of 
yellow flowers. Involucres of 2 or 8 rows of bracts, the outer ones 
often longer and leafy. Florets either all tubular, or the outer ones 
ligulate and radiating. Receptacle with chaffy scales between the 
florets. Achenes flattened, crowned by 2 or 3 (very rarely 4 or 5) short, 
stiff bristles or awns, which are rough, with minute deflexed prickles. 
A genus not very numerous in species, but diffused over the whole 
surface of the globe, some being among the commonest tropical weeds. 
Leaves undivided . { Salts oe : . erie) Bo cennuar 
Leaves deeply cut into 3 to 5 segments : : . 2 B. tripartiia. 
1. B. cernua, Linn. (fig. 521). Bur- - Masipeiltl —A rather stout, erect 
annual, 1 to 2 feet high, with spreading branches. Leaves lanceolate 
and serrate, but not divided. Flower-heads drooping, on terminal 
peduncles, from half an inch to an inch diameter ; the florets usually 
all tubular, but occasionally a few of the outer ones become ligulate. 
Inner bracts of the involucre broad, and often shining, and yellow on 
their edges ; outer ones more leafy, and often much longer, and spread- 
ing. Awns ‘of the achenes 2 or 3, very rarely 4. 
