Xanthium.] XLIII. COMPOSITE. 235 



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 Ambrosicce, the general habit and unisexual flowers 

 showing at first sight some analogy to Urticece and some other Monoch- 

 lamydce. 



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.] 



XL BIDENS. BIDENS. 



Glabrous herbs, with opposite leaves, and hemispherical heads of 

 yellow flowers. Involucres of 2 or 3 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 1. 2?. cernua. 



Leaves deeply cut into 3 to 5 segments 2. B. tripartite^ 



1. B. cernua, Linn. (fig. 521). Bur- Marigold. 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. 



