268 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



2. AMBROSIA L. 



Leaves opposite and alternate, once or twice lobed; receptacle chaffy; plants usually 



low, seldom over 1 meter high 1. A. elatior. 



Leaves all, except the uppermost among the inflorescence, opposite, palmately 3 or 

 5-lobed, pr often undivided; receptacle not chaffy; plants usually 1.5-2 meters 

 high. 



Leaves mostly 3-5-lobed 2. A, trifida. 



Leaves not lobed 2a. A. trifida integrifolia. 



1. Ambrosia elatior L. Common ragweed. 

 Waste and cultivated ground and roadsides; abundant. Aug.-Oct. Nearly 



throughout the U. S. (A. art emisiacf alia L.) 

 The abundant pollen of this plant is one of the chief causes of hay fever. 



2. Ambrosia trifida L. Great ragweed. 

 Damp soil near streams; abundant. Aug.-Sept. Eastern N. Amer. 



Known also as horseweed. 



2a. Ambrosia trifida integrifolia (Muhl.) Torr. & Gray. 

 With the typical form. 



153. ASTERACEAE. Aster Family. 



In this family, as well as in the Ambrosiaceae and Cichoriaceae (all three of which 

 are often united as the Compositae), the form of the inflorescence is conspicuously 

 different from that of our other flowering plants, so much so as often to deceive ama- 

 teurs, who mistake the head of flowers for a single flower. The flowers are borne in a 

 close head (except in the pistillate inflorescence of Ambrosiaceae) upon a flat or 

 convex receptacle, the latter often bearing chaff among the flowers. The head is 

 surrounded by an involucre of bracts. The corolla is tubular or strap-shaped. In the 

 Cichoriaceae all the flowers are strap-shaped. In the Asteraceae all the flowers may 

 be tubular, but more frequently the outer ones, or rays, are strap-shaped, while the 

 inner (forming the disk) are tubular. The fruit is an acheme. The calyx tube ia 

 united with the fruit, while the calyx limb is represented by bristles, awns, scales, 

 etc., called the pappus. 



A. Outer flowers of the heads never with strap-shaped (ray) corollas, the corollas with 



5 equal or nearly equal lobes, or the corollas of the outer flowers sometimes larger 



than those of the inner ones. 



Leaves opposite or whorled. 



Fruit covered with numerous slender hooked spines. (Ray flowers are present 



in this genus but they are so small as to be overlooked easily.) 



__ . A 23. ACANTHOSPERMUM. 



Fruit never with hooked spines. 



Flowers yellow or yellowish; involucre of 2 distinct series of bracts, the outer 

 ones green, the inner brownish or straw-colored. Leaves often lobed or 

 composed of several leaflets. 



Plants with viscid pubescence ; pappus none 22. POLYMNIA. 



Plants never with viscid pubescence; pappus of 2-6 long slender barbed 



awns 34. BIDENS. 



Flowers white, purplish, or blue; bracts all alike or similar. 



Stems climbing; bracts 4 4. MIKANIA. 



Stems erect; bracts more than 4 3. ETJPATORITTM. 



Leaves alternate. 

 B. Leaves with spiny teeth, or the bracts armed with hooked spines, or deeply 

 toothed. Plants usually with more or less woolly pubescence. 

 Bracts ending in hooked spines. Leaves mostly borne at the base of the stem, 

 very large, not spine-toothed 47. ARCTIUM. 



