KEY. 377 



Heads of two sorts, the sterile with many, the fer^ 

 tile with 1 or 2 flowers 935 



933. Heads corymbed. Pappus of bristles 934 



Heads solitary, terminal. Pappus of 5 or 6 fcales. 

 Anthers blue. Leaves entire, ovate-lanceolate, 3-nerved, ses- 

 sile. Idarshallia latifolia, Pursh. 



934:. Receptacle chafly throughout, with short, rigid, 



narrow, 3-nerved pales between the flowers. Flowers purple. 



Carphephorus tomentosus, Torr. and Gr. 



The columnar receptacle naked at the top, chaffy 



on the margins and toward the base. Heads woolly. Herb 



do wny-canescent 929 



935. Fertile involucres 1-flowered, oblong and, 4-angled. 

 Sterile heads crowded in a cylindrical spike, the cup-shaped 

 involucres produced on one side into a recurved hispid tooth. 

 Leaves lanceolate, partly clasping. 



Ambrosia bidentata, Mx. 



Fertile involucres 2-flowered, forming in fruit an 



oblong prickly bur. Sterile involucres globose, of distinct 



scales. (798) Xanthiiun. 



936. Flowers of the margin pistillate and fertile, 1-5, 

 those of the centre staminate, sterile. Shrubs or herbs. In- 

 volucral scales roundish, few. Heads small, greenish-white, 

 on short, recurved peduncles, forming leafy, panicled racemes 

 or spikes. Iva. 



{Marsh-Elder.) 



Flowers in the heads all of one sort 937 



937. Pappus of 2 or few retrorsely barbed awns. 



Bidens (985). 

 Pappus of 2 upwardly barbed, or naked awns. 



Coreopsis (985). 



938. (906.) Pappus composed of many bristles 939 



Pappus none, or a few scabrous bristles. Involu- 



cral scales fringed-margined, or ciliate, the middle ones some- 

 times spiny. Heads single. Flowers purple, rarely whitish, 

 the marginal often falsely radiate, large, blue. Centaurea. 



939. Receptacle deeply honeycombed. Stems winged by 



