H4 



In a few composites (see la below) the flowers have minute 

 corollas without colored parts. 



In identifying a composite, determine first whether the heads are 

 composed of tubular flowers, of strap-shape flowers, or of both sorts 

 together; and, secondly, observe the nature of the pappus, using pre- 

 ferably the ripe heads, or at least the oldest flower-heads available. 

 No further difficulties will be encountered. 



la. Flowers without petal-like or brightly colored parts; staminate 

 and pistillate flowers in separate heads (or rarely in the same 

 heads) ; coarse weeds with inconspicuous flowers (summer and 

 autumn) 2. 



Ib. Flowers with some petal-like parts, usually brightly colored or 

 white 10. 



2a. Leaves toothed or lobed 3. 



2b. Leaves deeply pinnatifid or dissected (4 15 dm. high; flow- 

 ers in erect spikes) (Ragweed) 7. 



3a. Fruit or pistillate flowers thickly covered with sharp hooked 



spines (310 dm. high) (Cocklebur) 4. 

 3b. Fruit not spiny 8. 



4a. With spines on the stem at the base of the leaves 



Cocklebur, Xanthium spinosum. 

 4b. Without any spines on the stem 5. 



5a. Body of the bur smooth or slightly hairy 



Cocklebur, Xanthium canadense. 



5b. Body of the bur and the spines densely pubescent 6. 



6a. Body of the bur more than twice as long as thick; a com- 

 mon weed Cocklebur, Xanthium commune. 

 6b. Body of the bur less than twice as long as thick 



Cocklebur, Xanthium echinatum. 



7a. Leaves twice-pinnatifid Ragweed^ Ambrosia artemisiifolia. 



7b. Leaves once-pinnatifid Rag-weed, Ambrosia psilostachya. 



8a. Leaves deeply 3-lobed (1 5 m. high) 



Giant Ragweed, Ambrosia trifida. 



8b. Leaves -serrate or obscurely lobed 9. 



9a. Stem simple or sparingly branched; pistillate heads in the axils 

 of the upper leaves (1 3 m. high) 



Giant Ragweed, Ambrosia trifida yar. integrifolia. 

 9b. Stem much branched; heads all alike, in panicles 



Marsh Elder, Iva xanthifolia. 



lOa. Flowers all strap-shape; juicy milky (The central flowers 

 must be examined carefully, since they are frequently much 

 smaller than the marginal ones) 11. 



lOb. Flowers all tubular, with regular, 4 5-lobed corollas 45. 



lOc. Flowers both tubular and strap-shape; heads radiate (in a 

 few species the rays are small and may be overlooked by mis- 

 take) 108. 



