28 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



Leaves alternate. ' 

 Flowers sessile in dense heads on a common receptacle surrounded 

 by an involucre of bracts. Corolla always present, of united 

 petals, usually colored and often showy; fruit an achene or 



rarely of 2 dry adherent carpels j (p. 37), 



Flowers not sessile in dense heads on a common receptacle sub- 

 tended by an- involucre of bracts. 



Leaves entire (sometimes cordate at the base) K (p. 38). 



Leaves toothed or lobed L (p. 39) 



A. 



Grasses or grasslike plants. 



Flowers not inclosed by husklike scales, composed of 3 sepals and 3 petals; stems 

 terete, usually simple, hollow or solid; fruit a dehiscent capsule containing few 



or many seeds JTTNCACEAE (p. 117). 



Flowers inclosed by husklike scales, without a proper calyx or corolla; stems some- 

 times branched ; fruit indehiscent, 1-seeded. 

 Leaves in 2 ranks on the stems; stems round or somewhat flattened, usually hollow; 

 leaf sheaths usually split; flowers with 2 bracts, one below and one above. 



POACEAE (p. 66). 



Leaves in 3 ranks on the stems; stems round or usually 3-angled; leaf sheaths not 



split; flowers with only one bract, this below CYPEB.ACEAE (p, 92). 



B. 



Plants floating on or submerged in water. 

 Plants very small, less than 1 cm. long, floating free, consisting of a frond bearing 1 

 or 2 minute flowers on the upper surface, often with 1 or more roots beneath. 



LEMNACEAE (p. 114). 



Plants larger, several centimeters long or very large, with conspicuous leaves and 



usually with branched stems. 



Leaves entire or finely toothed, never lobed, sometimes deeply cordate at the base. 



Blades of the leaves deeply cordate at the base, or peltate (the petiole attached 



to the middle of the blade). 



Leaves peltate, rounded, rarely more than 4 cm. wide; corolla of united petals, 



not more than 3 mm. long; flowers clustered Hydrocotyle (p. 219). 



Leaves not peltate, or, if so, oval and usually much more than 4 cm. in diameter; 

 corolla of distinct petals, much more than 3 mm. long; flowers not clustered.' 



NYMPHAEACEAE (p. 155). 

 Blades of the leaves not cordate at the base, never peltate. 

 Leaf blades spatulate in the floating leaves. Flowers minute, solitary or 2 or 3 

 together in the axils of the leaves, sessile; fruit nutlike, 4-lobed. 



T , . . . . . CALLITRICHACEAE (p. 197). 



Leaf blades never spatulate. 



Plants acaulescent. Leaves long, linear; fertile flowers borne on long, very 



slender scapes; fruit indehiscent, many-seeded Vallisneria (p. 66). 



Plants with stems. 

 Leaves alternate. 

 Stipules present; flowers in Bpikes, green; leaves linear or broad; fruit 



1-seeded.. POTAMOGETONACEAE (p. 62). 



Stipules wanting; flowers solitary on the end of a long slender peduncle, 



yellow; leaves linear; fruit many-seeded Heteranthera (p. 116). 



Leaves opposite or whorled. Flowers minute, green. 

 Flowers with a 3 or 6-parted perianth; leaves finely toothed; fruit 



several-seeded Anacharis (p . 66) . 



Flowers without a perianth; leaves entire or finely toothed ; fruit 1 -seeded. 



NAIADACEAE (p. 64). 



