236 MISC. PUBLICATION 4 2 3, U. S. DEPT. OF AGRICULTURE 



subtending bracts, with a thick central column expanded at apex into 

 a fleshy disk, the latter bearing under its margin in staminate flowers 

 the anthers, in pistillate flowers the ring-shaped stigma; ovary inferior; 

 fruit a many -seeded capsule. 



1. Pilostyles thurberi A. Gray, Amer. Acad. Arts and Sci. Mem. ser. 

 2, 5: 326. 1855. 



Known definitely only from the type locality, south side of the Gila 

 River near its confluence with the Colorado (Yuma County), but 

 reported to occur also in the Colorado Desert, Calif., flowering in 

 April. The type was collected by George Thurber in 1850. 



Parasitic on the stems of Dalea emoryi, a shrubby legume. All 

 specimens of infested branches of the host, collected by Harrison, 

 Loomis, and the writers, bear only pistillate flowers. It is noteworthy 

 that the family to which this tiny plant belongs includes a plant having 

 the largest of all known flowers, Rafflesia arnoldii, a native of Sumatra. 



33. POLYGONACEAE. Buckwheat family 



Plants herbaceous or woody; stems usually jointed; leaves simple, 

 often with stipules that are united into a sheath; flowers small, regular, 

 commonly perfect; perianth not differentiated into a calyx and a 

 corolla but usually corollalike; stamens 2 to 9; pistil 1, the ovary 

 superior, 1 -ceiled; style cleft or parted, the divisions 2 or 3; fruit an 

 achene, usually triangular. 



This family includes buckwheat (Fagopyrum) and rhubarb (Rheum), 

 also several common weeds of the genera Rumex (dock) and Polygonum 

 (kno tweed). 



Key to the genera 



1. Flowers subtended by a campanulate, turbinate, or cylindric involucre (2). 

 2. Teeth or lobes of the involucre not bristly or spiny at tip, _ 4. Eriogonum. 

 2. Teeth or lobes of the involucre ending in bristles or spines (3). 



3. Involucre subtending usually only 1 flower, its teeth tipped with hooked 

 or straight spines or bristles; bracts not connate-perfoliate. 



2. Chorizanthe. 

 3. Involucre subtending 2 or more flowers, its teeth tipped with straight 

 spines or bristles; upper bracts connate-perfoliate, forming a cup- 

 shaped disk - 3. Oxytheca. 



1. Flowers not subtended by an involucre of the foregoing character (4). 



4. Leaves opposite, without stipules 1. Pterostegia. 



4. Leaves alternate or basal, with sheathlike stipules (5). 



5. Sepals 4 or 6, the outer ones spreading or reflexed, remaining small, the 

 inner ones usually erect and enlarged in fruit (6) . 

 6. Leaf blades lanceolate to oblong-ovate; sepals 6, one or more of the inner 

 ones often with a large dorsal tubercle; styles 3 5. Rumex. 



6. Leaf blades reniform; sepals 4; styles 2 6. Oxyria. 



5. Sepals 5 (exceptionally 4) , all similar and erect in fruit (7) . 



7. Achene enclosed by the fruiting calyx or, if exserted, then the leaf blades 



narrow and attenuate at base 7. Polygonum. 



7. Achene long-exserted ; leaf blades broadly deltoid, hastate, or cordate. 



8. Fagopyrum. 



1. PTEROSTEGIA 



Plant herbaceous, annual; stems slender, weak, diffusely branched, 

 often prostrate; leaves opposite, petioled, the blades cuneate-obovate 

 or fan-shaped, usually deeply lobed; flowers solitary, subtended by a 



