366 MISC. PUBLICATION 42 3, U. S. DEPT. OF AGRICULTURE 



5. Descurainia richardsonii (Sweet) O. E. Schulz, Pflanzenreich IV. 

 105: 318. 1924. 



Sisymbrium richardsonii Sweet, Hort. Brit. ed. 2, 30. 1830. 



Apache, Coconino, and Greenlee Counties, 6,500 to 9,500 feet. 

 Great Lakes region to Yukon, south to northern Mexico and Baja 

 California. 



Kepresented in Arizona by the glandular subsp. viscosa (Rydb.) 

 Detling (Sophia viscosa Rydb.) which was collected at Hannigan 

 Meadow, Greenlee County (Kearney and Peebles 12366), and at Wal- 

 nut Canyon, Coconino County (MacDougal in 1891); also by the non- 

 glandular subsp. incisa (Engelm.) Detling (Sisymbrium incisum 

 Engelm., Descurainia serrata O. E. Schulz), which has been collected 

 in the Carrizo Mountains, Apache County (Standley 7377). 



32. ARABIS. 53 Rockcress 

 Contributed by Reed C. Rollins 



Biennial or perennial herbs with acrid juice; caudex simple or 

 multicipitally branched; young plants rosulate; leaves entire or 

 dentate, the basal ones petiolate, the cauline leaves sessile except in 

 A. tricornuta; inflorescence racemose; siliques narrowly linear, flat- 

 tened or at least slightly compressed parallel to the septum; valves 

 1-nerved at least below; seeds numerous, winged or rarely wingless; 

 cotyledons accumbent. 



Six of the nine Arizona species have a northerly distribution in the 

 Sierra Nevada, Great Basin, or Rocky Mountains and are rare in 

 Arizona. Two species, A. gracilipes and A. tricornuta, are endemic to 

 Arizona. A. perennans is widely distributed in the State and is 

 apparently the only species of the genus commonly encountered. 



Key to the species 



1. Mature siliques strictly erect, numerous, mostly crowded and appressed to the 

 rachis; petals white to ochroleucous, rarely pinkish (2). 

 2. Siliques subterete; stigma markedly expanded, conspicuously bilobed; 

 seeds biseriate, narrowly winged or rarely wingless; petals ochroleu- 

 cous 1. A. GLABRA. 



2. Siliques decidedly flattened parallel to the septum; stigma unexpanded, 

 entire or inconspicuously bilobed; seeds uniseriate or biseriate, winged 

 all around; petals usually white, rarely pinkish (3). 

 3. Stems sparsely to densely hirsute below; siliques about 1 mm. wide; seeds 



uniseriate; petals less than 6 mm. long 2. A. hirsuta. 



3. Stems glabrous throughout or appressed-pubescent at base; siliques 

 about 2 mm. wide; seeds biseriate; petals up to 1 cm. 



long 3. A. DRUMMONDII. 



1. Mature siliques spreading or deflexed, few, not crowded; petals white to 

 purple (4). 

 4. Lower cauline leaves petiolate; petals white; sepals glabrous; style about 



1 mm. long 4. A. tricornuta. 



4. Lower cauline leaves sessile; petals pink to purple; sepals at least sparsely 

 pubescent; stigma sessile or nearly so (5). 

 5. Siliques finely pubescent, strongly reflexed but the pedicles not geniculate, 

 straight, 2.5 to 3 mm. wide; seeds broadly winged, biseriate; petals 1.5 



to 2 cm. long 5. A. pulchra. 



5. Siliques glabrous, widely spreading to pendulous, arcuate, less than 2 mm. 

 wide; seeds narrowly winged or nearly wingless, uniseriate or biseriate; 

 petals less than 1 cm. long (6). 



5 3 Reference: Hopkixs, Milton, arabis in eastern and central north America. Rhodora 39: 

 63-98, 106-148, 155-186. 1937. 





