168 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



16. ARABIDOPSIS (DC.) Schur. 



1, Arabidopsis thaliana (L.) Britton. Mouse-ear cress. 



Fields; common. March-May. Native of Eur. and Asia; widely naturalized in 

 N. Amer. (Sisymbrium thalianum Gay; Stenophragma thaliana Celak.) 



17. CAMPE Dulac. Winter cress. 



Lobes of the leaves mostly 4-8 pairs; pods stout, sharply angled, on very stout short 



pedicels 1. C, verna. 



Tx>bes of the leaves mostly 1-4 pairs; pods slender or stout, obtusely angled, on slender 

 pedicels. 



Pods erect; petals pale yellow 2. C. stricta. 



Pods spreading or ascending; petals bright yellow 3. C. barbarea. 



1. Campe verna (Michx.) Heller. 



Woods, fields, and waste ground; abundant. Native of Eur.; extensively natural- 

 ized in eastern N. Amer. (Barbarea praecox R. Br.; B. verna Aschers.) 



This and the other species are much used as "greens," and are commonly seen in 

 the markets in spring. 



2. Campe stricta (Andrzej.) W. P. Wight. 



Fields; abundant. Apr.-May. Native of Eur. Asia, and northern N. Amer.; with 

 us doubtless adventive. (Barbarea vulgaris of Ward's Flora; B. stricta Andrzej.) 



3. Campe barbarea (L.) W. F. Wight. 



Fields; apparently not common. Apr.-May. Native of Eur. and northern N. 

 Amer. ; naturalized in eastern N . Amer. (Barbarea vulgaris R. Br. ; B. barbarea MacM. ) 



18. ARABIS L. Rock cress. 



Lower leaves pinnately divided; stem leaves mostly linear and entire, tapering at the 



base. Pods ascending 1. A. Iyrata. 



Lower leaves merely toothed; stem leaves all or mostly broader than linear, often 

 toothed. 

 Stem leaves, at least the upper ones, long- tapering at the base, not auricled, more 



or less toothed ; pods drooping. Plants hairy 2. A. canadensis. 



Stem leaves never tapering at the base, more or less clasping and auricled; pods 

 ascending or spreading. 

 Plants glabrous throughout; upper leaves mostly entire, long- tapering at the 



apex. Pods recurved-spreading — 3. A, laevigata. 



Plants hairy, at least below; upper leaves all or nearly all conspicuously toothed, 

 never long-tapering at the apex. 

 Petals more than twice as long as the calyx; pods strongly ascending; seeds 



narrowly winged 4. A. patens. 



Petals equaling or very slightly exceeding the sepals; pods spreading; seeds 

 not winged 5. A. dentata. 



Arabis hirsuta (L.) Scop, was reported from Sandy Landing by Ward , but the species 

 is not represented by specimens from our region. 



1. Arabis Iyrata L. 



On rocks along the upper Potomac; frequent; also collected once along the Eastern 

 Branch, probably adventive there. March-June. Widely distributed in N. Amer ; 

 also in eastern Asia. 



2. Arabis canadensis L. Sickle-pop. 

 Woods along the Potomac and Rock Creek; frequent. May-June. Eastern IT. S. 



3. Arabis laevigata (Muhl.) Poir. 



Woods, often on rocks; frequent along the upper Potomac, occasional elsewhere. 

 Apr.-May. Eastern N. Amer. 



