PLANTS OF IOWA 187 



2417. 0. comiculata, L. Yellow Procumbent Wood-sorrel. 

 Infrequent ; tropical species, about greenhouses. 



2418. 0. striata, L. Upright Yellow Wood-sorrel. 

 A common weed, in open places. 



2419. 0. cymosa, Small. Tall Yellow Wood-sorrel. 

 In woods and fields, not common. 



LINACE.E. FLAX FAMILY. 

 LlNUM, L. 



2420. L. usitatissimum , L. Flax. 



Escaped from cultivation, chiefly along railway right- 

 of-way. 



2421. L. sulcatum, Riddell. Grooved Yellow Flax. 

 Common, in dry places. 



2422. L. rigidum, Pursh. Large-flowered Yellow Flax. 

 Infrequent. 



ZYGOPHYLLACE.E. CALTROP FAMILY. 



Tribulus, L. 



2423. T. terrestris, L. Ground Bur-nut. 

 Introduced from Europe; infrequent. 



RUTACE.E. RUE FAMILY. 



Xanthoxylum, L. 



2424. X. americanum, Mill. Prickly Ash. 

 Common, in thickets and on rocky banks. 



Ptelea, L. 



2425. P. trifoliata, L. Three-leaved Hop-tree. 

 Dry woods, along streams ; frequent. 



SIMARUBACE^E. AILANTHUS FAMILY. 



Ailanthus, Desf . 



2426. A. glandulosa, Desf. Tree-of-Heaven. 

 Introduced from China. 



POLYGALACE^. MILKWORT FAMILY. 

 POLYGALA, L. 



2427. P. cruciata, L. Cross-leaved Milkwort. 

 In wet places, not common. 



