392 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



Stamen tube long-exserted ; bracts lanceolate, conspicuous, elongate; 



valves of the fruit very thick 3. P. calostachys. 



Flowers capitate. 

 Spines ascending; flowers densely pubescent; leaves pale green. 



4. P. dulce. 

 Spines divaricate ; flowers glabrate ; leaves usually bright green. 



5. P. unguis-cati. 

 Plants unarmed. 



Petiole winged 6. P. furcatum. 



Petiole not winged. 



Leaflets acute or acuminate; flowers subspicate 7. P. cog'natuin. 



Leaflets rounded at apex; flowers capitate 8. P. guadalupense. 



Leaflets small, the largest 5 mm. wide, usually much longer than broad ; leaflets 

 numerous. 

 Pinnae one pair. 



Valves of the fruit very hard and woody, 2.5 to 3 cm. wide__17, P. confine. 

 Valves of the fruit not woody, less than 2 cm. wide. 



Leaflets revolute, oblong 9. P. revolutum. 



Leaflets not revolute, oval or suborbicular. 



Leaflets pubescent 10. P. elastichophyllum. 



Leaflets glabrous 11. P. compactum. 



Pinnae 2 or more pairs. 



Corolla about 1.5 cm. long, densely white-sericeous. 



Calyx pilose; leaflets 2 to 2.5 mm. wide 12. P. acatlense. 



Calyx glabrous; leaflets 4 to 7 mm. wide 13. P. leiocalyx. 



Corolla less than 1 cm. long. 



Gland present on the rachis at the point of insertion of the lowest pair 

 of pinnae. 

 Plants unarmed ; valves of the pod contorted after dehiscence. Flowers 



capitate 14. P. arboreum. 



Plants armed with numerous spines ; valves not contorted. 

 Spines recurved ; flowers capitate. Leaflets narrowly oblong. 



15. P. leptophyllum. 

 Spines straight ; flowers capitate or spicate. 



Flowers spicate 16. P. flexicaule. 



Flowers capitate. 



Leaflets 4 or 5 pairs, oval or rounded-obovate, rounded at apex. 



17. P. confine. 



Leaflets 10 to 15 pairs, oblong, acute 18. P. schaffneri. 



Gland borne on the petiole below the pinnae. 

 Calyx glabrous or nearly so ; pods constricted between the seeds. 



19. P. tortum. 

 Calyx densely pubescent ; pods not constricted. 

 Pinnae 4 to 6 pairs in most of the leaves. 



Leaflets mostly 5 to 7 mm. wide ; plants unarmed. 



20. P. tomentosum. 

 Leaflets 1.5 to 2.5 mm. wide; plants usually armed with spines. 



Pods stipitate, 1.4 to 1.7 cm. wide 21. P. brevifolium. 



Pods sessile or nearly so, 2.5 cm. wide 22. P. albicans. 



Pinnae 2 or 3 pairs in most of the leaves. 



Corolla densely sericeous ; pinnae and leaflets crowded ; spines 



strongly recurved ; pods pubescent 23. P. sonorae. 



Corolla glabrate ; pinnae and leaflets not crowded ; spines nearly 

 straight; pods glabrous or nearly so 24. P. mexicanum. 



