STANDLEY TREES AND SHRUBS OF MEXICO. 371 



Leaflets small, tlie largest 6 mm. wide. 

 Petioles flat, striate, usually 10 to 20 cm. long ; leaflets soon deciduous. 



17. A. willardiana. 

 Petioles not flat and striate, commonly less than 10 cm. long ; leaflets per- 

 sistent. 

 Stipules spinelike, usually 1 to 3 cm. long. 



Leaflets glabrous 18. A. amentacea. 



Leaflets pubescent. 



Fruit densely hirtellous 19. A. bilimekii. 



Fruit glabrous or minutely puberulent 20. A. sonorensis. 



Stipules not spinelike, the spines all less than 1 cm. long. 

 Leaflets 3 to 7 pairs. 



Leaflets 2.5 to 4 mm. long; fruit about 1.5 cm. wide. 



21. A. greggii. 

 Leaflets mostly 6 to 10 mm. long ; fruit 2 cm. wide or wider. 



22. A. wrightii. 

 Leaflets 10 to many pairs in all or most of the leaves. 



Rachis of the leaves with recurved spines 23. A. iguana. 



Rachis of the leaves unarmed. 

 Pinnae much shorter than the rachis of the leaf, usually numerous 

 (commonly 15 or more). 



Fruit velvety-puberulent, about 2 cm. wide 24. A. macilenta. 



Fruit glabrous or nearly so, about 1.5 cm. wide. 



Rachis of the leaves glabrous 25. A. millefolia. 



Rachis of the leaves densely short-pilose 32. A. acatlensis. 



Pinnae nearly or quite as long as the rachis or often longer, 

 usually less than 10. 



Leaflets 7 to 10 pairs 26. A. rotundata. 



Leaflets 10 to many pairs. 

 Pinnae 2 to 5 pairs. 



Leaflets 8 to 11 mm. long 27. A. gaumeri. 



Leaflets 2 to 6 mm. long. 



Surfaces of the leaflets glabrous, the margins ciliate. 



28. A. dolichostachya. 

 Surfaces, at least the lower one, of the leaflets puberu- 

 lent or sericeous. 

 Leaflets about 2 mm. long ; fruit 16 mm. wide. 



29. A. compacta. 

 Leaflets 6 mm. long ; fruit 8 ram. wide. 



30. A. mammifera. 

 Pinnae 6 to 10 pairs in all or most of the leaves. 

 Pinnae mostly 5 to 8 cm. long ; fruit glabrous. 



31. A. coulteri. 

 Pinnae mostly 1.5 to 3 cm. long ; fruit pubescent. 



32. A. acatlensis. 

 BB. Flowers capitate. 



Stipules spinose, stout, straight, usually long. 



Fruit covered with thick stalked .:;l:in(ls___ 33. A. glandulifera. 



Fx'uit without glands, or the glands sessile. 



Fruit flat, bivalvate; bracts borne near the middle of the peduncle. 

 Fruit 2 to 3 cm. long, strongly curved ; spines very slender, yellowish. 



34, A. biaciculata. 

 Fruit 5 to 12 cm. long, straight or nearly so ; spines stout, brown, 

 white, or grayish. 



