ROBINSON AND GREENMAN. — GENUS POROPHYLLUM. 31 



v. — A PROVISIONAL KEY TO THE SPECIES OF PORO- 

 PHYLLUM, RANGING NORTH OF THE ISTHMUS 

 OF PANAMA. 



* Leaves with broad obtuse ovate or elliptic-oblong usually thin lamina, rather 



abruptly contracted at the base to a slender petiole. 

 ^- Heads very long ; involucral scales at maturity 8 to 12 lines in length : pe- 

 duncles conspicuously clavate : erect scarcely distinct annuals. 



1. P. macrocephalum, DC. Prodr. v. 648. 



2. P. ruderale, Cass. Diet. Sci. Nat. xliii. 56. 



3. P. ellipticum, Cass. 1. c. 



•(- ■*- Heads shorter ; involucral scales about half inch in length : peduncles 



scarcely or not at all thickened toward the summit. 



*+ Leaves with pellucid glands on the surface as well as at the margins. 



4. P. Ervendbergii, Gray, Proc. Am. Acad. xix. 35. 



5. P. nummularinm, DC. 1. c. 649. 



•t-f t-f Glands of the leaves marginal or none, 

 = Leaves broad, thickish : petioles rather short : probably a single species. 



6. P. viridiflomm, DC. 1. c. 648. 



7. P. Lindenii, Schz. Bip. in Seem. Bot. Herald, 308. 



= = Leaves thin and delicate. 

 a. Heads nodding or almost pendulous. 



8. P. nutans. Leafy shrub with slender terete fuscous branches 

 (marked with small light colored lenticels) and almost filiform branch- 

 lets : leaves thin, elliptic, entire, or crenulate, obtuse but often mucronu- 

 late at the apex, obtuse or acutish at the base, 8 to 14 lines long, half as 

 broad ; marginal glands 4 to 6 on each side the leaf; petioles filiform, 3 

 or 4 lines long : heads numerous, about 9 lines long : peduncles clustered 

 at the ends of the branches in 3's and 4's : involucral scales 5, oblong, 

 obtuse, 6 lines long, somewhat carinate, green, with double row of linear 

 glands : flowers white or nearly so, 9 lines in length ; limb of the corolla 

 sub-bilabiate, shallowly toothed : achenes 5 lines in length. — Collected 

 by 0. G. Pr ingle, in mountains near Lake Chapala, 16 December, 1889, 

 no. 2976, and by the same collector on rocky hills, Querendaro, Michoa- 

 can, 25 October, 1892 ; later by E. W. Nelson, between Chilapa and 

 Tixtla, Oaxaca, altitude 5,200 to 7,000 feet, no. 2170. Mr. Pringle's 

 plants were at first determined as P. Ervendbergii and so distributed. 

 On subsequent comparison with that species they appear thoroughly 

 distinct, having only marginal glands on the leaves, nodding instead of 

 erect peduncles, less deeply cleft corolla, and longer achenes. 



