116 CONTRIBUTIONS FROM THE NATIONAL. HERBARIUM. 



Florets glabrous, or the palea minutely hispidulous. 

 Panicle contracted, spikelike, not over 3 cm. long, the branches appressed or the 



lower sometimes ascending; plants low and spreading 3. I. pygmaea. 



Panicles open, the branches spreading or ascending. 

 Blades about 3 mm. wide, thick, rigid, pungent, with conspicuously thickened 



midrib 4. I. rigidifolia. 



Blades mostly 5 to 20 mm. wide, firm but not pungent nor with thickened midrib. 



Plants trailing; blades rarely over 5 cm. long 5. I. rigens. 



Plants clambering; blades mostly more than 5 cm. long. 

 Glumes pubescent; blades firm, not over 12 cm. long and 1 cm. wide. 



6. I. angustifolia. 

 Glumes glabrous (rarely obscurely pubescent at the tips); blades mostly 



over 15 cm. long and 1.5 cm. wide. 

 Spikelets aggregate toward the ends of the branches and branchlets. 



7. I. arundinacea. 

 Spikelets not aggregate; panicle loosely flowered 8. I. disperma. 



1. Isachne polygonoides (Lam.) Doell. 



Panicum polygonoides Lam. Encycl. 4: 742. 1798. "Cette plante crott a Cayenne, 

 & m'a et6 communiquee par le citoyen Leblond." The type, in the Paris Herbarium, 

 is an entire plant. 



Panicum trachyspermum Nees, Agrost. Bras. 212. 1829. "Habitat in graminosia 

 prope Para provinciae Paraensis." The type, collected by Martius, has been 

 examined in the Munich Herbarium. 



Isachne trachysperma Nees in Seem. Bot. Voy. Herald 224. 1857. Based on Panicum 

 trachyspermum Nees. 



Isachne polygonoides Doell in Mart. Fl. Bras. 2 2 : 273. 1877. Based on Panicum 

 polygonoides Lam. 



DESCRIPTION. 



Culms decumbent, branching and spreading, rooting at the lower nodes, the 

 flowering branches ascending, 10 to 30 cm. tall, glabrous, the nodes glabrous but the 

 base of the sheath hispid; sheaths mostly shorter than the internodes, papillose- 

 hispid or glabra te; ligule a ring of stiff hairs about 2 mm. long; blades ovate or ovate- 

 lanceolate, 2 to 5 cm. long, 7 to 17 mm. wide, cordate-clasping and ciliate at base, 

 acute or somewhat acuminate at apex, scabrous on the upper surface, puberulent or 

 pubescent beneath; panicles numerous, ovoid, partially inclosed in the sheaths or 

 finally exserted, the branches and branchlets slender, spreading, stiff and more or 

 less implicate, the pedicels 2 to 4 mm. long, enlarged at the summit; spikelets about 

 1.5 mm. long; first glume glabrous; second glume sparsely hispidulous; lower floret 

 ovate, only slightly turgid, greenish, glabrous, cartilaginous rather than indurate, 

 exceptional for the genus in resembling the glumes rather than the upper floret; 

 upper floret pubescent, whitish, indurate", hemispherical. 



This species appears to be an annual. It is distinguished from all our other species 

 by the ovate clasping blades and by the dissimilarity of the two florets. 



DISTRIBUTION. 



Moist ground, often in the water, Guatemala to Brazil. 

 Guatemala: Chupadero, Heyde & Lux 3916. 

 Costa Rica: Buenos Aires, Tonduz 4874. Boruca, Tonduz 4623. Turrialba, Tonduz 



8233. San Ramon, Tonduz 17909. 

 Panama: Porto Bello, Pittier 2454. Dolega, Hitchcock 8333. Cocle\ Pittier 4917. 



Chepo, Pittier 4531. Corozal, Hitchcock 9198. David, Hitchcock 8375. Gatun 



Lake, Amer. Gr. Nat. Herb. 599. Juan Diaz, Killip 4060. 

 Trinidad: Piarco Savanna, Amer. Gr. Nat. Herb. 598. 



