Fig. 37: Potamogeton clystocarpus: a, upper portion of leaf to show venation and 

 apex, X 5; b, portion of plant to show free margins of stipule, X IV-r, c, fruit, X 10; d, 

 fruit cut to show coil of embryo, X 10. 



clasping the stem but with margins free, becoming lacerate at the apex; peduncles 

 filiform, 15-65 mm. long; spikes short-cylindric, 8-10 mm. long, with 2 or 3 

 whorls of flowers; flowers with perianth broad-flabelliform and with slender claw, 

 2.5-3 mm. long, 1.5-2.5 mm. wide; anthers 0.8-1.2 mm. long; fruits obliquely 

 obovate to suborbicular, with 2 or more verrucose protuberances near the base, 

 2.5-2.8 mm. long, 1.8-2 mm. wide; dorsal keel rounded to prominently developed 

 and gibbous at base; lateral keels rounded or obscure; beak facial, recurved, 0.2- 

 0.5 mm. long; exocarp dark-olive-green; endocarp loop solid; apex of seed point- 

 ing slightly above the basal end or between the base and the middle of the opposite 

 side. 



In quiet pools and flowing streams, known only from Little Aguja Canyon, 

 Davis Mts., Jeff Davis Co., Tex., where it is endemic, in fruit from May to Oct. 

 and perhaps later. 



8. Potamogeton Berchtoldii Fieb. var. tenuLssimus (Mert. & Koch) Fern. Fig. 38. 



Plants often with winter-bud at base; rhizome absent; stem usually much- 

 branched, slender, terete or nearly so, usually with a pair of small translucent 

 glands at the nodes; branches (late in the season) often terminated by winter- 

 buds; stele of the one-bundled-type; endodermis of 0-cells; interlacunar bundles 

 absent; subepidermal bundles present; pseudohypodermis absent; leaves all sub- 

 mersed, linear to linear-setaceous, acute at apex, entire, light-green to deep-green, 

 translucent and flaccid, to 85 mm. long and 1 mm. wide; nerves 3, laterals often 

 obscure and (when not evanescent) joining the midrib one-fourth to 2 leaf-widths 

 below the tip; midrib bordered on each side (at least in the lower half) by a single 

 row of lacunae; stipules hyaline to subherbaceous, 3-14 mm. long, usually clasping 

 the stem but with margins free (or adhering because of adhesive materials in the 

 water); peduncles axillary, filiform, to 3 (rarely -4.5) cm. long; spikes sub- 

 globose, with 1 to 3 few-flowered whorls, 2-8 mm. long; flowers with perianth 

 round-flabelliform and with slender claw, 1-2 mm. long; anthers 0.5-0.8 mm. 

 long; fruits obliquely obovoid, 2-2.5 mm. long, 1.2-1.9 mm. wide; dorsal keel 

 obscure, very low and broad; lateral keels absent; beak facial, prominent, erect 

 or slightly recurving, 0.1-0.5 mm. long; exocarp dark-olive-green, smooth or 

 faintly rugulose when dry; endocarp loop solid; apex of seed pointing slightly 



106 



