NATADACEAE. 49 



Potamogeton fifif6rmis Macfcunii Morong. Leaves 2-7.5 cm> long, the largest i mm. 

 or more wide, obtuse, stiff, with a strong midrib and raised or slightly revolute margins; 

 fruit rarely more than 2 mm. long; peduncles only 1.5-2 cm. long; plant commonly 

 with a compact bushy habit. In brackish or salt water lakes, prairie region of Can. 



32. Potamogeton pectinatus L. FENNEL-LEAVED PONDWEED. (I. F. f- 

 I 73-) Stems slender, much branched, 0.31 mm. long, the branches repeatedly 

 forking. Leaves setaceous, attenuate to the apex, i-nerved, 2.5-15 cm. long, often 

 capillary and nerveless; stipules half free, 1.2-2.5 cm - l n g> their sheaths scarious 

 on the margins; peduncles filiform; the flowers in verticils; fruit obliquely obovoid, 

 with a hard thick shell, 3-4 mm. long, 2-2.5 mm - wide, without a middle keel, but 

 with obscure lateral ridges on the back, plump on the sides and curved or occa- 

 sionally a little angled on the face; style straight or recurved, facial; embryo apex 

 pointing almost directly toward the basal end. In fresh, brackish, or salt water, 

 Cape Breton to Br. Col., south to Fla., Tex., and Cal. Also in Europe. July-Aug. 



3?. Potamogeton interruptus Kitaibel. INTERRUPTED PONDWEED. (I. F. 

 f. 174.) Stems arising from a running rootstock which often springs from a small 

 tuber, 0.6-1.3 m - l n g> tne branches spreading like a fan. Leaves linear, 7-15 

 cm. long, 2-25 mm. wide, 3-5 -nerved, with many transverse veins; narrow, I- 

 nerved leaves occur on some plants and these are acuminate, much like those of 

 P. pectinatus; stipules partially adnate to the leaf-blade, sometimes with narrowly 

 scarious margins, the free part shorter and scarious, obtuse; spikes interrupted; 

 fruit broadly, obliquely obovoid, obtuse at the base, the largest 4 mm. long and 

 nearly as broad, prominently keeled and with rounded lateral ridges on the back, 

 the face nearly or quite straight; style facial, erect. In ponds and streams, Pr. 

 Edw. Isl. to N. Ind., Mich., and Neb. Also in Europe. Aug. 



34. Potamogeton Robbinsii Oakes. ROBBINS* PONDWEED. (I. F. f. 175.) 

 Stems stout, widely branching, 0.6-1.3 m. long, from running rootstocks sometimes 

 0.3 m. long. Leaves linear, 7-13 cm. long, 4-6 mm. wide, acute, finely many- 

 nerved, crowded in 2 ranks, minutely serrulate, auriculate at the point of attach- 

 ment with the stipule; stipules adnate, the free part 1.2-2.5 cm - l n g> acute, persist- 

 ent, white, membranous, mostly lacerate; peduncles 2-7 cm. long, the inflorescence 

 frequently much branched and bearing from 520 peduncles; spikes interrupted, 

 flowering under water; fruit obovoid, about 4 mm. broad and 3 mm wide, 3 -keeled 

 on the back, the middle keel sharp, the lateral ones rounded, the face arched, the 

 sides with a shallow depression which runs into the face below the arch; style sub- 

 apical, thick, slightly recurved; apex of the embryo pointing a little inside the 

 basal end. In ponds and lakes, N. B. to Or., south to Del., Penn., and Mich. The 

 plant is freely propagated by fragments of the stems which throw out rootlets from 

 each joint, but this is the rarest of our species to form fruit. Aug. -Sept. 



2. RUPPIA L. 



Slender, widely branched aquatics with capillary stems, slender alternate 

 i -nerved leaves tapering to an acuminate apex, and with membranous sheaths. 

 Flowers on a capillary spadix-like peduncle, naked, consisting of 2 sessile anthers, 

 each with 2 large separate sacs attached by their backs to the peduncle, having be- 

 tween them several pistillate flowers in 2 sets on opposite sides of the rachis, the 

 whole cluster at first enclosed in the sheathing base of the leaf. Stigmas sessile, 

 peltate. Fruit a small obliquely-pointed drupe, several in each cluster and pedi- 

 celled; embryo oval, the cotyledonary end inflexed, and both that and the hypo- 

 cotyl immersed. [Name in honor of Heinrich Bernhard Rupp, a German botanist.] 

 In the development of the plants the staminate flowers drop off and the peduncle 

 elongates, bearing the pistillate flowers in 2 clusters at the end, but after fertiliza- 

 tion it coils up and the fruit is drawn below the surface of the water. 



Three or four species, occurring in salt and brackish waters all over the world. 

 The following are the only ones known to occur in N. Am. 



Sheaths 6-8 mm. long ; drupes about 2 mm. long. i. R. maritima. 



Sheaths 12-35 mm, long; drupes 3-4 mm. long. 2. R. occidentalism 



