GENUS i. 



PONDWEED FAMILY. 



33. Potamogeton pectinatus L. Fennel- 

 leaved Pondweed. Fig. 206. 



Potamogeton pectinatus L. Sp. PI. 127. 1753. 



Stems slender, much branched, i-3 long, the 

 branches repeatedly forking. Leaves setaceous, atten- 

 uate to the apex, i-nerved, i'-6' long, aften capillary 

 and nerveless; stipules half free, \'-\' long, their 

 sheaths scarious on the margins; peduncles filiform, 

 2'-ia' long, the flowers in verticils; fruit obliquely 

 obovoid, with a hard thick shell, ii"-2" long, i"-ii" 

 wide, without a middle keel, but with obscure lateral 

 ridges on the back, plump on the sides and curved or 

 occasionally 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 Brit- 

 ish Columbia, south to Florida, Texas and California. 

 Also in Europe. Pondgrass. July-Aug. 



Inter- 



34. Potamogeton interruptus Kitaibel. 

 rupted Pondweed. Fig. 207. 



Potamogeton interruptus Kitaibel in Schultes, OEst. Fl. 



Ed. 2, 328. 1814. 

 Potamogeton flabellatus Bab. Man. Bot. Ed. 3, 343. 1851. 



Stems arising from a running rootstock which often 

 springs from a small tuber, 2-4 long, branched, the 

 branches spreading like a fan. Leaves linear, obtuse or 

 acute, 3'-$' long, i"-i" wide, 3-S-nerved with many 

 transverse veins; narrow, i-nerved leaves on some 

 plants, these acuminate, as P. pectinatus; stipules par- 

 tially adnate to the leaf-blade, the adnate part -l'-i' long, 

 sometimes with narrowly scarious margins, the free part 

 shorter, scarious, obtuse; peduncles i'-2 f long; spikes 

 interrupted ; fruit broadly and obliquely obovoid, obtuse 

 at the base, the largest 2" long and nearly as broad. 

 keeled and with rounded lateral ridges on the back, the 

 face nearly straight; style facjal, erect. 



In ponds and streams, Michigan, Nebraska, Saskatche- 

 wan and Wisconsin. Also in Europe. August. 



35. Potamogeton Robbinsii Oakes. 

 Robbins' Pondweed. Fig. 208. 



Potamogeton Robbinsii Oakes, Hovey's Mag. 

 7: 180. 1841. 



Stems stout, wide-branching, 2-4 long, 

 from running rootstocks. Leaves linear, 

 3'-S' long, 2"-3" wide, acute, many-nerved, 

 crowded in 2 ranks, minutely serrulate, 

 auriculate at attachment with the stipule ; 

 stipules with the adnate portion and sheath- 

 ing base of the leaf about \' long, the free 

 part i'-i' long, acute, persistent, white, 

 membranous, lacerate; peduncles i'-3' long, 

 inflorescence frequently much branched, 

 with 5-20 peduncles ; spikes interrupted, 

 i'-i' long, flowering under water; fruit 

 obovoid, about 2" broad and ij" wide, 

 3-keeled on the back, middle keel sharp, 

 lateral ones rounded, face arched, sides 

 with a shallow depression running into the 

 face below the arch ; style subapical, thick, 

 slightly recurved ; apex of the embryo 

 pointing a little inside the basal end. 



In ponds and lakes, New Brunswick to Oregon, south to New Jersey, Pennsylvania and 

 Michigan. 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. 



