_g NAIADACEAE 



33. Potamogeton interruptus Kitaibel. Interrupted Pondweed. (Fig. 174.) 



Potamwlon inteiriifilns Kitaibel in Schultes, OEst. Fl. 



K<1 > ^2S. 1*14. 

 I\,t\i,,iogetonftabellaius Bab. Man. Bot. Ed. 3. 324. && 



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 '-5 / long, i"-i#" 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, the adnate part '/I'-i' long, sometimes with 

 narrowly scarious margins, the free part shorter and 

 scarious, obtuse ; peduncles i'-2' long ; spikes slightly 

 interrupted ; fruit broadly and obliquely obovoid, ob- 

 tuse at the base, the largest 2" 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. Prince Edward Island to northern 

 Indiana and Michigan. Also in Europe. August. 



34. Potamogeton Robbinsii Oakes. 



Robbins' Pondweed. 



(Fig. 175.) 



Potamogeton Robbinsii Oakes, Hovey's Mag. 



7: 180. 1841. 



Stems stout, widely branching, 2-4 

 long, from running rootstocks sometimes 

 i long. Leaves linear, 3 '-5' long, 2 // ~3" 

 wide, acute, finely many-nerved, crowded 

 in 2 ranks, minutely serrulate, auriculate 

 at the point of attachment with the stipule ; 

 stipules with the adnate portion and sheath- 

 ing base of the leaf about l / 2 f long, the free 

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

 membranous, mostly lacerate ; peduncles 

 I '-3' long, the inflorescence frequently 

 much branched and bearing from 5-20 

 peduncles; spikes interrupted, ^'-iMong, 

 flowering under water; fruit obovoid, about 

 2" broad and i>" 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 subapical, 

 thick, slightly recurved ; apex of the em- 

 bryo 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. 



2. RUPPIA L. Sp. PI. 127. 1753. 



Slender, widely branched aquatics with capillary stems, slender alternate i-nerved 

 leaves tapering to an acuminate apex, and with membranous sheaths. Flowers on a capil- 

 lary, spadix-like peduncle, naked, consisting of 2 sessile anthers, each with 2 large sepa- 

 rate sacs attached by their backs to the peduncle, having between 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 pedicelled ; embryo oval, the cotyledonary end inflexed, and 

 both that and the hypocotyl immersed. [Name in honor of Heinrich Bernhard Rupp, a Ger- 

 man botanist.] 



In tin- development of the plants the staminate flowers drop off and the peduncle elongates. 

 iK-aritiK the pistillate flowers in 2 clusters at the end, but after fertilization it coils up and the fruit 

 i drawn ht-low tin- surface of the water. 



