344 94. POTAMOGETONACES. * 
slightly interrupted. Back of fr. without ridges, rounded when 
fresh ; inclosed nut with faint lateral ridges Ponds and ditches. 
P. VI. VII. E. 
19. P. pectinatus (L.); 1. formed of 2 interrupted tubes, lower 
narrowly linear flattened slightly grooved above, upper setaceous, 
fr. (2 lin.) roundish-obovate rounded on the back, nut with 2 
lateral ridges but (usually) no keel.—H. B. 323.  R. vii. 19.— 
St. branched, forming linear masses. L. very gradually acute, all 
l-nerved, no marginal nerves and scarcely thickened there ; up- 
per L. with an oval section. Spikes slightly interrupted. Back 
of fr. without ridges, rounded when fresh ; nut with strong lateral 
ridges. Varying greatly in length and size of st. and leaves— 
Ponds and streams. P. VI. VII. 
20. P. filiformis (Nolte !); 1. linear-setaceous 1-nerved with 
transverse veins, spikes greatly interrupted, fr. (12 lin.) obovate 
rugose rounded on the back without keel or ridges when dry, nut 
round-hacked.—L. like those of the preceding but longer. Fr. 
smaller. Whorls very distant on the spikes. Peduncles very 
long.—Rare. Forfarshire and Berwickshire. P. VI. VII. 8. 
wicket T, all opposite, submersed ; stipules none. 
21. P. densus (L.); @. ald opposite pellucid clasping elliptical- 
lanceolate or lanceolate, spikes shortly stalked ultimately reflexed. 
—E. B. 397. R. vii. t. 28.—L. crowded, rather recurved. Spike 
4-flowered. Sep. triangular.—Ditches. P. VI. VII. 
2. Ruppia Linn. 
1. R. maritima (L.); ped. elongating often spirally twisted, 
anth.-cells oblong 13 times as long as broad, nut ovate rather 
obliquely erect.—R. vii, 26.—Whole plart stronger than the next. 
L. very narrowly lmear. Sheaths large inflated—Salt marshes 
in the south. P. VI. VIII. E. 
2. R. rostellata (Koch); anth.-cells squarish as long as broad, 
nut gibbous at base obliquely ascending.— E. B.136?  R. vii. 25. 
—Whole plant very slender. L. rather filiform than linear. 
Sheaths small, close. Nut less obliquely ascending than in con- 
tinental specimens.—Salt marshes. P. VII. VIII. 
3. ZANNICHELLIA Linn. 
1. Z. palustris (L.); style at least half as long as the fruit.— 
E. B. 1844.—Floating. L. slender, opposite, filiform. Fl. 
axillary, sessile. Anth. 2—4-celled. Style 3 as long as fruit. 
Stigma generally notched at the margin.—B. Z. pedicellata (Fr.) ; 
style as long as the fruit. Clusters usually stalked. Pedicels 
