CV. NAIAD ACE AS. 
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Potamogeton . ] 
4 unguiculate scales. Stamens 4. Anthers 2-celled nearly sessile, 
opposite the scales of the perianth. Pistils 4, which become 4 
sessile ackenes. Stigmas nearly sessile, undivided. — Named 
from TTOTap-oQ, a river, and yn-wv, a neighbour ; all the species 
grow in the water. Chamisso and Schlechtendal have well 
illustrated this genus. (See Linncea, ii. p. 159.) 
* Leaves all opposite and submerged ; stipules none. 
1 . P. densus L. ( opposite-leaved P ) ; leaves crowded all 
opposite pellucid amplexicaul ovate-acuminate or lanceolate, 
spikes shortly stalked about 4-flowered finally reflexed. E. B. 
t. 397. 
Ditches, frequent. Tf.. 6,7. — Peduncles short. Head of flowers 
small, globose. Leaves keeled below, middle nerve or rib of many 
longitudinal cells, with 2 and sometimes 3 lateral parallel veins on 
each side, the inner one the strongest. 
** Leaves alternate, all submerged, with adnate stipules. 
2. P. pectindtus L. ( Fennel-leaved P.) ; leaves distichous se- 
taceous or linear sheathing by means of their adnate stipules, 
upper ones 1 — 3-nerved, spike interrupted, achenes large 3- 
ribbed at the back, the two lateral ribs sometimes obsolete. — 
а. leaves setaceous 1-nerved canaliculate, achenes with the 
lateral keels conspicuous. E. B. t. 323. P. marinus L . — 
/3. leaves broader, upper ones 3-nerved, lower 5-nerved, achenes 
with the latei’al keels inconspicuous. P. pectinatus var. vul- 
garis Ch. et Schl. P. flabellatus Bab. 
Rivers, lakes, ponds, and salt marshes. Near Ayr, Scotland. — 
б. Denver and Burnham, Norfolk ; Coventry ; Bath ; Gravesend ; 
Hull ; Yarmouth. If.. 6, 7. — P. flabellatus Bab. appears to us to 
be what Chamisso and Schlechtendal consider the common form of 
the species. 
3. P. filifdrmis Pers. ( slender-leaved P.) ; leaves distichous 
setaceous 1-nerved sheathing by means of their adnate stipules, 
spike interrupted, achenes (small) convex but not ribbed on 
the back. Cham, et Schlecht. 1. c. p. 167. 
Lakes and ponds in Forfarshire and Berwickshire. It. 6, 7. — 
So similar to our var. a. of the last that we know no distinguishing 
character except the smaller flowers and fruit, and the achenes being 
quite convex on the back after the cuticle has been removed : when 
this is not removed, they sometimes appear 3-ribbed externally. 
Kunth (En. iii. p. 136) and Mr. Bentham unite them. 
*** Leaves alternate, linear, all submerged ; stipules free. 
4. P. trichoides Cham. ( hair-like P.) ; leaves sub-setaceous, 
1-nerved finely pointed, peduncles elongate, spike at length 
