428 THE NAIAD FAMILY. [Potamogeton. 



through ; from 1 to 1 \ inches long by fully an inch broad. Stipules as 

 in the preceding species, but soon disappearing. Spike of flowers 

 seldom above 6 or 8 lines long. 



In rivers and ponds, all over the northern hemisphere, and in Australia. 

 Generally distributed in Britain. Fl. summer. 



6. P. crispus, Linn. (fig. 963). Curly P. — One of the most marked of 

 the alternate-leaved species. Leaves all submerged and thin, narrow- 

 oblong or broadly linear, obtuse, shortly tapering at the base, 1 to 2 

 inches long, 3 to 5 lines broad, always waved and sinuated on their 

 edges, and marked by 1 strong midrib and 2 parallel slender nerves at 

 some distance from it, but connected with it by a few transverse veins. 

 Spikes small, consisting of about 3 to 6 flowers, at some distance from 

 each other. 



In ponds, streams, and ditches of the northern hemisphere, except 

 the extreme north, and in Australia. Common in Britain. Fl. summer. 



7. P. densus, Linn. (fig. 964). Opposite P. — Eeadily known by its 

 numerous short leaves, all opposite, and arranged in 2 rows on the 

 opposite sides of the stems ; they are all submerged and thin, broadly 

 lanceolate, 6 to 9 lines long, folded and clasping the stem at their base, 

 with a strong midrib and 2 fainter parallel nerves, connected by a few 

 transverse veins. Stipules only under the peduncles or branches. Pedun- 

 cles very short, turned down after flowering, bearing a head of 2 or 3 

 flowers only. Eipe carpels rather large, rounded, and smooth. 



In shallow pools, and ditches, of Europe, except the extreme north, 

 temperate Asia and America. Common in Britain. Fl. summer. 



8. P. obtusifolius, Mert. and Koch. (fig. 965). Obtuse P.— Stems 

 slender. Leaves all submerged, alternate except under the branches, 

 1| to 3 inches long, and 1 to 2 lines broad, obtuse or scarcely acute, 

 usually 3-nerved, with a very few transverse veins. Sheathing scarious 

 stipules often persistent under the branches, but sometimes mostly 

 fallen away. Peduncles not long. Spikes usually \ inch or rather 

 longer, not very dense. Nuts broadly ovoid, about 1 J lines long, nearly 

 smooth or with a prominent often rugose dorsal rib and straight beak. 

 Seed much curved. 



In pools, ditches, and still fresh waters, over a great part of the globe. 

 Common in Britain. Fl. summer. The Linnean names P. gramineus and 

 P. compressus have been so variously applied by their author and others, 

 to this and the following species, and to some states of P. heterophyllus, 

 that the names given by German botanists are now generally adopted. 



9. P. acutifolius, Linn. (fig. 966). Acute P. — Very near P. obtusi- 

 folius, with the same habit and stipules. Leaves as in that species, 

 narrow-linear, but semi-amplexicaul and very acute, with 1 prominent 

 central nerve, and often 1 on each side not connected by transverse 

 veins, but occasionally numerous exceedingly fine longitudinal ones 

 may be seen under a lens. Spikes usually snorter than in P. obtusi- 

 folius. Nuts the same, but with a recurved beak. 



Appears to be as generally spread as P. obtusifolius, and perhaps more 

 common in Britain. Fl. summer. Both this and the preceding species 

 were included in early editions as robust varieties of R. pusillus. 



[P. zosterifolius, Schum., is a very closely allied plant with broad 

 almost winged stems, abruptly acuminate leaves, and 3-ribbed nuts.l 



10. P. pusillus, Linn. (fig. 967). Slender P.— Distinguished from 



