426 THE NAIAD FAMILY. [Potamogeton. 



on an axillary peduncle rising above the water. Perianth of 4 scale- 

 like segments. Stamens 4, opposite the segments ; the anthers sessile 

 and 2-celled. Capsules 4, each with a very short style or a sessile 

 stigma. Nuts small and seed-like, sessile, usually laterally compressed. 

 Seed much curved or almost coiled round an obovoid projection of the 

 endocarp. 



A considerable genus, most of the species spread over the greater 

 part of the globe, chiefly in fresh water, but some accommodating 

 themselves also to salt water, and many of them very variable in foliage. 

 In the species with axillary stipules, these are sometimes only to be 

 seen under the peduncles or under the branches of the stem. 



Upper leaves on long stalks, floating on the surface of the 



water. 



Lower submerged leaves stalked or reduced to mere leaf- 

 stalks . . . L P. natans. 



Lower submerged leaves sessile or nearly so. 

 Lower submerged leaves linear, 1-nerved or slightly 8- 



nerved 2. P. heterophyllui. 



Lowers submerged leaves lanceolate, with 6, 7, or more 



nerves 3. P.lucens. 



All the leaves under water and sessile. 



Leaves all opposite 7. P. dentus. 



Leaves alternate, except under the peduncles or forks. 



Leaves broadly ovate, clasping the stem all round . . 6. P. perfoliatus. 

 Leaves ovate-lanceolate or oblong, broad at the base and 



clasping the stem 4. P. pra&ongus. 



Leaves lanceolate or linear, tapering at the base, or not 



stem-clasping. 

 Leaves broadly linear or lanceolate, flat and entire, with 



many nerves 8. P. lucent. 



Leaves broadly-linear, waved, 1- or 3-nerved . . . 6. P. crispus. 

 Leaves narrow-linear, not waved, 1- or 3-nerved. 

 Leaves not dilated at the base, with a scarious stipule 



in most axils. 



Leaves 1 to 2 lines broad. Nuts l lines long. 

 Leaves obtuse or scarcely acute. Spikes usually J 



inch long 8. P. obtusifolius. 



Leaves very acute. Spikes short and few-flowered . 9. P. acuttfoliiw. 

 Leaves under 1 line broad. Nuts under 1 line long . 10. P. pusittus. 

 Leaves dilated at the base into a sheath, scarious at the 



edges 11. P. pectinatus. 



[The Potamogetons, like so many water plants, are very difficult of dis- 

 crimination, and nearly thirty species are enumerated in the London 

 Catalogue of 1886 as British, many of them distinguished by very 

 variable or minute characters. I have been unable to reduce these 

 satisfactorily to the leading types which Bentham has confined himself 

 to describing.] 



1. P. natans, Linn. (fig. 958). Broad P. One of the largest of our 

 Potamogetons. Leaves stalked, the upper ones floating on the surface 

 of the water, of a thick, opaque texture, ovate or oblong, 2 to 4 inches 

 long by 1 to 1^ broad, usually rounded at the base but sometimes 

 cordate or tapering, marked by several longitudinal nerves, with a few 

 cross veins often branched or slightly netted ; the submerged leaves 

 thinner and narrower, but stalked like the floating ones or reduced to 

 a mere stalk. Axillary stipules closely sheathing, often an inch long. 

 Spike dense and cylindrical, often an inch long or more, on a stout 

 peduncle of several inches. Nuts ovoid, above a line long, slightly com* 



