492 THE NAIAD FAMILY. 



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

 Lower submerged leaves stalked or reduced to mere leaf- 

 stalks 1- Broad P. 



Lower submerged leaves sessile or nearly so. ■ t> 



Lower submerged leaves linear, 1-nerved or slightly 3-nerved 2. Various-leaved P. 

 Lower submerged leaves lanceolate, with 5, 7, or more nerves 3. Shining P. 

 All the leaves under water and sessile. 



Leaves all opposite 7. Opposite P. 



Leaves alternate, except under the peduncles or forks. 

 Leaves broadly ovate, clasping the stem all round .... 5. Perfoliate P. 

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



clasping the stem 4-. Lo7ig P, 



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

 clasping. 

 Leaves broadly linear or lanceolate, flat and entire, with 



many nerves 3. Shining P, 



Leaves broadly linear, obtuse, waved, 1- or 3-nerved ... 6. Cu.rly P. 

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

 Leaves dilated at the base into a sheath scarious at the 



edges 9. Fennel P. 



Leaves not dilated at the base, with a scarious stipule in 



the aDl 8. Slender P. 



1. Broad Pondweed. Potamogeton natans^ Linn. 

 (Eng. Bot. t. 1822, P. ohlongus, Suppl. t. 2849. -P. ^olygonifolius, 



Bab. Man.) 



One of the largest of our Pondioeeds. Leaves stalked, the upper ones 

 floating on the surface of the water, of a thick, opaque texture, ovate or ob- 

 long, 2 to 4 inches long by 1 to 1^ broad, usually rounded at the base, some- 

 times 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 all stalked 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 compressed, nearly straight, the inner edge rounded 

 outwards, with 1 or sometimes 3 dorsal ribs. 



In stagnant or running waters, deep or shallow, sunny or shaded, in 

 almost all parts of the world, and varies accordingly in the size, shape, and 

 texture of the foUage, the size and number of the flowers, fruits, etc. Abun- 

 dant in Britain. Fl. summer. A variety with the leaves all under water, 

 thin and almost transparent, although stalked and broad, and with smaller 

 spikes and fruits, is often admitted as a species, under the name of P, 

 plantaginews (Eng. Bot. Suppl. t. 2848). 



2. Various-leaved Pondweed. Potamogeton heterophyllus, Schreb. 



(Eng. Bot. t. 1285.) 



Usually much smaller than the iroad P., which it resembles in the long 

 stalk and the ovate or oblong shape of its floating leaves, but these are only 

 1 to 2 inches long, and the submerged leaves are all narrow-lanceolate or 

 liaear, with the few veins of the slender P., tapering at both ends but not 

 distinctly stalked. Spikes and fruits like those of the smaller forms of the 

 broad P. 



Chiefly a North American species, not common in Europe, where it appears 

 to be rather a western plant. Occurs in many parts of Britain. Fl. sum- 

 mer. Sometimes the floating leaves are not developed, and then it is scarcely 

 to be distinguished from the slender P. except by its larger size, with a 

 denser spike, and generally a firmer consistence. 



