NAIADES. 787 



the whole globe, excepting perhaps South America. Common round 

 the British Isles. Fl. summer and autumn. 



Y. PONDWEED. POTAMOGETON. 



Aquatic herbs, with a perennial rootstock, long, floating, usually 

 forked stems, and alternate or rarely opposite leaves, either dilated 

 and sheathing at the base, or having all or some of them a sheathing, 

 scarious stipule in their axil. Flowers small, sessile in a spike or head, 

 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. Carpels 4, each with a very short style or a sessile 

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



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

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

 selves 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 



leafstalks 1. Broad P. 



Lower submerged leaves sessile or nearly so. 



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. Long 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. Curly 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 axil 8. Slender P. 



