2K) POTAMOGETON. [ctASS IV. order hi. 



Habitat. — Ponds and rivulets. Hovingliara, Yorlcsbire ; in tlie Lake 

 of Forfar, and also the Lake of Roscobie ; and Sir William Hooker, in 

 his British Flora, says he possesses specimens gathered in the Lake of 

 Forfar by Mr. Drummond, with peduncles three to four inches, and 

 spikes cylindrical an inch long. 



Perennial; flowering in July. 



**** Leaves alternate, ovate, lanceolate, or oblong, all submersed . Sti- 

 pules free. 



7. P. cris'pus, Linn. (Fig. 27L) curled Pondweed. Leaves lanceo- 

 late, waved, serrated, three-nerved. 



English Botany, t. 1012.— English Flora, vol. i. p. 233.— Lindley, 

 Synopsis, p. 249. — Hooker, British Flora, vol. i. p. 7G. 



p. serra'lus ; (Fig. 272.) leaves serrated, not waved. 



Leaves alternate below, opposite above, nearly sessile, crisped at the 

 edges, and about two inches in length, slightly reticulated on each 

 side the rib. Spikes short, and loose. 



13. serra'tns is P. serra'tinn of Hudson, acknowledged by himself to 

 he too near P. crispum to deserve to be regarded as a species. It is 

 marked as a variety, with more of the leaves opposite, serrated, and less 

 undulated, than a-ispus. 



Habitat. — Ditches, ponds, and rivulets; common. 



Perennial ; flowering in June and July. 



8. P. perfolia'tiis, Linn. (Fig. 273.) perfoliate Pondweed. Leaves 

 heart-shaped, clasping the stem, with five to seven principal and 

 numerous smaller intermediate nerves. 



English Botany, t. 168.— English Flora, vol. i. p. 230.— Lindley, 

 Synopsis, p. 249. — Hooker, British Flora, vol. i. p. 76. 



Leaves sessile, rather ovate, crowded at the upper part of the stem, 

 mostly under two inches in length, brittle to the touch, and of an olive 

 colour, presenting a pellucid oily appearance. Stems rather turned 

 upwards. The whole plant is submersed, except the flower-spikes, 

 which, during July and August, rise a little above the surface of the 

 water, and become for a time conspicuous, displaying on the surface 

 their little spikes, cov^qjed with a profusion of white pollen. 



Habitat. — Ponds afid rivers ; common. 



Perennial ; flowering in July and August. 



It is justly remarked by Sir James Smith, that the respiration of 

 aquatic plants differs greatly from that of such as inhale atmospheric 

 air, and may be said to bear some analogy to that of fishes when com- 

 pared with beasts and birds. In aquatic plants, the leaves are altoge- 

 ther diff'erent in texture, their cuticle being, like that of fishes, invari- 

 ably destitute of hairs. 



9. P, tu'cens, Linn. (Fig. 274.) shininn Pondweed, Leaves elliptic- 



