SPECIES AND VARIETIES. 319 



L. minor: aquatic; fronds fioating, small, broadly ovate or 

 orbicular, thickish, cohering three or four together, with one 

 root under each, not stalked. — Ponds and still waters. Fl. 

 June, July, not unfrequent. 



L. gibba : aquatic ; fronds floating, obovate, larger and 

 much thicker than the last, flat above, spongy and almost 

 hemispherical beneath, with a single root to each. — Ponds 

 and still waters, rather rare. Fl. June, July, rarely. 



L. polyrrhiza : aquatic ; fronds larger, floating, broadly 

 ovate or orbicular, thickish, each with a cluster of roots be- 

 neath. — Still waters. Fl. not observed. 



(272) Potamogeton. Pondweed. 



* Upper leaves floating on the surface of the water. 



P. natans : aquatic ; leaves opaque, ovate or oblong, usually 

 rounded, sometimes cordate or tapering at the base, marked 

 by several longitudinal nerves, tfce submerged ones thinner 

 and narrower, all stalked or reduced to a mere stalk ; spike 

 dense and cylindrical. — Stagnant or running waters. Fl. 

 June, July. 



P. heterophyllus resembles this, but is smaller, and has 

 the lower submerged leaves sessile or nearly so, linear, and 

 1-3 nerved ; while P. lucens, which is larger, has the lower 

 leaves lance-shaped, with 5-7 longitudinal nerves. 



** Leaves all submerged. 

 t Leaves alternate. 



P. perfoliatus : aquatic ; leaves broadly ovate, thin, obtuse, 

 completely clasping the stem, the auricles united on the oppo- 

 site side, spike shortish. — Rivers and ponds. Fl. July. 



P. crispus : aquatic ; leaves thin, narrow-oblong or broadly 



