SPECEES AND VARIETIES. 319 



L. minor : aquatic ; fronds floating, 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, the 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 stern, the auricles united on the oppo- 

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



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



