602 HTDROPTERIDES. (WATER-PEKN FAMILY.) 



Order 164. HYDROPTERIDES.* (Water-Fern Family.) 



Aquatic herbs, with the sporangia of two forms, borne at the base of 

 the leaves and bursting irregulairly. 



1. ISOETES, L. 



Plants composed of fibrous roots and filiform cellular leaves, without any ap- 

 parent stem. Sporangia sunk in an excavation of the dilated base of the leaves, 

 plano-cpnvex, membranaceous, filled with transverse threads and minute pow- 

 dery spores ; those of the central leaves filled with larger spores. 



1. I. flaccida, Shuttlw. Immersed; leaves very long (l^°-2°), slender, 

 flaccid, yellowish-green ; spores very small, minutely pulverulent, not reticulated. 

 — In lakes and clear streams, Middle and West Florida. 



2. AZOLLA, Lam. 



Minute floating plants, with pendent roots, pinnately branching stems, and 

 thick imbricated cellular leaves. Sporangia of two kinds, ovoid, sessile on the 

 under side of the branches, and covered with a thin membrane ; the smaller kind 

 opening transversely, containing several angular grains, attached to a central col- 

 umn, the larger bursting irregularly and containing numerous globular stalked 

 spores. 



1. A. Caroliniana, Willd. — On Stillwater, chiefly near the coast, Flor- 

 ida, and northward. — Plant reddish, circular in outline, ^'-1' in diameter. 

 Leaves ovate, obtuse, rounded and roughened on the back. 



