140 IIYDKALES. [Endogens. 



Alliance X. HYDRALES. — The IIydral Alliance. 



Diagnosis — Unisexual aquatic Endogens, with perfect or imperfect floioers, not arranged 

 on a spadix, and without albumen. 



The essential character of the Hydral AlUance consists in its <J $ flowers and exal- 

 buminous seeds ; it is therefoi'e necessary to expel all those genera, which, like Potamo- 

 geton, have been placed among the Naiads although they are ; for in tnith there is 

 notliing except the dicHnous character which can distinctly divide the Arrow-grasses from 

 the Naiads. Among the Frogbits, however, a couple of genera occm' which are described 

 as being truly and yet cannot be referred to any other Order, and they therefore con- 

 stitute real exceptions to the otherwise positive distinction. The Hydi*als are all, as 

 their name indicates, strictly aquatic, no instance of a land-plant occui'ring among them. 

 They divide into tlu'ee well-marked Orders, namely : 



Stamens epigynous ; ovary adherent 39. Hydrocharidace^. 



Stamens hypogrjnous j ovary free ; pollen globose ... 40. Naiadace.e. 

 Stamens hypogynous ; ovary free ; pollen confervoid . . 41. Zosterace^. 



The genera of these Orders demand, however, a much more careful examination than 

 they have yet had, and considerable changes may be expected among them ; for it is 

 uncommon to find in the same Order so much diversity of condition as occurs among 

 the Naiads and Frogbits as at present constituted. 



