140 HYDRALES. [Endogens. 



Alliance X. HYDRALES. — The Hydiial Alliance. 



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



on a spadix, and without albumen. 



The essential character of the Ilydral Alliance consists in its $ $? flowers and exal- 

 huininous seeds ; it is therefore necessary to expel all those genera, which, like Potamo- 

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

 nothing except the diclinous character which can distinctly divide the Arrow-grasses from 

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

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

 Btitute real exceptions to the otherwise positive distinction. The Hydrals are all, as 

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

 They divide into three well-marked Orders, namely : 



Stamens epigynous ; ovary adherent 39. Hydrocharidace.i£. 



Stamens hypogynous ; 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. 



The order of Triurids, in the former editions referred to Dictyogens, upon 

 erroneous descriptions, now finds a place here with the following diagnosis : 



Stamens hypogynous ; ovaries 00, free ; embryo rudimentary ; 



pollen globose 40 bis Triuridace.e. 



