Family 2. HYDROCHARITACEAE 
By PER AXEL RYDBERG 
Aquatic or mud-inhabiting herbs, monoecious or dioecious, usually with 
rootstocks or stolons. Leaves clustered on the short crown. Flowers enclosed 
in spathes of distinct or united bracts. Hypanthium well developed, at least 
in the pistillate flowers. Perianth regular or nearly so. Petals and sepals 
each 3. Stamens 6-12. Filaments distinct or partly united. Gynoecium of 
6-15 united pistils. Ovary inferior, usually 6-15-celled. Styles or stigmas as 
many as the cells of the ovary. Ovules numerous. Fruit usually indehiscent, 
somewhat fleshy. 
Marine plants with distichous leaves which are sessile and linear; anthers 4-celled. 
1. Twavassia. 
Fresh-water plants with spirally arranged clustered leaves, which are distinctly 
petioled, with ovate, orbicular, or reniform blades ; anthers 2-celled. 
Filaments distinct; ovary 1-celled; petals in the pistillate flowers wanting 
or rudimentary. 2. HYDROMYSTIA. 
Filaments monadelphous; ovary 6-9-celled ; petals in the pistillate flowers 
present. 3. LIMNOBIUM. 
1. THALASSIA Solander; Konig, Ann. Bot. K. & S.2: 96. 1805. 
Marine dioecious herbs with creeping rootstocks. Leaves several, clustered at the nodes, 
sheathing at the base, linear, elongate. Scapes arising from the clusters of the leaves; 
spathes tubular, 2-cleft. Staminate flowers pedicelled. Perianth of 3 petaloid sepals. Sta- 
mens 9; filaments very short or obsolete, distinct; anthers opening laterally. Pistillate 
flower nearly sessile in the spathe; ovary 6~-1l2-celled. Fruit stalked, mammillate or echi- 
nate, opening by valves. Seeds numerous. 
Type species, Thalassia testudinum Konig. 
1. Thalassia testudinum Konig, Ann. Bot. K. & S. 2: 96. 1805. 
Thalassia vitrariorum Pers. Syn. Pl. 2: 563. 1807. 
Base of the short stem covered by the remains of old leaves; leaf-blades linear-strap- 
shaped, 1-3 dm. long, 7-10 mm. wide, glabrous, obtuse, rounded at the apex, withering- 
persistent ; spathe 2-cleft, its lobes elliptic, papillose-dentate on the margins; perianth- 
lobes in both kinds of flowers oblong, rounded above, 10-12 mm. long; anthers 8 mm. 
long, linear; stigmas 9-12, linear-filiform, pilose, grooved on the inside, about 1 cm. 
long ; fruit (young) elliptic-fusiform, short-stalked and short-beaked, densely mammillate. 
TYPE LOCALITY: Sea near the shores of Antigua [West Indies]. 
DISTRIBUTION : Coasts of Peninsular Florida, Bermuda, Bahamas, Antilles, and Venezuela. 
2. HYDROMYSTIA G. Meyer, Fl. Esseq. 152. 1818. 
Trianea Karst. Linnaea 28: 424. 1856. 
Stoloniferous dioecious or monoecious water-plants. Leaves basal, petioled, clustered ; 
blades ovate or ovate-orbicular, tapering into the petiole, somewhat spongy beneath. 
Flowers solitary or the staminate ones 2 or 3, enclosed in a 2-cleft spathe. Sepals 3, lanceo- 
late. Petals 3, in the staminate flowers linear, nearly filiform, longer than the sepals, in 
the pistillate flowers wanting or rudimentary. Stamens 6-12, distinct; anthers linear. 
Gynoecium of about 6 united pistils; ovary one-celled with 6 placentae; styles 6, subulate ; 
stigmas curved. Fruit ellipsoid, one-celled, many-seeded. 
Type species, Hydromystia stolonifera G. Meyer. 
VOLUME 17, Part 1, 1909] 73 
