NATURAL ORDERS. 287 



margin of the frond, inclosed in a spatha without a spadix. 

 Stamens 1 or 2. Ovary 1-celled ; ovules 2 or more. Fruit 

 membraneous or capsular. (See Plate VIII., Fig. 2.) 



Genera. — Pistia, Lemna. 



535. Kaiadace^, tJie Pond-weed Tribe. — Water plants. Leaves 

 very cellular, with parallel veins. Flowers inconspicuous, some- 

 times perfect. Perianth of 2 or 4 pieces, or wanting. Stamens 

 definite, hypogynous. Ovaries 1 or more ; ovule solitary. Fruit 

 dry, usually indehiscent. Seed erect or pendulous, exalbumi- 

 nous ; embryo straight or curved. 



Geneea. — Potamogeton, Najas, Ruppia, Zostera, Zannichellia. 



536. ALisMACEiE, tJie Water-Plantain Tribe. — Floating or 

 swamp plants^ usually with a creeping, fleshy rhizoma. Leaves 

 with parallel veins. Flowers regular, perfect, or polygamous, 

 usually in racemes or panicles. Sepals 3. Petals 3. Ovaries 

 superior, several, 1-celled. Fruit dry, 1 or 2 seeded. Seeds 

 straight or curved, destitute of albumen ; embryo shaped like a 

 horse-shoe. 



a. Properties : some are acrid, others have edible rhizomes. 

 Genera. — Alisma, Sagittaria. 



537. Sub-order Juncagine^. Seed and embryo straight. 



Genera. — Triglochin Scheuchzeria. 



538. HYDROCHARiDACEiE, tlie FrogMt Tribe. — Aquatic herbs. 

 Leaves parallel-veined, sometimes spiny. Flowers dioecious ot 

 polygamous, inclosed in a spatha. Sepals 3, herbaceous. PetaU 

 3. Stamens definite or indefinite. Ovary adherent, 1 or many 

 celled ; ovules '^vqc^qwHj attached to parietal placentae. Fruit 

 dry or succulent. Seeds numerous, exalbuminous ; embryo 

 straight. 



Genera. — Udora, Vallisneria, Hydrocharis. 



539. BuRMANNiACE^. — Llerbs with radical leaves. Perianth 

 colored, tubular, 6-cleft, the three outer (calyx) sometimes keel- 

 ed at the back, the 3 inner (petals) minute. Stamens 3, oppo- 

 site the petals. Ovary adherent, 1-3-celled, with 3 placentce 

 Fruit a capsule. Seeds innumerable, minute, without albumen 



Genus. — Apteria. 



540. Orchidace^, the Orchis Tribe.— ^Llerbs or shrubs., with 

 fibrous or tuberous roots. Stem simple or wanting. Leaves 

 mostly radical, sheathing, cauline ones sessile. Flowers irregu- 

 lar, bracted, commonly in a spike, or racemose, or panicled, 

 seldom solitary. Perianth 6-parted, 3 divisions external, 3 in- 

 ternal ; a lower one in the form of a lip (labellum), often spur- 

 red. Stamens 3, opposite the se^Dals, coherent with the style 

 (composing the column) ; pollen powdery, or cohering in waxy 



