NAIADES. 489 



broadly ovate or orbicular, ratber thick, with a cluster of roots under each 

 one. Flowers of the gibbous D. 



As widely dispersed as the other species, and rather more frequent than 

 the gibbous D., but much less so than the two others. The flowers appear 

 to have been but once observed. 



LXXVII. THE NAIAD FAMILY. NAIADES. 



Aquatic floating or submerged plants ; the leaves either 

 sheathing at the base or accompanied by sheathing stiptdes, 

 alternate or sometimes opposite. Flowers axillary, incon- 

 spicuous, solitary or spiked, usually proceeding from a sheath- 

 ing bract. Perianth none, or consisting of 4 small, scale-like 

 segments. Stamens 1, 2, or 4. Ovaries either of 2 or 4 dis- 

 tinct carpels, each with a single ovule and a separate stigma, 

 or single, with 1 ovule and 2 to 4 stigmas. Fruit consisting 

 of 1, 2, or 4 seed-like nuts, each with one seed, without 

 albumen. 



An Order not numerous in species, but abundantly dtfiused over aU parts 

 of the world, in the sea as well as in fresh waters. 



Stems creeping in sand or mud under salt-water. Leaves very long 

 and linear. Flowers within the base of a long, linear bract, 



like the stem-leaves 1. ZosTEBA. 



Stems floating. Flowers axillary. 

 Ovaries simple, with 1 style and 3 or 3 stigmaa. Leaves opposite 



or whorled, and often. toothed 2. Naiad. 



Ovaries of 4 carpels, each with a separate stigma. 

 Flowers and carpels aiiUary and sessile. Leaves opposite, very 



slender 3. Zannicheilia. 



Flowers usually 2, on an axillary peduncle. Carpels stalked, 



pear-shaped. Leaves alternate, very slender 4. Ruppia. 



Flowers in pedunculate spikes or heads. Carpels sessile. 



Leaves alternate or rarely opposite 5. Pondweed. 



I. ZOSTERA. ZOSTEEA. 



Marine herbs, the stem creeping and rooting in the sand or mud, with 

 long, grass-like, alternate leaves. Flowers enclosed in a sheath near the base 

 of leaves similar to the others, but usually smaller. Within this sheath is 

 an oblong or linear, thin, leaf-like peduncle, on one side of which are 

 arranged in two rows a few sessile anthers, with 3 or 4 sessile or nearly 

 sessile ovaries, tapering into a deeply 2-cIeft, linear style. Embryo spht 

 longitudinally into 2 valves, which fold over a long, curved, linear radicle. 



A genus hitherto hmited to the two British species. 



Leaves seldom a foot long. Ripe seeds smooth 2. DvjarfZ, 



Leaves usually more than a foot long. Ripe seeds furrowed .... 1. Common Z, 



¥ 



1. Common Zostera. Zostera marina, Linn. 

 (Eng. Bot. t. 467. Qrass-wracH.) 

 Creeping stems or rootstocks often very long and rather fleshy. Leaves 

 varying from near a foot to several feet in length, and from 2 to 3 or 4 lines 



