LXXVIII. NAIADEJ5. 425 



perianth, from Ruppia in the usually opposite leaves, in the single 

 stamen, and in the shape of the fruit. 



1. Z. palustris, Linn. (fig. 956). Common Z. Stems slender, branched 

 and floating. Leaves finely, linear, bright green, 1 to 2 inches long, 

 mostly opposite, with a small, sheathing, membranous stipule embrac- 

 ing the stem withinside. At the time of flowering there are usually 

 about 4 ovaries together, almost sessile within the stipule, each with a 

 short style and a broad, disk-shaped stigma, and a solitary stamen with 

 a slender filament in the same or in a separate axil ; the anthers 2- or 

 4-celled. When ripe the carpels are 1 to 1 lines long, sessile or shortly 

 stalked, somewhat curved and flattened, tipped by the remains of the 

 style ; the ribs on the back often crenated, warted, or slightly winged. 



In ponds, or lagoons of fresh, or brackish or even salt water ; dis- 

 persed over a great part of the globe. Common in Britain. PL. the 

 whole summer. 



[There are four well-marked forms of this. 



a. Z. palustris proper. Stamens long ; anther 4-celled. Carpels 2-4, 

 sessile, style half as long, stigma small. 



b. Z. brachystemon, Gay. Stamens short; anther 2-celled. Carpels 

 2-4, snbsessile, their bracts crenated ; stigma large. 



c. Z. pedunculate, Reichb. Stamens short ; anther 2-celled. Carpels 

 pedicelled, their backs muricate ; stigma large. 



d. Z. polycarpa, Nolte. Stamens very short ; anther 2-celled. Carpels 

 4-6, subsessile, their backs smooth ; stigma large.] 



IV. RUPPIA. RUPPIA. 



A single species, distinguished from ZannicheUia by the alternate 

 leaves, 2 sessile anthers, and the ripe carpels all stalked and ovoid. 



1. B. maritima, Linn. (fig. 957). -Sea R. A slender, branched, 

 floating plant, much resembling Potamogeton pectinatus. Leaves almost 

 capillary, with a sheathing base. Peduncles axillary, at first very short, 

 bearing 1 or 2 flowers, each consisting of 2 almost sessile anthers, with 

 2 distinct cells, and 4 carpels, at first nearly sessile. As the fruit ripens, 

 the carpels become little, obliquely pointed nuts, 1 to 1$ lines long, on 

 pedicels from 2 or 3 lines to an inch in length, the common peduncle 

 often becoming spirally coiled, and also lengthening. 



In salt marshes, lagoons, and shallow creeks and bays, dispersed over 

 nearly the whole globe. Common round the British Isles. PI. summer 

 and autumn. [There are two British forms usually regarded as species. 



a. R. maritima proper. Sheaths inflated. Fruiting peduncles spiral. 



b. R. rostellata, Koch. Sheaths not inflated. Fruiting peduncle* 

 short, flexuous. Nut beaked, gibbous.] 



V. POTAMOOETON. PONDWEED. 



Aquatic herbs, with a perennial rootstock, long, floating, usually 

 forked stems, and alternate or rarely opposite leaves, either dilated and 

 sheathing at the base, or having all or some of them a sheathing, 

 Carious stipule $n their axil- Flowers Buaajl, sessile in a spike oj bead 



