470 NAIADACE^:. (PONDWEED FAMILY.) 



4. THALASSIA, Solaud. 



Flowers dioecious. Spathe of the sterile flower terminating the scape, 

 2-cleft, the lobes obtuse. Sepals 3, ovate-oblong, obtuse. Anthers 9, sessile, 

 shorter than the calyx, connivent. Fertile flowers unknown. Marine herbs, 

 with linear sheathing leaves surrounding the central scape. 



1. T. testudinum, Koenig. Immersed; stem creeping; leaves 1 

 long, the outer ones withering-persistent. Shallow water along the coast 

 of Florida. 



5. RUPPIA, L. DITCH-GRASS. 



Flowers perfect, naked, two or more on a sleuder spadix, enclosed in the 

 spathe-like sheaths of the leaves, but soon loug-exserted. Stamens 2, closely 

 sessile: anther-cells large, distinct. Ovaries 4, sessile, containing a single 

 suspended campylotropous ovule. Stigma peltate. Achenium stalked, ob- 

 liquely ovate. Embryo pointed by the short plumule. Salt-water herbs, 

 with filiform branching stems, and alternate linear or bristle-like sheathing 

 leaves. 



1 . R. maritima, L. In shallow water, along the coast. May - August. 

 Stems immersed, l-3 long, mostly creeping at the base. Leaves filiform, 

 1'- 3' long, with dilated membranaceous sheaths. Fruiting peduncles l'-4' 

 long. Acheuium pointed. 



6. POTAMOGETON, Tourn. POXDWEED. 



Flowers perfect, spiked. Sepals 4, roundish, valvate in the bud. Stamens 4, 

 opposite the sepals : filaments short ; anthers 2-celled. Ovaries 4, sessile. 

 Ovules ascending, campylotropons. Style short or none : stigma peltate. 

 Achenia 1 -4, compressed. Embryo curved or coiled. Aquatic herbs, with 

 immersed slender and jointed stems. Leaves stipulate, alternate and opposite, 

 either all immersed and commonly membranaceous, or the upper ones floating 

 and more rigid. Spikes peduncled, axillary and terminal. 



* Leaves all immersed and alike. 

 >- Leaves filiform. 



1. P. pectinatus, L. Stem flexuous; the branches diffusely forking, 

 distichous; leaves long, thickish, slightly channelled, approximate on the 

 branches ; stipules small, united with the long and sheathing base of the 

 leaves; spikes slender, interrupted, on long filiform peduncles; achenium 

 obovate, smooth, slightly compressed, keeled on the back. Fresh or brack- 

 ish water. June - August. Stems 2 - 3 long. Leaves 3' - 4' long. Spikes 

 1'- 2' long. 



2. P. pauciflorus, Pursh. Stem very slender, flattened, sparingly 

 branched ; leaves scattered, thin, 3-nerved, sessile ; stipules free from the 

 leaves, connate, sheathing ; spikes short-peduncled, 4 - 6-flowered, globose in 

 fruit; acheuium round-obovate, short-pointed, keeled and sinuate-toothed on 

 the back. Shallow ponds, Georgia, and northward. July - August. Stem 

 l-2long. Leaves 1'- 2' long. Peduncles \' -\' long. 



