78 NAJADACEAE (PONDWEED FAMILY) 



36. P. pectinatus L. Stem filiform, repeatedly dichotomous ; leaves very 

 narrowly linear or setaceous, attenuate to the apex, 1-nerved with a few trans- 

 verse veins ; peduncles filiform ; spikes of 2-6 remote whorls ; fruit obliquely 

 broad-obovoid, compressed, 3.5-4.5 mm. long, rounded on the back, obscurely 

 ridged on the sides ; embryo spirally incurved. Chiefly in brakish water, e. 

 Que. to B. C., s. along the coast to Fla., aud in the interior to Pa., the Great 

 Lake region, Kan., Col., etc. July-Sept. (Cosmop.) 



37. P. interriiptus Kitaibel. Similar; leaves usually broader (0.5-2 mm. 

 wide) ; edges of the stipules less scarious ; fruit more compressed, sharply keeled. 

 Coast of e. N. B. ; Mich. ; probably of wide distrib. July-Sept. (Eu.) 



38. P. Robbinsii Oakes. Stem ascending from a creeping base, rigid, very 

 branching, invested by the bases of the leaves and stipules; leaves crowded in two 

 ranks, recurved-spreading, narrow-lanceolate or linear, 7-12 cm. long, acuminate, 

 ciliate-serrulate with translucent teeth, many-nerved ; stipules obtuse when 

 young, their nerves soon becoming bristles ; spikes numerous, loosely few- 

 flowered, on short peduncles ; fruit oblong-obovoid, keeled with a broadish wing, 

 acutely beaked ; embryo stout, ovally annular. In quiet water, N. B. to B. C., 

 s. to Del., Pa., Ind., Wyo., Ida., and Ore. ; rarely fruiting. July-Sept. 



2. RUPPIA L. DITCH GRASS 



Flowers 2 or more (approximate on a slender spadix, which is at first inclosed 

 in the sheathing spatbe-like base of a leaf), consisting of 2 sessile stamens, each 

 with 2 large and separate anther-cells, and 4 small sessile ovaries, with solitary 

 campylotropous suspended ovules ; stigma sessile, depressed. Fruit small ob- 

 liquely ovoid pointed drupes, each raised on a slender stalk which appears after 

 flowering ; the spadix itself also then raised on an elongated thread-form 

 peduncle. Embryo ovoid, with a short and pointed plumule from the upper end, 

 by the side of the short cotyledon. Marine herbs, growing under water, with 

 long and thread-like forking stems, and slender almost capillary alternate leaves 

 sheathing at the base. Flowers rising to the surface at the time of expansion. 

 (Dedicated to H. B. Ruppius, a German botanist of the 18th century.) 



1. R. maritima L. Leaves linear-capillary ; fruit obliquely erect ; fruiting 

 peduncles capillary (1-3 dm. long) ; stipes 0.5-4 cm. long. Shallow bays and 

 streams, along the entire coast ; also occasionally hi saline places in the interior. 

 (Cosinop.) 



3. ZANNICHELLIA [Mich.] L. HORNED PONDWEED 



Flowers monoecious, sessile, naked, usually both kinds from the same axil ; 

 the sterile consisting of a single stamen, with a slender filament bearing a 2-4- 

 celled anther ; the fertile of 2-5 (usually 4) sessile pistils in the same cup-shaped 

 involucre, forming obliquely oblong nutlets in fruit, beaked with a short style, 

 which is tipped by an obliquely disk-shaped or somewhat 2-lobed stigma. Seed 

 orthotropous, suspended, straight. Cotyledon taper, bent and coiled. Slender 

 branching herbs, growing under water, with mostly opposite long and linear 

 thread-form entire leaves, and sheathing membranous stipules. (Named in 

 honor of 0. G. Zannichelli, a Venetian botanist.) 



1. Z. palustris L. Style at least half as long as the fruit, which is flatfish, 

 somewhat incurved, even, or occasionally more or less toothed on the back (not 

 wing-margined in our plant), nearly sessile; or, in var. PEDCNCI;L.\TA J. Gay, 

 both the cluster and the separate fruits evidently peduncled. Ponds and slow 

 streams, chiefly brackish, throughout N. A. July. (Cosmop.) 





 4. ZOSTERA L. GRASS WRACK. EEL GRASS 



Flowers monoecious; the two kinds naked and sessile and alternately ar- 

 ranged in two rows on the midrib of one side of a linear leaf-like spadix, which 

 is hidden in a long and sheath-like base of a leaf (spathe) ; the sterile flowers 

 consisting of single ovate or oval 1-celled sessile anthers, as large as the ovaries, 



