[*][*] Stipules united with the sheathing base of the leaf.
27. P. pectinàtus, L. Stem filiform, repeatedly dichotomous; leaves very narrowly linear, attenuate to the apex, 1-nerved with a few transverse veins; spikes interrupted, on long filiform peduncles; fruit obliquely broad-obovate, compressed, bluntly keeled; shell of nutlet very thick; embryo spirally incurved.—N. Brunswick to Fla., westward across the continent. Aug.–Oct. (Eu.)
28. P. marìnus, L. Resembling narrow-leaved forms of the last species, low and very leafy; peduncles much elongated; fruit much smaller (1´´ long) and thinner, round-obovate, not keeled upon the rounded back, tipped with the broad sessile stigma; embryo annular.—Western N. Y., Ill., Mich., and southward. Probably the range of this species is much more extensive than indicated, as it has been confounded with P. pectinatus.
29. P. Robbínsii, 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 (3–5´ long and 2–3´´ wide), 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-obovate (2´´ long), keeled with a broadish wing, acutely beaked; embryo stout, ovally annular.—In ponds and slow streams, N. Brunswick to N. J., the N. shore of L. Superior, and far westward.
4. RÚPPIA, L. Ditch-grass.
Flowers perfect, 2 or more approximated on a slender spadix, which is at first enclosed in the sheathing spathe-like base of a leaf, entirely destitute of floral envelopes, 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 obliquely ovate 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 botanical author of the early part of the 18th century.)
1. R. marítima, L. Leaves linear-capillary; nut ovate, obliquely erect, 1½´´ long; fruiting peduncles capillary (3–6´ long); stipes 1–12´´ long.—Shallow bays, along the entire coast; also occasionally in saline places in the interior. (Eu., Asia, etc.)
5. ZANNICHÉLLIA, Micheli. Horned Pondweed.
Flowers monœcious, 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 Zannichelli, a Venetian botanist.)
1. Z. palústris, L. Style at least half as long as the fruit, which is flattish, somewhat incurved, even, or occasionally more or less toothed on the back (not wing-margined in our plant), nearly sessile; or, in var. pedunculàta, both the cluster and the separate fruits evidently peduncled.—Ponds and slow streams, throughout N. America, but not common. July. (Eu., Asia.)