432 NAIADACE^E. (PONDWEED FAMILY.) 



3. ZOSTERA. Pistils and anthers alternately sessile in 2 rows on one side of a linear spadlx 



enclosed in a leaf. Stigmas 2. 



* Flowers perfect. 



4. RUPPIA. Flowers naked on a spadix : each of 4 large anther-cells, and 4 ovaried which 



are raised on long stalks hi fruit. 

 6. POTAMOGETON. Flowers and fruit spiked. Sepals, stamens, and sessile ovaries each 4. 



1. IV A I AS, L. NAIAD. 



Flowers dioecious (or sometimes monoecious), axillary, solitary and sessile ; 

 the sterile consisting of a single stamen enclosed in a little membranous spathe : 

 anther at first nearly sessile, the filament at length elongated. Fertile flowers 

 consisting of a single ovary tapering into a short style : stigmas 2-4, awl- 

 shaped : ovule erect, anatropous. Fruit a little seed-like nutlet, enclosed in a 

 loose and separable membranous epicarp. Embryo straight, the radicular end 

 downwards. Slender branching herbs, growing entirely under water, with 

 opposite linear leaves, somewhat crowded into whorls, sessile and dilated at the 

 base. Flowers very small, solitary, but often clustered with the branch-leaves 

 in the axils. (Nat'as, water-nymph ; an ill-chosen name for these insignificant 

 water- weeds ; from their place of growth.) 



1. N. flexilis, Rostk. Leaves membranaceous, spreading, very narrowly 

 linear, entire, or sparingly very minutely denticulate (under a lens) ; stigmas 

 usually 3 -4. (N. Canadensis, Michx. Caulinia flexilis, Wttld.) Ponds and 

 slow streams ; common. July -Sept. (Eu.) 



N. MINOR (Caulinia fnigilis, Willd.}, with the more rigid and recurved frag- 

 ile leaves rather strongly toothed, is not identified in this country. 



2. ZANNICHEL.L.IA, Micheli. 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 up. 

 Slender branching herbs, growing under water, with very slender stems, op- 

 posite or alternate long and linear thread-form entire leaves, and sheathing 

 membranous stipules. (Named in honor of Zannichelli, a Venetian botanist.) 



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

 tish, somewhat incurved, even, or occasionally more or less toothed on the back 

 (not wing-margined in our plant), nearly sessile, or, in var. PEDDNCUL!TA, both 

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

 streams ; rather rare. July. (Eu.) 



3. ZOSTER A, L. GRASS-WRACK. EEL-GRASS. 



Flowers monoecious ; the two kinds naked and sessile and alternately arranged 

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

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



