FLOWERING PLANTS AND FERNS OF ARIZONA 73 



5. NAIADACEAE. Pondweed family 



Plants aquatic; flowers perfect or unisexual, axillary, sometimes 

 subtended by an involucre or a spathe; true perianth none (in 

 Potamogeton a false perianth formed by the sepallike appendages of 

 the 4 stamens); stamens 1 to 4; pistil 1 or more, the ovary 1 -celled, 

 the ovule usually solitary. 



Key to the genera 



1. Flowers perfect, in axillary spikes or clusters; anthers 4, sessile, the connec- 

 tives expanded dorsally into herbaceous sepallike bodies; ovaries 4, 

 sessile; leaves alternate or nearly opposite 1. Potamogetox. 



1. Flowers unisexual, axillary; anther solitary, not appendaged; leaves mostly 

 opposite or whorled (2). 

 2. Leaves linear-filiform, entire; pistillate flowers 2 to 5, subtended at base by a 

 scarious, cup-snaped involucre; stigma one, cup-shaped, sometimes slightly 



2-lobed 2. Zannichelli a. 



2. Leaves linear, serrulate to spinulose-dentate or pinnatifid, dilated at base; 

 pistillate flower solitary, not subtended by an involucre; stigmas 2 to 4, 

 subulate 3. Na ias. 



1. POTAMOGETON. Pondweed 



Leaves with membranous stipules, mostly alternate, all much alike 

 or the lower ones narrower and submersed and the upper ones with 

 broad floating blades; flowers perfect, in axillary spikes, these often 

 emersed; perianth technically none but closely simulated by the 

 concave sepallike appendages of the 4 stamens; anthers sessile; 

 pistils 4, the style short or the stigma sessile. 



Plants of ponds and sluggish streams, flowering mostly in summer. 

 In irrigation ditches the plants are sometimes so abundant as mate- 

 rially to retard the flow. The species are at best difficult to iden- 

 tify and the difficulty is especially great with Arizona specimens, 

 which rarely have good fruit. The following treatment is necessarily 

 provisional. 



Key to the species 



1. Leaves dimorphic, some floating, with broad, thickish, long-pet.ioled blades, 



others submersed, narrower, thin, commonly sessile or short-petioled (2) . 



2. Submersed leaves mostly filiform or narrowly linear, mostly not more than 



2 mm. wide, or a few of the uppermost submersed leaves often broader 



but soon disappearing (3). 



3. Blades of the floating leaves commonly broadly ovate and cordate at base, 



more than 3 cm. long, with more than 15 nerves; spikes all alike, 



cylindric, in fruit commonly 2 cm. long or longer; stems usually 



StOUt 1. P. NATANS. 



3. Blades of the floating leaves elliptic, rounded or short-cuneate at base, 



seldom more (usually less) than 3 cm. long, with fewer than 15 

 (commonly 7) nerves, these deeply impressed beneath; spikes of 2 

 kinds, emersed, several- to many-flowered, cylindric, less than 2 em. 

 long, and submersed, few-flowered, subglobosc; stems very slender. 



2. P. DIVERSIFOI.irs. 



2. Submersed leaves lanceolate or broader, if linear more than 2 mm, wide, 

 mostly 7-nerved; nutlets 3-keeled (4). 



4. Nutlets less than 3mm. long, not prominently keeled- . _ 3. P. gramineus. 

 4. Nutlets 3 to 4 mm. long, prominently keeled, the middle keel crested or 



winglike 4. P. /.mi. 



1. Leaves all much alike and submersed (5). 



5. Blades lanceolate or broader; mature spikes 3 cm. long, or longer; nurle's 

 3-keeled, the middle keel crested or winglike 4. P. eizn. 



