2. Fruit broader than high; stigmas to 1 mm. long; stamens to 1.2 mm. long and 



not elongating as the fruit develops; anthers 0.1-0.2 mm. wide; 

 flowers without bracteoles; leaves essentially uniform; plants mostly 

 terrestrial or on mud (4) 



3(2). Height of fruit exceeding the width by 0.2 mm.; lower end of mericarps 

 curved outward so that the fruit is thickest at base 2. C. verna. 



3. Height of fruit equaling width or not varying by more than 0.1 mm.; lower end 



of mericarps straight so that fruit is thickest a little below the 

 middle; commisural grooves very narrow 3. C. heterophylla. 



4(2). Fruit 0.3-0.8 mm. wide, more or less gibbous at base 4. C peploides. 



4. Fruit 0.5-1.2 mm. wide, not gibbous at base (5) 



5(4). Fruit pedicelled; wing and thin margin of carpel turned outward at right 

 angles to the surface of the fruit or revolute and appearing like a 

 thickened margin 5. C. Nuttallii. 



5. Fruit almost sessile; margin of carpel appearing as if not winged but under high 



magnification showing a minute wing; styles usually deflexed 



6. C. terrestris. 



1. Callitriche hermaphroditica L. Fig. 510. 



Fruit 1-2.5 mm. wide, about as high as wide; surface of carpels obscurely and 

 irregularly pitted; margins of carpels with a narrow wing but the outer part of 

 the carpel itself strongly compressed and winglike; style sharply reflexed from 

 between the mericarps, usually breaking and leaving the persistent base; floral 

 bracts absent; leaves lance-linear, uniform, dark-bright-green, with bases clasping 

 the stem. 



Submersed aquatic in N.M. (Taos Co.) and Ariz. (Coconino Co.), June-Oct.; 

 Greenl., across Can. to Alas., n. U. S., s. in the w. to Ariz, and N.M. 



2. Callitriche verna L. emend. Kiitz. Fig. 511. 



Fruits sessile, suborbicular, 0.6-1.4 mm. wide, the height always exceeding the 

 width, the width greatest above the middle, the thickness at base greater than at 

 summit; face of mericarps sharply reticulate, the reticulations appearing more or 

 less clearly in vertical rows; margins of carpels widely spreading with a definite 

 scarious wing that is always widest at summit and runs a greater or lesser distance 

 down the sides; pale green submersed aquatic or sometimes terrestrial on the mar- 

 gins of ponds or streams; stems to about 5 dm. long; leaves very variable, the 

 lower submersed ones often linear, 0.3-1 mm. wide, 1 -nerved, shallowly bidentate 

 at apex, the upper ones often dilated and the terminal leaves petioled and narrowly 

 obovate to spatulate and commonly in a floating rosette, various intermediate 

 leaves are present on many plants. C. palustris L. 



In quiet shallow water or stranded on mud in e. and cen. Tex. through N.M. 

 (Catron, Taos and Sandoval cos.) to Ariz. (Apache, Coconino and Pima cos.), 

 Mar.-June; from Greenl. to Alas., s. to Tex., N.M., Ariz, and Calif.; also Mex., 

 Eur. and Asia. 



Young fruits of C. verna, when pressed, sometimes widen as they flatten to 

 simulate in outline those of C. heterophylla. 



3. Callitriche heterophylla Pursh emend. Darby. Fig. 512. 



Fruit 0.6-1.2 mm. wide, the height equaling the width or not more than 0.1 mm. 

 greater or 0.1 mm. less than the width; carpels more broadly rounded at summit 

 than at base so that the outline of the fruit is slightly heart-shaped, convex on the 

 face and thickest just above the base; margins of fruits wingless or rarely with a 

 narrow wing at the summit; styles 1-6 mm. long, erect or spreading, persistent or 

 caducous; plants rather dark green; leaves of many types, often linear and one- 



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