240 



but with a more limited range, occurring in sandy ponds along 

 the Atlantic coast from Southern New England to Florida and 

 Louisiana. 



In order to compare the number of spinulose teeth in the sub- 

 merged leaves of the two species, fourteen of each were taken at 

 random and examined. In P. paliistris thirteen plants had them 

 on nearly all the leaf segments and one bore them occasionally, 

 while in P. pectinata eleven were entirely destitute of them, and 

 three showed them occasionally, and these so minute as to be 

 scarcely discernible under a lens. 

 4. Myriophylbim. L. Gen. PL n. 724 (1737). 



Leaves for the most part verticillate (alternate in No. 4), the 

 emersed bract-like, entire, toothed or pectinate, the submerged 

 long, pectinate-pinnatifid, with fine capillary divisions. Flowers 

 axillary, commonly monoecious, the staminate above with a very 

 short calyx tube, and two to four-lobed limb or none. Petals two 

 to four. Stamens four to eight. The intermediate flowers not 

 unfrequently perfect. Calyx of the fertile flowers with a more or 

 less deeply four-grooved tube, and four minute lobes or none. 

 Ovary two to four-celled, having a single pendulous ovule in each 

 cell. Styles four, short, often plumose and recurved. Drupe 

 four-sided, splitting at maturity into four crustaceous, one-seeded, 

 indehiscent carpels, which are smooth, angled or tuberculate on 

 the back. 



Fifteen or twenty species are known, inhabitants of fresh 

 water in all parts of the world, both in the tropics and the frigid 

 zone. 



Twelve species occur in North America, which may be briefly 

 characterized as follows : 

 Carpels smooth. 



Flowers on emersed spikes. 



Floral leaves shorter than the flowers. Spikes nearly 

 naked. 



Flowers in verticils. i. M. spicatum. 



Flowers alternate, or subverticillate below. 



2. M. alternifloriim. 

 Floral leaves longer than the flowers, pectinate-pinnatifid. 



3. M. verticillatum. 



