64 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



10. Potamogeton diversifolius Raf. 



Common in shallow pools and at margins of quiet waters; rock pools at Great Falls: 

 edge of canal. Fr. July-Aug. Nearly throughout the U. S. (P. hybridus Michx.) 



The characters supposed to distinguish P. dimorphus Raf. from P. diversifolius are 

 neither prominent, constant, nor important. Specimens possessing the diagnostic 

 points of both forms are readily found. 



11. Potamogeton pectinatus L. Sago pondweed. 

 Abundant in quiet waters. Fr. July to fall. Cosmopolitan. 



Notable for its abundant tubers which are eagerly sought by waterfowl. 



12. Potamogeton robbinsii Oakes. 



Common in the mouthB of large creeks below Washington. Northern U. S. 

 Rarely fruits. 



Potamogeton natans L., recorded by Ward from the Eastern Branch, is not repre- 

 sented by specimens. 



2. ZANNICHELLIA L. Horned pondweed. 



1. Zannichellia palustris L. 

 Occasional along the edge of the river and canal. Fr. July -Sept. Cosmopolitan. 



13. NAIADACEAE. 



1. NAIAS L. Bushy pondweed. 



Basal auricles of the leaves rounded, toothed on the even margin; leaves linear, the 

 teeth frequent. 



Stem slender, many-leaved, usually much branched 1. N. flexilis. 



Stem stout, few-leaved, sparsely branched, elongate la. N. flexilis robusta. 



Basal auricles of the leaves pectinate, toothed on the irregular divisions; leaves 

 threadlike, the teeth sparse 2. N. gracillima. 



1. Naias flexilis (Willd.) Rostk. & Schmidt. 



Abundant in the canal and other quiet waters. Fr. July. Northern Hemisphere. 

 Entire process of fertilization takes place under water. 

 la. Naias flexilis robusta Morong. 

 Found with the species. 



2. Naias gracillima (A. Br.) Magnus. 



Abundant in a pool at Widewater. Northeastern U. S. 



14. ALISMACEAE. Water plantain Family. 



Flowers 2-2.5 mm. wide, in large panicles; achenes borne in a single whorl; leaves 



ovate, cordate. Flowers all perfect 1, ALISMA. 



Flowers 4-15 mm. wide, usually in whorls of 3 (sometimes 2 in Lophotocarpus); 

 achenes borne in a dense head; leaves arrow-shaped, or else acute at the base. 

 Pedicels very stout; lower flowers of the inflorescence perfect. 



2. LOPHOTOCARPUS. 

 Pedicels slender; lower flowers of the inflorescence pistillate 3. SAGITTARIA. 



1. ALISMA L. 

 1. Alisma subcordatum Raf. Water plantain. 



Shallow water and mud; along the canal, and flats below Chain Bridge; probably 

 fairly common in similar locations. June-Sept. Eastern U. S. (Listed as A. plan- 

 tago, A. plantago omericanum, and A. plantago-aquatica.) 



Echinodorus radicans (Nutt.) Engelm. has been reported by Steele, " along a de- 

 pression in the flats below Chain Bridge, perhaps a dozen specimens, some well devel- 

 oped, August 1, 1900." 



