POND- WEED TRIBE. 645 



4. ZOST^RA (Grass-wrack). Flowers composed of 

 stamens and pistils alternately arranged in 2 rows in a 

 long leaf-like sheath. (Name from the Greek zoster, a 

 girdle, which the leaves resemble in form.) 



1. POTAMOG^TON (Pond-weed). 



1. P. natans (Floating Pond- weed). Upper leaves 

 elliptical, ribbed, and cellular, lower submersed, linear. 

 Ponds and ditches, common. An aquatic plant, with 

 cord-like stems, proportioned to the depth of the water 

 in which it grows ; smooth, floating leaves, on long 

 stalks j and cylindrical spikes of small green flowers, 

 which rise above the surface of the water. The upper, 

 or floating leaves are 2 3 inches in length, the lower, 

 which are not always present, are very narrow and a 

 foot long, or more. Fl. June August. Perennial. 



2. P. pe?folidtus (Perfoliate Pond-weed). Leaves 

 alternate, all submersed, egg-shaped, embracing the 

 stem, pellucid, 7 -nerved. Ponds and lakes, common. 

 Remarkable for its brown, almost transparent leaves, 

 2 3 inches long, which when dry have the appearance 

 of gold-beaters' skin, and are so sensitive of moisture, 

 that they will curl when laid on the palm of the 

 hand. EL June August. Perennial. 



3. P. densus (Opposite-leaved Pond- weed). Leaves 

 opposite, all submersed, embracing the stem, pellucid. 

 Ponds and rivers, common. Like the last in habit, 

 but smaller. Fl. June August. Perennial. 



4. P. pusillus (Small Pond- weed). Leaves linear, 

 very narrow ; flowers in a long-stalked, loose spike. 

 Ponds and lakes, common. A tangled mass of thread- 

 like stems, and dull, olive-green leaves, with numerous 

 spikes of brownish flowers, which are either submersed, 

 or partially rise above the surface of the water. Fl. 

 June August. Perennial. 



* From 18 to 20 species of Pond-weed are described 

 as natives of Britain ; they all, more or less, resemble 



