518 naiadAce^e 



short-stalked spike ; carpels compressed, keeled, with a straight 

 beak. — Ditches ; frequent. — Fl. July, August. Perennial. 



22. P. Friesii. — Stem thread-like, compressed; leaves often in 

 tufts, linear, i — 3 in., 5 — 7-nerved ; stipules small, acute ; flowers 

 few, minute, in an interrupted spike on a slender, compressed 

 stalk ; carpels small, swollen, bluntly keeled, with short, stout 

 beak. — Ponds and ditches ; local. — Fl. July, August. Perennial. 



23. P. pusillus (Small Pond-weed). — A tangled mass of thread- 

 like stems; and dull, olive-green, narrowly-linear, 1 — 3-nerved, 

 acute leaves ; with numerous, small, loose, long-stalked spikes of 

 brownish flowers, which are either submersed, or rise partially 

 above the surface of the water. — Ponds and lakes ; common. — 

 Fl. June — August. Perennial. 



24. P. Sturrbckii, a very slender, delicate, and elegant form, 

 closely allied to the preceding, with bright green, pellucid, 3 — 5- 

 nerved, obtuse leaves 2 — 3 in. long ; flowers in a very small spike 

 on a very slender stalk ; and carpels very small and shortly 

 beaked. Occurs only in Forfar and Perth. 



25. P. trichoides (Hair-like Pond-weed). — Stem thread-like, with 

 numerous spreading dichotomous branches : leaves bristle-like, 

 spreading, stiff, acute, i-veined, dark green, 1 — 2 in. long, with 

 slender, acute stipules ; flowers 3 — 6, very minute, on a slender 

 stalk ; carpels 1 in each flower, compressed, with a short beak. — 

 Ponds ; rare. — Fl. August — October. Perennial. 



**** Leaves all similar, submerged, linear, with flat margins and 

 stipules united into a sheath 



26. P. pectindtus (Fennel-leaved Pond-weed). — Forming dense 

 tangled masses of thread-like stems with alternate branches ; 

 leaves 3 — 8 in. long, upper 1 -nerved, of 2 hollow tubes, lower flat, 

 3-n^rved ; flowers few, in interrupted spikes on long, slender 

 stalks ; carpels large, swollen, slightly keeled, with short beaks. — 

 Streams ; common. — Fl. June — August. Perennial. 



27. P. interruptus, a closely allied form, with flat, 1 — 5-nerved 

 leaves and obscurely keeled carpels, is less common. Often in 

 brackish waters. 



28. P. filiformis (Slender-leaved Pond-weed). — Stem thread-like, 

 with short branches below; leaves hair-like, 1 -nerved, channelled; 

 flowers in dense, separate whorls ; carpels small, with very slight 

 keels and beaks. — Lakes; local. — Fl. June — September. Pe- 

 rennial. 



4. RrjppiA (Tassel Pond- weed). — Slender, aquatic herbs with 

 thread-like, sheathing leaves and an umbellate group of minute, 



