WEEDS OF PONDS, RIVERS, DITCHES 323 



are poisonous and dangerous to stock which have access 

 to them, and others entail much expense and annoyance 

 by invading cultivated watercress beds. 



Water plants exhibit very striking adaptations to their 

 environment. Some of them, such as Water Lilies and 

 certain kinds of Potamogeton and Cham, are only met 

 with in fairly deep water, while others, such as Rushes, 

 Reed Grass, Yellow Iris, and Marsh Marigold, are chiefly 

 found in shallow water by the margins of ponds and 

 streams. Brooklime and some forms of Water Crow- 

 foot inhabit running streams ; other plants are adapted 

 to the stagnant or slow-moving water of ponds and 

 canals. 



A fairly marked succession of zones of vegetation is 

 frequently observable at the sides of ponds, lakes, and 

 slow-moving rivers. At the extreme edge, where the 

 water joins the dry land, the ordinary herbage of the 

 field is intermingled with plants such as Caltha palustris, 

 Ranunculus Flammula, Pedicularis palustris, and various 

 kinds of Juncus and Carex, which can grow freely in 

 water-logged soil. In somewhat deeper water other 

 species of Carex, Yellow Iris, Water Mint, and Phrag- 

 mites communis often predominate. Further in are seen 

 the Bulrushes (Scirpus lacustris and S. Taberncemontani) 

 and Equisetum limosum ; the yellow and white Water 

 Lilies appear usually in from 6 to 12 feet of water, 

 while many of the Potamogetons are found only where 

 the water is 12 to 24 feet deep. 



Fresh water Algae and species of Chara and Nitella 

 frequently cover the bed of the pond or lake in the 

 deepest parts. 



In the surface water all over the pond, down to a 

 depth of 8 or 9 feet, there are often present a vast 

 number of Diatoms, Desmids, and other very minute 

 Algae, forming the plankton or free floating vegetation. 



