236 BRITISH PLANTS 



occur other submerged species of Potamogeton (Fig. 18), 

 Elodea canadensis, and other submerged, rooted aquatics 

 mentioned in the next group. 



In the centre of much-used canals the vegetation is 

 very similar to that of streams, the motion of the barges 

 producing a well-marked current. Out of reach of the 

 barges the vegetation may be that typical of still water. 



3. Standing Water. 



Aquatic plants reach their greatest development in 

 still water. Unless the water is much disturbed by 

 animals, as in cattle- and duck-ponds, or when polluted 

 by sewage or the refuse from chemical works or copper- 

 mines, it becomes the home of a vast assemblage of 

 plants, and it is a common sight in summer to see the 

 whole surface of a pond covered with the white blossoms 

 of the water-crowfoot. 



Ponds and lakes in the lowlands are always better 

 inhabited than those in the highlands. The chief reason 

 for this difference is that the water of the former is 

 usually derived from the- drainage of large areas of soft 

 rock, and consequently contains a considerable amount of 

 nutritive material in solution. The water of a highland 

 loch, on the other hand, comes from a comparatively 

 small area of hard rock, and the amount of material in 

 solution is conspicuously less. The highland type of 

 vegetation is not necessarily restricted to high altitudes ; 

 it may occur at lower levels in water poor in mineral salts. 

 At the same time, altitude does play, some part in bringing 

 about differences in the vegetation. At high levels the 

 water is often frozen in winter to a depth below the limits 

 of plant-life, and few plants can survive such extreme 

 conditions. In the lowlands the water is seldom frozen 

 to a depth of more than a few inches, and the plants can 

 hibernate in the warmer layers below. In some cases 

 special hibernating shoots are produced (p. 54), and 

 many possess rhizomes. Annual plants are extremely 

 rare (p. 54). 



The plants of a lake or pond usually show a more or 

 less pronounced distribution in zones depending on the 

 depth of the water. Each zone is often dominated by a 

 single species, in all cases a rooted plant, forming a 



