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 
