POLLUTED RIVERS 



Let us, for instance, notice some of the ordinary plants to 

 be found along a riverside. Willows and Alders are the 

 ordinary trees, because they are specially fitted to stand the 

 danger of being regularly overflowed. They easily take root, 

 so that branches broken off and floated down are enabled to 

 form new trees without much difficulty. In the United 

 States, it has become a custom to plant Willows along the 

 banks, because they are then not so liable to be broken 

 down and worn away. Yet when a big Willow tree has 

 become undermined, the weight of the trunk may cause it to 

 fall over towards the water, so that a large section of the 

 bank may be loosened and serious damage may be done if it 

 is torn away by a heavy flood. 



Amongst such Willows, should be mentioned the "cricket 

 bat *" kind, which has to be grown with the very greatest care, 

 and of which a single tree may be worth £28. 



Many of our rivers are, alas, sadly polluted by artificial 

 and other impurities which kill the fishes and destroy the 

 natural vegetation. When this happens a horrible-looking 

 whitish fungus {Apodytes lacted) coats the stones and banks 

 under water and the water swarms with bacteria. This 

 fungus and the bacteria are really purifying the water, for 

 they break up the decaying matter in it. 



The oily or slimy character of the outside skin of all sub- 

 merged plants is of very great importance to them. It allows 

 the water to glide or slip over them without any friction. 



Still keeping to our river bank, let us look for submerged 

 plants. What is that dark green feathery plume ? It is 

 the Hornwort (Ceratophyllum) gently wriggling or moving 

 from side to side. It has probably never been still for a 

 moment since it first began to grow. Take it out of the 

 water, and it collapses into a moist, unpleasant little body, 



208 



