A WORLD IN THE WATER 87 



water with the greatest ease. It is difficult, 

 indeed, to think that they are plants at all, 

 were it not that the colouring matter indicates 

 the presence of the wonderful chlorophyll. 

 Each of the tiny Diatoms is enclosed in a 

 hard flinty shell, most exquisitely sculptured. 

 These minute shells have played a big part in 

 changing the level of the earth. In some parts 

 of the world, notably in Virginia, deposits of 

 Diatom shells have been found forty feet in 

 depth. How many generations of these little 

 plants must have contributed to this vast mass 

 it is not easy to conceive. 



Many kinds of tiny algae hang together in 

 threads ; others may be attached to some 

 water plant or have found a resting place on a 

 rock. They evince an enormous variety of 

 colour, ranging from the most vivid green to a 

 dull brown. Yet in all it is possible to detect 

 the presence of chlorophyll, and each atom is 

 playing a part in the re-oxygenising of the 

 water. As well it is quite certain that these 

 plants use up a good deal of impure matter, so 

 that many a stagnant pool is reaUy more pure, 

 so far as its water is concerned, than the run- 

 ning stream where the number of algse is more 



