FRESH-WATER ALG. 201 
they contain ; while their own tissues form food 
and shelter to myriads of animalcules, that wander 
over these—to them—trackless fields and endless 
mazes, and convert the waste pools and ditches of 
the wayside into scenes of busy life and enjoyment. 
This mutual adjustment between the economies of 
the animal and vegetable kingdoms, whereby the 
vital functions of each are maintained in the ut- 
most efficiency, is one of the most beautiful and 
striking phenomena of organic nature. 
The largest of the fresh-water alge is the River 
Lemania (Lemania fluviatilis), which bears a close 
relation to the lower Fucoids of our rocky sea- 
shores. It is never found growing in stagnant 
waters. Indeed, it is said to languish and die, 
when the streams in which it is produced have, by 
some cause or other, been converted into motion- 
less pools. It loves to grow in clear swift rivers, 
flowing with a strong current over a rough and 
rocky bed, and in Alpine streamlets, on the very 
verge of the numerous cascades which they form 
during their descent from the hills. It is a matter 
of surprise how it can sustain the immense force 
and weight of the impetuous waters, without being 
uprooted and carried away. Examination will, 
however, discover that it has been wonderfully 
provided with means to enable it to brave the 
dangers to which in such situations it is exposed. 
