PLANTS FOR THE AQUARIUM 83 



a fern or a flowering plant with its leaves, stem, and roots, algae 

 do not possess true roots, leaves, or stems. The whole plant 

 body is called a thallus, and when in the following pages we speak 

 of the " roots," this is merely by analogy, for the fixing organs 

 have neither the structure nor the function of true roots. Algse 

 reproduce by spores, which often possess the power of locomotion. 

 It not infrequently happens when an aquarium is, by accident or 

 design, well stocked with algae, that these reproduce so rapidly 

 that the water actually becomes turbid with the minute spores, 

 and these settle on the sides so thickly as to form a slimy green 

 scum. It is the habit of aquarium-keepers to call this scum 

 " confervoid growth," because it is often due to the development 

 of the spores of algae belonging to the Confer voidese. The ex- 

 pression has, however, only a generalised accuracy, for the effect 

 may be produced by a variety of algae. The cure, in both the 

 fresh and salt water aquarium, is to keep vegetarian gasteropods 

 (i.e. snails, periwinkles, etc.), who lick up the deposit with con- 

 siderable rapidity. If this is not sufficient the tank should be 

 emptied and carefully cleaned. 



Again, it sometimes happens that specimens, plant or animal, 

 taken from freshwater pools, are coated with a brownish deposit. 

 This is due to the presence of a great number of the little brownish 

 algae called diatoms, which occur both in the sea and fresh water, 

 and are sometimes extraordinarily abundant. They furnish the 

 food supply of a large number of aquatic animals, and when 

 not in excess are not objectionable. They are objects of great 

 beauty under the microscope, as are also the Desmids, micro- 

 scopic green algae found in freshwater pools, especially on moors, 

 but the microscopic forms of life are beyond our scope here. 



Considering the marine macroscopic algae first, we find that 

 they group themselves into three great divisions, according to 

 the colour, which corresponds here to certain peculiarities of 

 structure. These groups are the Red, the Brown, and the Green 

 Algae. All three groups are necessarily confined to relatively 

 shallow water, because all contain chlorophyll and must have 

 sunlight. Of the three the red weeds extend into the deepest 

 water, for which their colouring specially fits them, and they 

 are therefore on the whole not well adapted for the aquarium. 



