THE FEESII-WATER AQIJARIIJM. 49 



No. 8. The Naiad, grows in ponds and slow streams, but 

 we cannot recommend it highly. 



No. 9. Pond-weeds exhibit twelve distinct varieties in 

 the United States, most of which may be seen in the vici- 

 nity of New York, and some in brackish water, as well as 

 fresh. They differ considerably in size, and in the form 

 of their leaves ; and while the leaves of a few float on 

 the water, those of the remainder are submerged. The 

 latter kind of any plant is always to be preferred for an 

 Aquarium, the reader should remember, and for an obvi- 

 ous reason. The plants in an Aquarium are chiefly desira- 

 ble for their ability to purify the water by oxygenizing it. 

 Those whose leaves lie on the surface, or rise above it, 

 necessarily waste upon the atmosphere just so much of the 

 oxygen they exhale, while those that live wholly sub- 

 merged diffuse all their vitalizing property where it is most 

 needed. When ornament and utility may be combined in 

 any plant the acme of essential service is at once reached 

 in its employment in the Aquarium ; when that is impos- 

 sible, mere beauty must be sacrificed on the altar of the 

 practical. 



The Potamogeton natans, has a broad, green, pointed leaf, 

 and might be mistaken for a lily. Occasionally we meet 

 a P. natans with brown, ovate leaves, floating on leaf- 

 stalks rooted at the bottom. This makes a good resting- 

 place for the little Tritons, or Water Newts, and for simi- 

 lar animals that require now and then an introduction to 

 the atmospheric air. The P. densus is a small plant 



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