AQUATIC PLANTS OF FRESHWATER 



of the plant, they should be removed, the aquarium cleaned and new plants 

 introduced; though this may only occur in large tanks or basins. Ex- 

 cessive growth of some of the larger varieties, especially the Confervse, may 

 also form such compact masses and mats that even large fishes become en- 

 tangled and may die of suffocation. These larger algas and confervse are 

 best removed with tweezers, the hand, or by entangling the growth about 

 a stick. Aquatic plants taken directly from the pond, especially Myrio- 

 phyllum and Nitella, are the usual sources of introducing these objection- 

 able algae. The microscopic green algae are sometimes present in aquaria 

 in such numbers as to obscure the contents; often the decomposition ot 

 their chlorophyll makes the water brown. A small quantity of a solution 

 of permanganate of potassium in water diluted to a claret color and then 

 added to the aquarium water, will cause their entire disappearance without 

 injury to the animal life. 



FLOATING WATER PLANTS 



Some of the native and tropical floating plants may be introduced 

 into the aquarium with very pleasing effect. Of these the native species 

 are Duckweed, Floating Pondmoss, and Crystalwort, and the more com- 

 monly known tropical species, Salvinia, Trianea, Frog-bit, Water Lettuce, 

 and the Water Hyacinth. 



DUCKWEED 



Lemnas or Duckweeds, Fig. 133, occur on every ditch and pool; the 

 five most generally distributed in the United States being L. minor or 

 Lesser Duckweed; L. perpusilla or Tiny D'uckweed; L. gibba or Thick- 

 leaved Duckweed; L. polyrhiza (Spirodela poly- 

 rhiza) or Greater Duckweed; and L. trisulea or 

 Ivy-leaved Duckweed, the last two least often 

 found in the Eastern and Middle States. They 

 are small flowering plants having pendant roots, 

 and grow by the extension of offshoots from 

 the clusters; and in winter sink to the bottom 

 when the growth of the leaves is checked by 

 frost. 



L. minor has nearly round leaves of even, 

 small size and bright green color, with usually 

 one rootlet attached to each leaf It is very 



FIG. 133. Duckweed. Reduced one- j j • i i 



third. \. Lcmnamivor. common and durmg the Summer almost covcts 



;• \Z:MltirHi.a. the surface of ditches and other slow flowing 

 4. Lemna perpusilla. or Stagnant Water. L. perpusilla has a still 



208 



