
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 Conferve, may 
also form such compact masses and mats that even large fishes become en- 
tangled and may die of suffocation. ‘These larger alge and conferve 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 alge. The microscopic green alge 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 Duckweed; L. gibba or Thick- 
leaved Duckweed; L. polyrhiza (Spirodela poly- 
rhiza) or Greater Duckweed; and ZL. ¢risulea 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 
common and during the summer almost covers 

FIG. 133. Duckweed. Reduced one- 
third. 1. Lenina minor. 
CLE the surface of ditches and other slow flowing 
3. Spirodela polyrhiza. : : 
4. Lemna perpusilla. or stagnant water. JL. perpusilla has a still 
g 
208 
