pond scum

pond scum

noun
any free-floating freshwater alga that forms a green scum on water.

Origin:
1885–90

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
pond scum
 
n
a greenish layer floating on the surface of stagnant waters, consisting of various freshwater algae

Collins English Dictionary - Complete & Unabridged 10th Edition
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Encyclopedia Britannica
Encyclopedia

pond scum

genus of green algae, found only in fresh water and usually free-floating. The slippery unbranched filaments are composed of cylindrical cells containing one or more beautiful spiral green chloroplasts, from which the genus gets its name. The nucleus is suspended in the central vacuole by fine cytoplasmic filaments. Vegetative reproduction is by fragmentation of the filaments. In sexual conjugation, cells of two strands lying side by side are joined by the outgrowths, or conjugation tubes, and the contents of one cell pass into and fuse with the contents of the other. The resulting fused cell (zygote) becomes surrounded by a thick wall and overwinters, while the vegetative filaments die

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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