algae

[al-jee]
plural noun, singular alga [al-guh] .
any of numerous groups of chlorophyll-containing, mainly aquatic eukaryotic organisms ranging from microscopic single-celled forms to multicellular forms 100 feet (30 meters) or more long, distinguished from plants by the absence of true roots, stems, and leaves and by a lack of nonreproductive cells in the reproductive structures: classified into the six phyla Euglenophyta, Crysophyta, Pyrrophyta, Chlorophyta, Phaeophyta, and Rhodophyta.


Origin:
< Neo-Latin, plural of Latin alga seaweed

algal, adjective
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World English Dictionary
algae (ˈældʒiː)
 
pl n , sing alga
unicellular or multicellular organisms formerly classified as plants, occurring in fresh or salt water or moist ground, that have chlorophyll and other pigments but lack true stems, roots, and leaves. Algae, which are now regarded as protoctists, include the seaweeds, diatoms, and spirogyra
 
[C16: from Latin, plural of alga seaweed, of uncertain origin]
 
algal
 
adj

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Etymonline
Word Origin & History

algae
(plural), 1794, from alga (sing.), 1550s, from L. alga "seaweed," of uncertain origin, perhaps from a PIE base meaning "to putrefy, rot."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

algae al·gae (āl'jē)
pl.n.
Any of various chiefly aquatic, eukaryotic, photosynthetic organisms, ranging in size from single-celled forms to the giant kelp.

The American Heritage® Stedman's Medical Dictionary
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American Heritage
Science Dictionary
alga   (āl'gə)  Pronunciation Key 
Plural algae (āl'jē)
Any of various green, red, or brown organisms that grow mostly in water, ranging in size from single cells to large spreading seaweeds. Like plants, algae manufacture their own food through photosynthesis and release large amounts of oxygen into the atmosphere. They also fix large amounts of carbon, which would otherwise exist in the atmosphere as carbon dioxide. Algae form a major component of marine plankton and are often visible as pond scum and blooms in tidal pools. Land species mostly live in moist soil and on tree trunks or rocks. Some species live in extreme environments, such as deserts, hot springs, and glaciers. Although they were once classified as plants, the algae are now considered to be protists, with the exception of the cyanobacteria, formerly called blue-green algae. The algae do not form a distinct phylogenetic group, but the word alga serves as a convenient catch-all term for various photosynthetic protist phyla, including the green algae, brown algae, and red algae.
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American Heritage
Cultural Dictionary
algae [(al-jee)]

Primitive organisms that contain chlorophyll but do not have structures, such as xylem and phloem, to transport fluids. Algae sometimes contain only a single cell, and nowadays they are not considered members of the plant kingdom.

Note: The most familiar algae are the greenish scum that collects in still water.
Note: Algae supply a considerable part of the world's oxygen.
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
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Example sentences
The algae floating in the sea are microscopic plants of great consequence on a
  global level.
Green algae do better than red and brown algae in the strong light of shallow
  water.
When the algae die and decompose, the water is depleted of oxygen.
They are toothless herbivores who live off the plants and algae in the river.
Synonyms
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