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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: cl-ssified into the six phyla euglenophyta, crysophyta, pyrrophyta, chlorophyta, phaeophyta, and rhodophyta.
historical examples

this alga decomposes the hard coral limestone, making the surface of the rock soft and powdery.
the solomon islands and their natives h. b. (henry brougham) guppy

but the longest of all known plants is the alga macrocystis.
the sea-beach at ebb-tide augusta foote arnold

each lichen consists of a plasmodomous plant (sometimes a protophyte, sometimes an alga) and a plasmophagous fungus.
the wonders of life ernst haeckel

yet it is at least probable that the first real plant on this world was a seaweed or alga.
the romance of plant life g. f. scott elliot

an alga (halosphria viridis) has been brought up from depths of one thousand to two thousand metres.
appletons’ popular science monthly, december 1898 various

the alga, chromophyton rosanoffii, is another example of apparent luminosity, due to reflection from almost spherical cells.
the nature of animal light e. newton harvey

when young this alga consists of an annulated tube formed of a single cell.
the sea-beach at ebb-tide augusta foote arnold

alga (seaweed) suggests algere, ‘to be cold,’ one of the symptoms of the ague (querceram).
the apologia and florida of apuleius of madaura lucius apuleius

it has recently been shown, however, that this organism is most probably an alga.
encyclopaedia britannica, 11th edition, volume 9, slice 3 various

the alga polysiphonia violacea floats in long feathery tufts from the stakes.
the sea-beach at ebb-tide augusta foote arnold

plural noun (sing) alga (ˈælɡə)
unicellular or multicellular organisms formerly cl-ssified 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
n.

see algae.
n.

(plural), 1794, from alga (singular), 1550s, from latin alga “seaweed,” of uncertain origin, perhaps from a pie root meaning “to putrefy, rot.”

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.
alga
(āl’gə)
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 sc-m 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 cl-ssified as plants, the algae are now considered to be protists, with the exception of the cyan-bacteria, 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.
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 sc-m that collects in still water.

note: algae supply a considerable part of the world’s oxygen.