noun, plural prothalli [proh-thal-ahy] . Botany.

1850–55; < Neo-Latin; see pro-, thallus Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
prothallus or prothallium (prəʊˈθæləs, prəʊˈθælɪəm)
n , pl -li, -lia
botany the small flat free-living gametophyte that bears the reproductive organs of ferns, horsetails, and club mosses. It is either a green disc on the soil surface or it is colourless and subterranean
[C19: from New Latin, from pro- before + Greek thallus a young shoot]
prothallium or prothallium (prəʊˈθæləs, prəʊˈθælɪəm, -laɪ, -lɪə)
[C19: from New Latin, from pro- before + Greek thallus a young shoot]
pro'thallic or prothallium
pro'thallial or prothallium

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American Heritage
Science Dictionary
prothallus   (prō-thāl'əs)  Pronunciation Key 
Plural prothalli (prō-thāl'ī)
The gametophyte of homosporous ferns and some other plants. Prothalli have chlorophyll for photosynthesis, but they are not differentiated into roots, stems, or leaves. They are usually small, flat, and delicate. Prothalli develop from germinated spores, and they bear both archegonia for producing eggs and antheridia for producing sperm. See more at alternation of generations.
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