thallus

[Latin, green stalk, from Greek thallos, from thallein, to sprout.]

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A plant body or fungus undifferentiated into stem, root, or leaf.

1. a simple plant body not differentiated into root, stem and leaf, characteristic of mycelial fungi and some algae.
2. the actively growing vegetative organism as distinguished from reproductive or resting portions, as in fungi.

The noun has one meaning:

Meaning #1: a plant body without true stems or roots or leaves or vascular system; characteristic of the thallophytes