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Rhizoids are protuberances that extend from the lowerepidermal cells ofbryophytes andalgae. They are similar in structure and function to theroot hairs ofvascularland plants. Similar structures are formed by somefungi. Rhizoids may be unicellular or multicellular.[1]
Plants originated in aquatic environments and gradually migrated to land during their long course of evolution. In water or near it, plants could absorb water from their surroundings, with no need for any special absorbing organ or tissue. Additionally, in the primitive states of plant development, tissue differentiation and division of labor were minimal, thus specialized water-absorbing tissue was not required. The development of specialized tissues to absorb water efficiently and anchor the plant body to the ground enabled the spread of plants onto land.[2]
Rhizoids absorb water mainly bycapillary action in which water moves up between threads of rhizoids; this is in contrast to roots in which water moves up through a single root. However, some species of bryophytes do have the ability to take up water inside their rhizoids.[2]
In land plants, rhizoids aretrichomes that anchor theplant to the ground. In theliverworts, they are absent or unicellular, but they are multicellular inmosses. Invascular plants, they are often calledroot hairs and may be unicellular or multicellular.
In certainalgae, there is an extensive rhizoidal system that allows the alga to anchor itself to a sandy substrate from which it can absorb nutrients.[3] Microscopic free-floating species, however, do not have rhizoids at all.[4]
Infungi, rhizoids are small branchinghyphae that grow downwards from thestolons and anchor the fungus to thesubstrate, where they release digestiveenzymes and absorb digested organic material.[citation needed]
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