Rhizoids are protuberances that extend from the lower epidermal cells of bryophytes and algae. They are similar in structure and function to the root hairs of vascular land plants. Similar structures are formed by some fungi. Rhizoids may be unicellular or multicellular. Roots are multicellular organs composed of multiple tissues that collectively carry out a common function.
Plants originated in water, from where they 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 was minimal, thus the requirement for specialized water absorbing tissue was not required. Once plants colonized land however, they required specialized tissues to absorb water efficiently, and also to anchor themselves to the land.
Rhizoids absorb water by capillary action, in which water moves up between threads of rhizoids and not through each of them as it does in roots.
In fungi, rhizoids are small branching hyphae that grow downwards from the stolons that anchor the fungus to the substrate, where they release digestive enzymes and absorb digested organic material. That is why fungí are called heterotrophs by absorption. In land plants, rhizoids are trichomes that anchor the plant to the ground. In the liverworts, they are absent or unicellular, but multicelled in mosses. In vascular plants they are often called root hairs, and may be unicellular or multicellular.
In certain algae, there is an extensive rhizoidal system that allows the alga to anchor itself to a sandy substrate from which it can absorb nutrients. Microscopic free-floating species, however, do not have rhizoids at all.