Found in fungi and certain species of sponges and plants, rhizoids are small filaments that function in a manner that is similar to that of the roots of vascular land plants. The primary duty of rhizoids is to help organisms absorb nutrients, minerals and water.
Rhizoids and roots are both necessary for many organisms to sustain life, but the two differ in the way they form and function. Rhizoids are generated from a single cell, while roots are the result of multiple cells with a common purpose combining together. Rhizoids absorb water between threads in a process called capillary action, while roots absorb water through their own cell wall in a process called osmosis.
In fungi, rhizoids are located in the thallus, which is the body of cells that make up the organism. The thallus is a structure in the plant kingdom that lacks specialized components such as leaves or a stem. In several species, the rhizoid serves an additional purpose beyond nutrient absorption by anchoring the thallus to the substratum. In biology, the substratum is the object on which a organism exists. For example, the substratum for a specimen of moss growing on a tree trunk would be the tree trunk.