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Rhizoids are thin filaments that anchor certain plants and absorb water and nutrients, serving as a root system for plants without traditional roots. They are not to be confused with true roots, which are vascular and carry water and nutrients. Rhizoids are found in bryophytes and seedless vascular plants, and are believed to be the first evolution of a plant root. The term “rhizoid” can also refer to other structures in scientific usage.
A rhizoid, with a name derived from the Latin prefix rhizo- for “root,” isn’t actually a root. Rhizoids are short, thin filaments that anchor certain types of plants and absorb water and nutrients from the plant environment. Rhizoids, while not technically a root, serve as a root system for plants that lack a traditional root system.
A true plant root is vascular. It has hollow tubes for carrying water and nutrients to various parts of a plant, such as leaves, where they are metabolized for growth. Xylem carries water and phloem carries nutrients.
In many microscopic fungi and algae, a rhizoid may be unicellular, a single elongated plant cell. Most multicellular rhizoids are also relatively undifferentiated single cells connected end to end. Cell membranes are porous, so water and nutrients can pass from one cell to another.
The function of the rhizoids is to be the root system of bryophytes, plants without vascular tissue, such as mosses and liverworts. Like a tangled mass of silky white hair, rhizoids can bond a plant to its substrate, whether it’s soil, solid rock, or other material in which it grows. Likewise, the greatly increased surface area of many hair strands effectively absorbs water and dissolved minerals.
The rhizoids of the liverwort are very long unicellular structures. Moss rhizoids are multicellular, and some moss species may have a deep, extensively branching rhizoid system. A rhizoid of most mosses cannot absorb water directly. Rather, it transports water by surface capillarity. Some fungal rhizoids secrete digestive enzymes to absorb the resulting organic material from its host.
Rhizoids are also crucial for a class of seedless vascular plants, such as ferns, during the gametophyte stage, when their reproductive cells have a single set of genetic chromosomes. The young plant survives, thanks to its rhizoids, until a fertilized egg begins to develop a vascular system including true roots. It then enters its sporophyte stage, when reproductive cells contain a complete set of chromosomes within the spores that are released into the wind. Moss also propagates asexually during its sporophyte stage. Ferns and mosses have only one parent.
Rhizoids support primitive plants and are not found in most sexually reproducing vascular plants, so it is widely believed that a rhizoid is the first evolution of a plant root. Algae and other plants in a liquid medium may have evolved specialized cells devoted to absorbing water and nutrients, while other cells evolved to absorb sunlight. Life on Earth became the next logical step. The whip fern, reminiscent of fossil plants from the earth’s early Cambrian Period, has a vascular system but no roots or leaves – only rhizoids – to support it.
The term rhizoid is sometimes used loosely to define “root hairs,” the singular filaments that are extensions of special hair-forming cells on the outer layer of the roots of a vascular plant. Both are trichomes, any thin hair-like appendage or outgrowth of a plant. Both also have almost the same functions.
A rhizoid is not to be confused with a rhizome. Rhizomes, also called rootstocks, are nodes along the underground stem of some plants from which a new root system and stem bud can originate. Irises are an example of plants that can be propagated by rhizomes.
The term “rhizoid” also has other meanings in scientific usage. He came to describe any filamentous root-like structure with fractal branching as how some colonies of bacteria grow. It has also been used to describe the structures in a cell or organism that allow it to anchor or adhere to its environment.
