root hair

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A thin hairlike outgrowth of an epidermal cell of a plant root that absorbs water and minerals from the soil.

from The Century Dictionary.

• noun A delicate filament developed from a single cell (thus distinguished from a root-fibril) on the epidermis of the young parts of a root; a unicellular trichome borne on a root.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology The rhizoid of a vascular plant; a tubular outgrowth of a trichoblast, a hair-forming cell on the epidermis of a plant root.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun thin hairlike outgrowth of an epidermal cell just behind the tip; absorbs nutrients from the soil