Source : Webster's Revised Unabridged Dictionary (1913)

   2. An edible or esculent root, especially of such plants as
      produce a single root, as the beet, carrot, etc.; as, the
      root crop.

   3. That which resembles a root in position or function, esp.
      as a source of nourishment or support; that from which
      anything proceeds as if by growth or development; as, the
      root of a tooth, a nail, a cancer, and the like.
      Specifically:
      (a) An ancestor or progenitor; and hence, an early race; a
          stem.

                They were the roots out of which sprang two
                distinct people.                  --Locke.
      (b) A primitive form of speech; one of the earliest terms
          employed in language; a word from which other words
          are formed; a radix, or radical.
      (c) The cause or occasion by which anything is brought
          about; the source. ``She herself . . . is root of
          bounty.'' --Chaucer.

                The love of money is a root of all kinds of
                evil.                             --1 Tim. vi.
                                                  10 (rev. Ver.)
      (d) (Math.) That factor of a quantity which when
          multiplied into itself will produce that quantity;
          thus, 3 is a root of 9, because 3 multiplied into
          itself produces 9; 3 is the cube root of 27.
      (e) (Mus.) The fundamental tone of any chord; the tone
          from whose harmonics, or overtones, a chord is
          composed. --Busby.
      (f) The lowest place, position, or part. ``Deep to the
          roots of hell.'' --Milton. ``The roots of the
          mountains.'' --Southey.

   4. (Astrol.) The time which to reckon in making calculations.

            When a root is of a birth yknowe [known]. --Chaucer.

   {A["e]rial roots}. (Bot.)
      (a) Small roots emitted from the stem of a plant in the
          open air, which, attaching themselves to the bark of
          trees, etc., serve to support the plant.
      (b) Large roots growing from the stem, etc., which descend
          and establish themselves in the soil. See Illust. of
          {Mangrove}.

   {Multiple primary root} (Bot.), a name given to the numerous
      roots emitted from the radicle in many plants, as the
      squash.

   {Primary root} (Bot.), the central, first-formed, main root,
      from which the rootlets are given off.

   {Root and branch}, every part; wholly; completely; as, to
      destroy an error root and branch.

   {Root-and-branch men}, radical reformers; -- a designation
      applied to the English Independents (1641). See Citation
      under {Radical}, n., 2.

   {Root barnacle} (Zo["o]l.), one of the Rhizocephala.

   {Root hair} (Bot.), one of the slender, hairlike fibers found
      on the surface of fresh roots. They are prolongations of
      the superficial cells of the root into minute tubes.
      --Gray.

   {Root leaf} (Bot.), a radical leaf. See {Radical}, a., 3
      (b) .

   {Root louse} (Zo["o]l.), any plant louse, or aphid, which
      lives on the roots of plants, as the Phylloxera of the
      grapevine. See {Phylloxera}.

   {Root of an equation} (Alg.), that value which, substituted
      for the unknown quantity in an equation, satisfies the
      equation.

   {Root of a nail}
      (Anat.), the part of a nail which is covered by the skin.
              

   {Root of a tooth} (Anat.), the part of a tooth contained in
      the socket and consisting of one or more fangs.

   {Secondary roots} (Bot.), roots emitted from any part of the
      plant above the radicle.

   {To strike root}, {To take root}, to send forth roots; to
      become fixed in the earth, etc., by a root; hence, in
      general, to become planted, fixed, or established; to
      increase and spread; as, an opinion takes root. ``The
      bended twigs take root.'' --Milton.

Source : WordNet®

root hair
     n : thin hairlike outgrowth of an epidermal cell just behind the
         tip; absorbs nutrients from the soil