Source : Webster's Revised Unabridged Dictionary (1913)

Pollen \Pol"len\, n. [L. pollen fine flour, fine dust; cf. Gr.
   ?]
   1. Fine bran or flour. [Obs.] --Bailey.

   2. (Bot.) The fecundating dustlike cells of the anthers of
      flowers. See {Flower}, and Illust. of {Filament}.

   {Pollen grain} (Bot.), a particle or call of pollen.

   {Pollen mass}, a pollinium. --Gray.

   {Pollen sac}, a compartment of an anther containing pollen,
      -- usually there are four in each anther.

   {Pollen tube}, a slender tube which issues from the pollen
      grain on its contact with the stigma, which it penetrates,
      thus conveying, it is supposed, the fecundating matter of
      the grain to the ovule.

Source : WordNet®

pollen tube
     n : slender tubular outgrown from a pollen grain that penetrates
         the ovule and releases male gametes