The Collaborative International Dictionary of English v.0.48:

Grass tree \Grass" tree"\ (Bot.)
   (a) An Australian plant of the genus Xanthorrh[oe]a, having
       a thick trunk crowned with a dense tuft of pendulous,
       grasslike leaves, from the center of which arises a long
       stem, bearing at its summit a dense flower spike looking
       somewhat like a large cat-tail. These plants are often
       called "blackboys" from the large trunks denuded and
       blackened by fire. They yield two kinds of fragrant
       resin, called Botany-bay gum, and Gum Acaroides.
   (b) A similar Australian plant (Kingia australis).
       [1913 Webster]