The Collaborative International Dictionary of English v.0.48:

Stirps \Stirps\, n.; pl. Stirpes. [L., stem, stock.]
   1. (Law) Stock; race; family. --Blackstone.
      [1913 Webster]

   2. (Bot.) A race, or a fixed and permanent variety.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

STIRPES, descents. The root, stem, or stock of a tree. Figuratively, it
signifies, in law, that person from whom a family is descended, and also the
kindred or family.
     2. It is chiefly used in estimating the several interests of the
different kindred, in the distribution of an intestate's estate. 2 Bl. Com.
517 and vide Descent; Line.