The Collaborative International Dictionary of English v.0.48:

Cow tree \Cow" tree`\ (kou" tr?`). [Cf. SP. palo de vaca.]
   (Bot.)
   A tree (Galactodendron utile or Brosimum Galactodendron)
   of South America, which yields, on incision, a nourishing
   fluid, resembling milk.
   [1913 Webster]