The Collaborative International Dictionary of English v.0.48:

Basal \Ba"sal\, a.
   Relating to, or forming, the base.
   [1913 Webster]

   Basal cleavage. See under Cleavage.

   Basal plane (Crystallog.), a plane parallel to the lateral
      or horizontal axis.
      [1913 Webster]

WordNet (r) 3.0 (2006):

basal
    adj 1: especially of leaves; located at the base of a plant or
           stem; especially arising directly from the root or
           rootstock or a root-like stem; "basal placentation";
           "radical leaves" [syn: radical, basal] [ant:
           cauline]
    2: serving as or forming a base; "the painter applied a base
       coat followed by two finishing coats" [syn: basal, base]
    3: of primary importance [syn: basal, primary]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "basal":
   ab ovo, aboriginal, basic, basilar, beginning, bottom, bottommost,
   central, constituent, constitutive, crucial, elemental, elementary,
   embryonic, essential, foundational, fundamental, generative,
   genetic, germinal, gut, in embryo, in ovo, lowermost, lowest,
   material, nethermost, of the essence, original, pregnant, primal,
   primary, primeval, primitive, primordial, pristine, protogenic,
   radical, rudimental, rudimentary, seminal, substantial,
   substantive, underlying, undermost