The Collaborative International Dictionary of English v.0.48:

Plani- \Plan"i-\, Plano- \Plan"o-\ [L. planus. See Plane, a.]
   Combining forms signifying flat, level, plane; as
   planifolious, planimetry, plano-concave.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plano- \Pla"no-\
   See Plani-.
   [1913 Webster]