The Collaborative International Dictionary of English v.0.48:

Thermotaxis \Ther`mo*tax"is\, n. [NL.; thermo- + Gr. ? an
   arranging.] (Physiol.)
   (a) The property possessed by protoplasm of moving under the
       influence of heat.
   (b) Determination of the direction of locomotion by heat.
       [Webster 1913 Suppl.]