megistothermBot.|mɪˈgɪstəʊθɜːm| [f.Gr. µέγιστο-ς, superl. of µέγας great + θέρµη heat.] A plant requiring a very high temperature for growth. Cf. megatherm, meiotherm. 1879Stormonth Man.Sci. Terms, Megistotherms, plants requiring extreme or a very high degree of heat.