The most important ecological factor for mosses is thought to be correlation of leaf costa length with moisture. The present work offers the results of ten years' study of this phenomenon in members of the Bryidae from the central parts of the Podillian upland (227 species). Seven categories of leaf costa length are proposed: A, costa short or none; B, costa about half the leaf length; C, costa ending some distance below the leaf apex; D, costa ending at the leaf apex; E, costa excurrent; F, costa excurrent into the awn for about half the leaf length; G, costa excurrent into the awn for more than one leaf length. The percentage incidence of these categories in different ecological groups varies as follows: hydrophytes (1 species) 0, 0, 0, 100, 0, 0, 0; hygrohydrophytes (7) 0, 57.1, 28.6, 14.3, 0, 0, 0; hygrophytes (17) 11.8, 35.3, 23.5, 29.4, 0, 0, 0; mesohygrophytes (17) 5.9, 11.8, 47.1, 35.3, 0, 0, 0; hygromesophytes (15) 0, 0, 53.3, 20.0, 20.0, 6.7, 0; mesophytes (86) 25.6, 13.9, 11.6, 37.2, 10.5, 1.2, 0; xeromesophytes (29) 10.3, 10.3, 10.3, 41.4, 24.1, 3.5, 0; mesoxerophytes (37) 2.7, 5.4, 16.2, 18.9, 37.8, 13.5, 5.4; xerophytes (18) 5.6, 5.6, 5.6, 27.8, 22.2, 33.3, 0. The most numerous pattern for hygrohydrophytes and hygrophytes is B; for mesohygrophytes and hygromesophytes, C; for mesophytes and xeromesophytes, D; for mesoxerophytes, E; xerophytes, F. In general leaf costa length increases as the xerophytic condition is approached, indicating a distinctly adaptive function of this feature.