Rheophytic mosses : their morphological, physiological, and ecological adaptation.

Abstract

The word "rheophytic mosses" applies to species which mainly, but not exclusively, inhabit the periodically flooded zones of rivers and streams. Though van Steenis did not refer to rheophytic mosses intensively, more than 130 moss species belonging to 31 families and 62 genera have been reported worldwide by various authors. The adaptive features of the morphology and physiology of rheophytic mosses are discussed in this article, especially their preference in habitat. Rheophytic mosses have certain morphologies that appear adaptive for a rheophytic environment, some of which differ from those of rheophytes of vascular plants. For example, mosses almost never have linear leaves, which are characteristic of the latter. In contrast, shoots of rheophytic mosses are terete or complanate and appear adapted to lessen water resistance. In addition, leaves of rheophytic mosses have obtuse apices, which causes the terete appearance of the shoots in wet conditions. The obtuse leaf apices might be related to the laminar cells of the leaf apex being shorter than those of the median part of the leaves. Another characteristic is the multi-layered leaf margins, particularly found in members of the Neckeraceae. They would give the leaves mechanical strength against damage caused by water. Since rheophytic mosses do not have thin-walled laminal cells that are characteristic of aquatic mosses, we concluded that rheophytic mosses are derived not from species inhabiting stable water but directly from the terrestrial species. We categorized the rheophytic mosses into three groups, each of which is inferred to have been derived from different "mother" groups of land species. Members of the three groups are as follows ; (1) epiphytic species totally restricted to the upper rheophytic zones where prolonged dessication occurs repeatedly ; (2) terrestrial species growing in permanently wet, lower rheophytic zones ; (3) saxicolous species inhabiting open and mostly dry places, such as on boulders in sunny riverbeds. The members of the first group are derived from land species growing on branches or trunks of shrubs and trees in forests (e.g., Cryphaeaceae, Brachytheciaceae); the second from species of forest floors (e.g., Neckeraceae, Thamnobryaceae); and the last from species inhabiting much drier places (Pottiaceae, Grimmiaceae). Since these three groups have different characteristics in morphological adaptation to rheophtytic environments, they should be treated separately whenever we study their speciation from land species.