Synopsis top ↑
The species of the pantropical genus Leucophanes are
characterized by a whitish-green appearance, mainly corticolous,
turf-forming habit; multistratose structure of the leaf; production of
leaf-tip gemmae, and short branching systems. A definition of leaf
structure is based primarily on comparative morphology of young and
adult leaves, perigonial bracts, and scale leaves of Leucophanes and
those of the related genera Exodictyon, Exostratum,
Arthrocormus, and Syrrhopodon. The leaf is a complex organ
composed of a costa (with stereids, guide cells and hyalocysts) and a
multistratose lamina. The multistratose lamina is considered to be
composed of two sections, a basal unistratose, hyaline area and a
multistratose section with a central layer of quadrangular
chlorophyllous cells (living photosynthetic elements), the
chlorocysts, arranged in a net-like pattern and surrounded, on each
surface, by one or more layers of porose hyaline cells (devoid of
protoplasm at maturity), the hyalocysts. Growth-habit and structural
features displayed by the gametophytes of Leucophanes are
considered adaptations for water retention. It is most likely that the
life-form characteristic of the genus developed in relation to
environmental stresses imposed by the corticolous habit in seasonally
dry tropical forests.
Three subgenera, twelve species and six subspecies are recognized. Important characters in defining these taxa include leaf apex, shape and arrangement of chlorocysts as observed in surface view, size and shape of hyalocysts, composition of the multistratose lamina as seen in cross-sections (layers of hyalocysts, stereids and guide cells), pore size on the transverse walls, sexual condition, perichaetial leaves, and peristome and spore ornamentation.
Leucophanes is distributed in the tropical and subtropical areas of the world, particularly in central Africa, Malesia, Southern Japan, Australia, the islands of the tropical Pacific and the Neotropics. There are two centers of species diversity, one in Africa and the other in Malesia and adjacent areas. Of these, Africa has the highest number of endemics with five species, while Malesia has two. The species are able to grow on both acidic and basic substrata. An active radiation to littoral forests appears to have taken place in some species, e.g., L. glaucum and L. octoblepharioides, while others appear to be strictly elements of inland tropical rain forests.
The genus is hypothesized to have originated in Laurasia (Southeast Asia) as early as the Lower to Upper Cretaceous. Its present day distribution reflects not only its paleohistory but also its efficient dispersal capabilities.