The Leucobryaceae have thick, whitish leaves that consist mostly of an expanded costa. The costa is differentiated into outer layers of enlarged, hyaline leucocysts and a central layer of smaller green chlorocysts. A somewhat similar costal modification is found in the Dicranaceae (Campylopus, Brothera, Paraleucobryum). Furthermore, the peristome of Leucobryum is identical to that of Dicranum. For these reasons the two families are sometimes merged. Robinson (1985, 1990) attributed basic structural and functional features to the leucobryaceous leaf and on this basis redelimited the family. The important functional aspect of the leucobryaceous leaf involves its ability to generate and hold air within the leucocysts. This is thought to increase the photosynthetic efficiency of the completely enclosed chlorocysts.