Lateral growth patterns in the Cladoniaceae

Lateral growth from the apex of vertical structures is widespread in cladon iiform lichens. In the family Cladoniaceae, it is accomplished through a de velopmental shift in the meristem, in which growth orientation changes from isotropous to anisotropous. En anisotropous development, the more or less constant relationship among the axial, radial, and circumferential planes o f growth is altered during ontogeny. The result is pronounced lateral elong ation of the apical meristem, a departure from the isotropous body plan of early ontogeny. Development that favors radial and circumferential growth o ver axial growth is an innovation that provides ontogenetic flexibility but which also entails the loss of control from a single centralized meristem to one or more meristems. A shift from the constraints of symmetry to the r isks and potential of asymmetry and a subsequent diversity of heritable tha llus forms reflect evolutionary processes in the Cladoniaceae. Similar morp hogenetic activities, which are apparently highly conserved, are shared by species that are presumably only distantly related.