Ontogeny and morphogenesis of the fruticose lichen Usnea florida (L.) Weber ex F.H. Wigg

The development of the fruticose lichen Usnea florida was described in accordance with the concept of a discrete approach to the description of higher plant ontogeny proposed by T.A. Rabotnov and A.A. Uranov. The ontogeny of the U. florida thallus, represented by 4 periods and 11 ontogenetic states, includes the development of fungal hyphae from a spore, its association with alga into a primary thallus, thallus morphogenesis (the life form formation and destruction of the thallus), and all related morphological changes in the apothecia and other structures generated by the thallus. Based on an analysis of morphogenetic processes during thallus formation, we proposed morphological criteria for the separation of ontogenetic states. The formation of the main frame of the thallus and its intensive branching occur before the development of the apothecia because of the switching to the reproductive function. Thallus aging and destruction start even before fruit bodies formation (v₂ state); later, these processes become more intensive and reach a maximum at the g₃ and ss states. Changes in thallus morphogenesis can include both delay and acceleration of morphogenetic processes, representing the manifestation of adaptation mechanisms in response to different environmental factors.