Symbiosis is now widely accepted as a source of evolutionary innovation (Margulis and Fester 1991) that has  stimulated an enormous morphological radiation in ascomycetes. Vegetative structures have especially   developed to a complexity that is not reached elsewhere in the fungal kingdom (Honegger 1991b). Lichen  morphology and anatomy are now understood as being highly adapted to constraints imposed by the   environment on themutualistic symbiosis,where the mycobiont is the exhabitant and the cyanobacterial or   green-algal photobiont is the inhabitant (Hawksworth 1988b). A very wide range of different thallus structures have been described and a complete outline of lichen morphology is not the scope of this chapter. However,  detailed reviews are given by Henssen and Jahns (1973) and Jahns (1988). Common mycological terms also  used in lichenology are not always explained here. Readers are referred to recent mycological textbooks, to Hawksworth et al. (1983), or to a glossary of a recent lichen flora. Irrespective of lichen growth form, it must function as a photosynthetically active unit in a manner that allows positive net photosynthesis and  subsequently sufficient growth rates. This implies that the photobiont has to be supplied with just the right  amount of light, even in the deep shade of rain forests or under fully exposed conditions of deserts. Carbon  dioxide (CO2) diffusion to the photobiont needs to occur readily, even under fully hydrated conditions.Water  loss should be adapted to the specific environment: minimized in dry environments, and maximized in very wet environments. Thereby optimal CO2 gain may be realized.