Whereas in most plant species the anatomy of the organism is identified with structures associated with a single vegetative body, the "lichen body" is more aptly described as a colony of cells that share a variety of associations with one another that vary from one species of lichen to the next. In some species of lichen, fungal and algal cells merely coexist. Coenogonium leprieurii, for example, is a lichen that lives in low-light tropical and subtropical forests in which the filamentous green algal partner (Trebouxia) is dominant.
In most lichen species, however, the relationship between the symbiotic partners is more intimate, with the lichen body appearing to be a single entity.
In these species the algal symbionthas no cell walls and is penetrated by filaments from the fungal symbiont called haustoria, which pass sugars from the algal cell to the fungal cell and may have a role in the transportation of water and nutrients from the fungal cell. This integration is so complete that many naturalists prior to the nineteenth century mistakenly classified lichens as mosses.
In most lichen species it is nevertheless possible, with a good magnifying device, to identify several distinct regions of the thallus or lichen body. The outermost region is the cortex, a compacted layer composed of short, thick hyphae (widely dilated filaments) of the fungal symbionts that protect the lichen from abiotic factors in its environment. These hyphae extend downward into a second region, the photobiont layer, where they surround the algal sym-bionts. Below this is a third region, the medulla, composed of a loosely woven network of hyphae.
Underneath this is a fourth region, the under-cortex, that is similar in appearance and structure to the cortex. The bottom of the lichen body is com posed of rhizines, rootlike structures composed of bundles of hyphae that attach the lichen to its substrate (the rock, bark, or other support on which it resides). This arrangement of regions into layers serves to prevent water loss. Many species can survive complete desiccation, coming back to life when water becomes available again. The cortex also contains pseudocyphellae, which are pores that allow for the exchange of gases necessary for photosynthesis.
Lichens typically live for ten years or more, and in some species the lichen body can survive for more than a hundred years. Reproduction in most fungal species proceeds by the development of a cup- or saucer-shaped fruiting body called an apo-thecium, which releases fungal spores to its surroundings. Procreation in lichens is more problematic in that the fungal offspring must also receive the right algal symbiont if they are to survive. The most common form of dispersion in lichen is by the accidental breaking off of small pieces of the thallus called isidia, which are then spread by wind to new substrates. In some species, small outgrowths of the thallus known as soralia arise, composed of both fungi and algae and surrounded by hyphae, to form soredia, which after dispersion give rise to a new thallus.
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