Lichens are always described as a fungus and one or more algae (or cyanobacteria) living in symbiosis. The fungus provides structure and anchoring for the partnership and the algae transform sunlight into food via photosynthesis. I'm not so convinced that the partnership is quite as symbiotic as we're led to believe: in a symbiotic relationship, the partners cooperate to mutually benefit each other, but I think the fungus has the upper hand in a lichen for a number of reasons:
1) The methods of reproduction are all either purely fungal (via fungal fruitbodies that produce fungal spores), or are controlled by the fungal partner (broken or ejected fragments of lichen contain the fungus with a convenient bundle of algae already confined, ready for the next specimen).
2) The algae that are found in lichens can happily exist independently of the partnership: the fungal component cannot.
3) The fungal partner benefits from the food produced via photosynthesis which it cannot perform by itself, but what does the alga receive in return, apart from a place to live?
4) The alga can only reproduce within the confines of the lichen body: as the lichen grows, there is more space for the alga to grow.
I rather think the partnership is heavily weighted in favour of the fungal partner.
This is a specimen of Graphis scripta (so-called because the spore-producing fruitbodies closely resemble handwriting):
|Thallus of the lichen Graphis scripta|
Here's another example:
|Thallus of the lichen Pertusaria hymenea|
(The dark blotches to the upper right and lower centre are the liverwort Frullania dilatata.)
I took a close-up of a much younger specimen of the Pertusaria, and you can see that it is much more rounded at this stage, but the vertical cracks that will spread it horizontally have started to appear:
I suppose the spaces get filled in with new thallus body as the lichen grows. So here we have a fungus that not only knows how to garden (it cultivates the alga), but has also learned a bit of DIY to plaster over the cracks.
(Click here for an image of the Pertusaria from 2004)