Monday, 31 January 2011

All you have to do is look

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
Notice the shape of the lichen: it is much wider than it is tall, and for a while I wondered why this characteristic shape is the norm for so many of our crustose lichens. The answer is quite simple: in percentage terms, a tree expands in girth much more quickly than it expands in height. The lichen grows at a common rate (perhaps a millimetre per year) in all directions from its starting point, but the bark of the tree expands more quickly horizontally than it does vertically. Over the years the lichen takes on this characteristic shape in response to the growth of the tree.

Here's another example:

Thallus of the lichen Pertusaria hymenea
I have actually been photographing the above specimen for perhaps 8 years now, and it's beginning to show its age: The thallus has been eaten away in a number of areas, most likely by slugs or snails. The bark of the tree has also split on the left, and this has almost split the original lichen into multiple specimens. Finally, Ivy has been growing on the tree and some of its tendrils have been pulled away, taking some of the thallus with them (you can see traces of the Ivy tendrils at the lower edge of the lichen, to the left of centre, looking like a fossil, and in the pale line to the right of the image).

(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)


The Weaver of Grass said...

I think plants are so much cleverer than we imagine Stuart. I read somewhere the other day that
amoeba farmed in a simple way (I have probably used the word 'plant' in the completely wrong place - we greenhorns know so little about it really.

Caroline Gill said...

Thank you, for this, Stuart. We went on a winter walk some weeks ago, hoping to see birds. There were very few about, so I ended up observing Lichens, but knowing practically nothing about them. This has been a great introduction, and I shall view them with more interest from now on!

Stuart said...

Lichens are actually crucial to the welfare of virtually everything on the planet: they are our pioneer species, converting rock and wood into soil so that other plants and animals can gain a foothold. They are also food for micromoths, molluscs, larger moths and some parasitic fungi. It's amazing how many food chains start or end with a fungus.