Though some seem to regard the bloom of yellow Xanthoria parietina which seems to adorn so many of our trees now as something of a weed, I have to confess that I rather like the brightness it brings to bare tree trunks during winter, when we are desperate for colour.  Looking at it growing on elder trees along the river Wear in Durham yesterday, I was struck that its distribution is not uniform.

It is clear that the lichen colonies start to grow mainly at the points where new branches emerge.  Not surprising, I suppose; although I talked, in an earlier post (The wonderful world of lichens) about why these organisms make such efficient pioneer species, spores or vegetative propagules still need to alight somewhere suitable to grow.

Xanthoria parietina growing on elder trees on the banks of the River Wear

Most lichens can reproduce either sexually, by producing spores, or vegetatively by means of propagules produced in special structures called soralia or isidia. In the case of X. parientia, reproduction is mostly sexual.  The cup-like fruiting bodies visible more clearly in the picture below are the apothecia responsible for the production of sexual spores or asci.

Developing apothecia on the surface of X. parietina – the mature discs become orange in bright light

It’s worth mentioning at this point, perhaps, that lichens are named for their fungal partner; a mutualistic association cannot be named under the Botanic Code.  The majority of lichens in the UK, including X. parietina, have an Ascomycete fungal partner hence their production of spores in the sac-like asci seen above.  The ascospores produced in the asci contain only the fungal partner in the relationship so, once they germinate, they need to obtain an appropriate algal partner quickly to survive. What more likely place on a tree to find one than at the junction of two branches, where moisture is likely to be retained?