Two Verrucaria lichens

I am an amateur naturalist attempting to identify all the life in my garden.

February is seeing Oxford hit by a succession of clear, frosty days. The cold weather means that many of my garden's plants, fungi and animals are tucked away out of sight or sitting out the winter in a dormant state. As I've remarked before, this is a good time for the amateur to take an interest in those humble lifeforms the mosses and lichens which, far from lying dormant, seem to positively relish the winter and the lack of competition it affords. My garden wall for example, is currently layered with a vibrant mat of fruiting mosses.
Photo 1 shows a closeup of two more denizens of my wall's stonework. For the uninitiated, no, this is not a mistake! What appears from a distance to be little than a collection of stains on the stonework is in fact my blog's first example of a crustose (=flat, crustlike - see here for more explanation) lichen. In fact there are two in photo 1. Photo 2 shows a zoomed-out image of the same region. Personally, I find it little short of miraculous to see life thriving on bare rock.

Photo 3 shows a 40x magnified view of the tiny black dots peppering the surface of the uppermost lichen in photo 1. These are perithecia - tiny flask shaped 'salt cellars' from which the lichen shoots out spores.

When it comes to species, as always, my identifications come with a health warning: I'm far from expert in identifying lichens and furthermore, without going to the lengths of studying spores under the microscope and applying chemical tests (lichenologists take these seriously: I even have a book - Microchemical Methods for the Identification of Lichens, Orange, James and White - that teaches the amateur how to perform chromatography!), it can be well-nigh impossible to definitively identify some lichens by visual appearance alone. With this disclaimer, let me say that my lichen's fruit bodies being set in little craters in the surface, and the black line delineating the thallus perimeter in photo 1, are both features that fit with the description of Verrucaria baldensis in my copy of Lichens:An Illustrated Guide to the British and Irish Species (F.S.Dobson).

A similar analysis for the lower, 'less spotty' (though you'll still see some perithecia if you look closely) lichen in photo 1 inclines me to an identification as Verrucaria muralis.

The Verrucaria genus of lichens is noteworthy for including a number of freshwater and maritime lichens. The next time you're at the seashore, look towards the high-tide line and in all likelihood you find the rocks everywhere stained black. This isn't oil (as is sometimes thought) but a result of the ubiquitous seashore lichen V.maura.

Amongst the many extreme features of the life of a crustose lichen is the extraordinarily slow growth of some. According to my copy of Lichens (O. Gilbert, New Naturalist series) lichens are amongst the oldest and slowest growing organisms in the world: Rhizocarpon geographicum, for example, puts on a mere 0.09mm of growth-radius per year. A patch of the lichen Aspicilia calcerea, growing on the Rollright Stones, a stone circle in Oxfordshire, is estimated to have begun life there in 1195AD. It's thought some Icelandic lichens might be 9000 years old! I'll have nothing quite so extreme in my garden, but I am curious to track the growth rates of at least a few of the crustose patches on my garden wall. Watch this space!