I am an amatuer naturalist trying to learn something about everything living in my garden.
Photo 1 shows a lichen growing, amongst a number of similar patches, on a wooden bird table in my garden.
I am very far from being an expert on lichen identification but in the course of doing this blog over several years I have picked up a few tricks. One is to examine your lichen through a hand lens for any characteristic surface lumps on bumps. Some lichen species become decorated with powdery granules called soralia. Others with tiny sausage shaped protuberances called isidia. For my lichen, the latter are present in abundance as can be seen in the close-up Photo 2 (click on photos to enlarge).
Another trick to help with lichen identification is to check the under surface. In photo 2 I have peeled back a small section to reveal a black underside with a covering of tiny, root-like hairs. In the jargon, these are known as rhizines. Not all lichens have them. They are not roots in the traditional sense, since they do not function to suck-up water as do the roots of plants. Rather their job seems to be to help anchor lichens to surfaces.
Armed with the features above and my trusty copy of Lichens (Dobson, Richmond Publishing) I'm confident to identify my lichen as Melanelia (Parmelia) subaurifera. The book suggests a final test: gently rubbing the surface should leave a pale yellow-white abrasion. Photo 4 shows this.
I am fond of lichens and so was very pleased when someone recently made me a present of the new edition of Lichen Biology (Ed. Thomas H Nash III, publ. Cambridge). This book is primarily aimed at professionals and I don't pretend to have followed some of the very detailed sections on e.g. lichen biochemistry, but I was able to follow others and came away with a new respect for the intricacy with which nature adapts these little creatures to their environment. Take for example the construction of the little air-filled spaces often found inside the bodies of lichens:
Under a microscope a lichen is revealed to be a mass of long, spaghetti-like fungal cells ('hyphae'), mixed-through with a sprinkling of green, single-celled algae or sometimes cyanobacteria. (The fungus carries out various tasks such as water storage. The algae or cyanbacteria do what no fungus can: photosynthesise food from sunlight). In considering this description however it would be wrong to picture things as a random tangle of fungal threads and algal cells. Photo 5 shows a lichen cross section I made and discussed some time ago (here) and reveals that things are far from random. Of particular relevance for today's posting is the layer known as the medulla that contains a lot of air-filled voids (see the region of the box in photo 5 for example). What is the purpose of these voids? The answer of course is that algae, like all plants, 'feed' (via photosynthesis) on a diet of sunlight and gaseous carbon dioxide. This is the key to understanding the voids: they are present to allow the flow of gaseous CO2 gas to the algae.
So far so good, but possibly it might occur to you to wonder what happens when it rains?! Do these voids fill up with water and in so-doing stifle CO2 flow, and hence photosynthesis, in the lichen? As I learnt from the book above Nature, of course, has an answer. In one chapter, a remarkable photograph taken by Rosmarie Honeggar with an electron microscope reveals how the fungal threads in the medulla carefully coat themselves and their precious cargo of algal cells in a minuscule layer of water-repellent proteins. This remarkable water repellent 'jacket' prevents the medulla from becoming saturated with water and so maintains the algae in a gaseous environment conducive to photosynthesis.
The water-repelling proteins the fungal hyphae secrete are known as hydrophobins and their discussion would make a lengthy posting in its own right. They were unknown to science until the 1990's when they were discovered by Wessel and co-workers in a fungus called Schizophyllum commune. Since their discovery these remarkable molecules have turned out to be widespread amongst fungi, with fungi using them in a variety of ingenious ways to 'manipulate' the surface tension of watery environments. For example, in order to help their spores get airborne, some plant-infecting fungi coat their spores in hydrophobins so as to prevent them becoming trapped or stuck together by thin films of water. Hydrophobins also help the infectious spores stick to the waxy, water-repellent leaves of targeted host plants. You can find a short introduction and further references on hydrophobins in lichens here ( P.S. Dyer, New Phytologist (2002) 154 : 1–4).
So there you have it! Minuscule, air-filled voids in a wafer-thin lichen...but take a closer look and as so often in natural history, one finds a hitherto unimagined world of subtlety and sophistication.