Saturday, January 30, 2010

A lichen Lecanora dispersa

I am an amateur naturalist trying to discover everything living in my garden.

I have a manhole cover in my garden. (Try to contain your excitement at hearing this news dear reader!) This may seem an unpromising place to search for life, but a closer look reveals a host of those most enterprising (in terms of the habitats they are willing to conquer) of creatures - the lichens.

Zooming in, photo 1 (click on photos to enlarge) reveals at least two species: The yellow lichen (which looks like our old friend Caloplaca citrina - though I've not taken time to carefully check this) and a second species with numerous white, frosted fruits ('apothecia').

A little time spent with my trusty copy of Lichens (Frank Dobson, Richmond Publishing) and browsing some similar images on the internet, and I'm tolerably confident the latter is the lichen Lecanora dispersa.

There are at least 20 British species of Lecanora lichen. Dobson describes L.dispersa as 'very common and found even in the centres of cities'. It will grow on a variety of basic substrates including bark, stone, iron and leather.

Photo 3 shows a magnified view of the apothecia, with their white, wrinkled rims.

For photo 4 I took one apothecium and made a squash between a microscope slide and a coverslip. As I've discussed previously, lichens are a partnership between fungi and algae. The small green, algal cells are clearly seen in photo 4. That algae can be present throughout a lichen's cup-shaped apothecia was a new discovery for me - I'd hitherto only associated them with the non-fruit parts of a lichen.

The main purpose of a lichen's apothecia is to act as surfaces from which to liberate numerous reproductive spores. I've talked about this before, but I was not previously in a position to complement my descriptions with photographs. Recently however, I've been lucky enough to come into possession of a Cambridge rocking microtome. For those unfamiliar, this is a device for taking extremely thin (~few microns) slices of a specimen, which can then be viewed under a microscope. Actually things are not entirely straightforward, since, in order to avoid said slice ripping and disintegrating as it's cut, it's necessary to first provide support to the cellular structure of the sample by embedding the whole thing in a block of wax. The recipe for doing this is a little involved: Starting from pure water, you pass the sample through half-a- dozen water+alcohol baths. The baths are arranged so that the alcohol concentration steadily increases until finally your sample is left sitting in pure alcohol, the water in the sample's cells having, by then, been replaced with alcohol. Next, you substitute the alcohol for the solvent toluene (a solvent of wax) before dropping it into a bath of molten paraffin wax and leaving for several hours. Finally, you pour the wax+sample into a mould and leave to cool. (In fact, there are still futher steps that finesse this process - but you get the general idea! Should you wish to try it yourself the best guide I've found is the booklet Practical Microscopy (Eric Marson, Northern Biological Supplies) and/or you could join one of the hobbyist microscopical societies such as the redoubtable Postal Microscopical Society.)

Photos 5 and 6 show my rocking microtome. The blue handled blade is the extremely sharp sectioning razor. The white object is the wax-block-embedded sample. Pulling the lever at the rear of the microtome (photo 5) raises the sample and releasing (photo 6) forces it down onto the blade shaving off a wafer thin slice and simultaneously ratcheting the sample forward ready to take the next.

And the consequence of all this labor?... Photo 7 shows a cross section through a lichen apothecium. Strictly, for this posting it ought to be that of L.dispersa, but working on the principle 'walk before you can run!' I instead sectioned a sample of our old friend X.parietina from my apple tree, since the latter has large apoethecia and a robust surrounding thallus (= 'body'). I've labelled up photo 7 (click to enlarge) and you can clearly see that the apothecium surface (technically termed the hymenium) comprises an assemblage of sausage-shaped 'asci' packed full of spores.

One thing puzzles me: I have not seen the granular bodies I see here labelled up in the handful of books I have on lichen. Possibly these are air bubbles. They turned up very frequently in my samples however and I don't believe they are. Can anyone comment?

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