Asterostroma cervicolor

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Asterostroma is a corticioid genus and corticioid fruitbodies are flat and with very little in the way of surface features so, in most cases (though not all), identifying even the genus is impossible unless a specimen is examined microscopically. It is under the microscope that you are able to see the great variety of structures possessed by corticioid fungi. Some people may be curious as to what microstructures are present and the aim of this sighting is to show one particularly striking microstructure. The first photo shows a macroscopic view of a fruitbody that was growing on well-rotted wood, which was already breaking up even before I collected this specimen. The paired photos show some microscopic detail. On the left you see a mass of brown, star-like structures (asterosetae) and on the right you a closer view of several. An asteroseta consists of a stem atop which there are a number of sharply-pointed, radiating arms (which need not all be in the same plane). In this specimen the large asterosetae had a span of almost a tenth of a millimetre. If you have a corticioid fungus with asterosetae you have either the genus Asterostroma (fruitbody with a smooth surface) or Asterodon (surface covered by numerous short teeth). Another feature of most Asterostroma species is that the colourless spores are amyloid. This means that the spores turn blue when put into Melzer’s reagent (a particular iodine-containing solution). These photos were taken of a small sample mounted in Melzer’s reagent and the numerous small blue blobs are spores. In Asterostroma cervicolor the spores bear a small number of short, rounded knobs but to see those clearly I would have needed to go to a higher magnification. The scattered, dark-rimmed circles in the left hand photo are air bubbles.