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Classification

Microscopic structures

We will now look at some of the important microscopic structures.

Asci and basidia

You will have noted that the assignment of a fungus to either the Ascomycota or Basidiomycota makes no use of the features of the asci or basidia - merely the presence of one or the other. However, both asci and basidia show considerable variation in features such as size, shape, internal structure, number of spores produced or reactions with chemicals. All of these features are used in the further classification of ascomycetes and basidiomycetes. Different ascal and basidial features are used to help define categories from classes down to species.

Asci may be single-walled or double-walled; they may open to release spores or they may remain closed and not release the spores; in ascomycetes where the asci release the spores, each ascus may either rupture at the apex or have an apical lid that opens to allow the spores out. There is in fact considerable variation in the structure of the apex, best seen with an electron microscope, and this information is used in high level ascomycete classification.

Tremella fuciformis

Auricularia sp.

In most cases the basidia are elongated and club-like, though there is variation in shape. Commonly, each basidium has four projections and four spores - but some species may have just one projection and spore per basidium and others up to eight. In most basidiomycetes the basidia have no dividing walls (or septa) but a small number of genera have septate basidia. Two examples are Tremella <<one of 081 and 082>> and Auricularia <<079>>. The Tremella basidium has two septa (at right angles to one another) aligned along the long axis of the basidium. In Auricularia each basidium has septa at right angles to the long axis of the basidium.

DRAWINGS - TREMELLA AND AURICULARIA BASIDIA

Hyphae

The detailed structure of the wall (or septum) between neighbouring hyphal cells is an important high level classificatory feature. This wall is not solid, but has some form of opening, and there's a small number of septum types. The nature of the septum is used to help define the high level fungal groups, especially within the basidiomycetes. However, septal structure is not used in routine identification, since it can be seen only with an electron microscope. In some fungi the opening in the septum is just a simple hole, but this three-dimensional cutaway diagram <<[http://www.apsnet.org/education/IllustratedGlossary/PhotosA-D/doliporeseptum.htm]>> shows the complex type found in many basidiomycetes. This two-dimensional electron microscope image <<[http://www.bsu.edu/classes/ruch/msa/blackwell/9-33.jpg]>> shows the same structure. Note the central doughnut-like opening that is surrounded by a perforated, spherical shell.

While the septal structure cannot be seen with a light microscope, such a microscope does show that hyphae are not uniform in appearance. They may be narrow or broad, with thin or thick walls, coloured or colourless, smooth or encrusted, straight or contorted, clear or with oily contents. Unlike the septal structure, these features are not used for high level classification, but are used to help define genera or species. The nature of the hyphae is particularly important in the classification and routine identification of polypores. This is explained in the {{HYPHAE IN POLYPORES SECTION}}.

A FEW HYPHAL DRAWINGS

Jafneadelphus ferrugineus

In some fungi the hyphae throughout the fruiting body are much the same in appearance, but usually there are some differences through the fruiting body. For example, in many mushrooms the composition of the skin of the cap is markedly different to that of the flesh in the cap. In the corticioid fungi, the hyphae near the wood substrate are often different to those near the basidia. In ascomycetes with cup or disk-like fruiting bodies (such as Jafneadelphus ferrugineus <<??105>>) the spore-bearing asci are usually confined to a relatively small layer near the upper surface of the cup-like fruiting bodies. There is some variety in the tissue structure of the remainder of the fruiting body and that variety is a very useful feature in both the classification and routine identification of ascomycetes.

Spores

The {{SPORE SECTION}} illustrates a number of types of spores. Given the variation in form shown there, you should not be surprised to learn that spores play an important role in fungal classification. While fruiting body form of a particular species can sometimes vary in response to environmental conditions, spores are a stable microscopic feature and this helps explain their importance in both classification and identification. As with asci and basidia, different spore features are used at different levels in the classification hierarchy.

Routine spore examination requires a compound microscope allowing magnification up to a thousand times. Electron microscopes are used to show very fine detail in spore structure and so help in classification work. Spores have several ways of germinating and this information has also proven useful in understanding fungal relationships.

The colour of a spore print [SPORE PRINT SECTION] is a macroscopic feature that is used in fungal identification. A spore print shows a mass of spores. It is important to note that spore colour as seen through a compound microscope will usually be paler than the colour of the mass of spores in a spore print. For example, spores that are white in a spore print will usually be colourless under the microscope. Looking at a spore print, you are using light that falls onto the spores and is reflected back to your eyes. Under the microscope you are using light that passes through the spores. Similarly, you see differences in a piece of coloured cellophane, depending on whether the cellophane is lying on a table or held up in front of a light.

Other microscopic structures

Scutellinia sp. (click to enlarge and see hairs)

We will now look at three other microscopic structures. You should remember that all of these are hyphal in origin - but sometimes they have become very highly modified hyphae.

The photo of Scutellinia <<104>> shows a number of hairs around the margin of the orange disk. While it’s easy to see the hairs with the naked eye (or magnifying glass), you need a microscope to see that the hairs are thick-walled with forked bases. Numerous fungi have hairs in various parts of the fruiting bodies and the micro-structure of these hairs is a useful identification feature, for there is a much variety in hair structure.

DIAGRAM OF SCUTELLINIA HAIR

In mushrooms, the basidia stick out from the gill surfaces. Many mushrooms also have protruding cystidia, interspersed with the basidia. Cystidia are also single-celled structures, of varying shapes and sizes but with largely unknown functions.

A COUPLE OF MUSHROOM CYSTIDIAL DIAGRAMS (AND LINKS TO SPOROCARPS??)

In most cases the cystidia finish within the air space between the gills. In a number of Coprinus species there are very large, club-like cystidia that reach from one gill to the next, as shown schematically in the accompanying diagram. It’s very similar to a diagram early in the ASCOMYCETES AND BASIDIOMYCETES SECTION, the only addition being the two large cystidia (coloured blue for ease of recognition) that extend from one gill to another. At one stage the commonly accepted idea was that the large Coprinus cystidia helped keep the gills apart, so acting like beams under compression. However, it’s now more likely that the cystidia are under tension and act like struts, holding the gills together during expansion of the mushroom cap.

<<Reference: Moore, D. Fungal morphogenesis. Cambridge University Press, 1998. See the discussion starting on page 322. >>

DIAGRAM – coprinus cystidiumg

Incidentally, if you’d like to know why the Coprinus spores are shown in several colours, see {{ INKCAP MUSHROOMS in the BALLISTOSPORE SECTION}}.

You can also find cystidia on the caps and stems of many mushrooms and on various other types of basidiomycete fruiting bodies. The corticioid fungi in particular show great variety of cystidial forms and cystidia are very useful in the identification of these fungi. These drawings show a few examples of cystidia in corticioid fungi.

CORTICIOID CYSTIDIA DRAWINGS

Numerous ascomycetes have paraphyses between the asci, as indicated by the accompanying diagram, where the spore-filled asci are represented by the blue-dotted yellow areas. There’s more about this diagram (and ascomycete structure) in the ASCOMYCETES AND BASIDIOMYCETES SECTION. The paraphyses may simply be unmodified hyphae - but often they are highly modified, as shown by these drawings of paraphyses from several ascomycetes.

DIAGRAM – apothecium2g

DRAWINGS OF PARAPHYSES – AND LINKS TO SPOROCARPS