microscopic Characters

Flesh characters

Homoiomerous v. heteromerous

Most fungal flesh is composed entirely of cells that form filamentous hyphae and is called a homoiomerous trama. The flesh of Russula and Lactarius however, contains rounded elements called sphaerocysts as well as filamentous hyphae – this is called a heteromerous trama.

Gill tramas

Filamentous hyphae are aligned into four basic patterns in the central part of the gills (photos below). These can be seen in thin sections of the gill ie the gill trama, and are a great help in identifying the genus when used in combination with the spore colour, gill attachment and the type of cap cuticle (see below for cap cuticle):

• • • Regular eg Inocybe, Agaricus, Psathyrella, Entoloma, Lepiota

    • Irregular eg Omphalina

    • Divergent = bilateral eg Amanita. Boletales, Gomphidius, Paxillus

    • Inverse eg Pluteus, Volvariella

Inevitably there are intermediate forms particularly between regular and irregular gill trama. This can be clearly seen in some species of Hygrocybe e.g. H. quieta.

Gill tramas are often best observed at a lower magnification e.g. x 100.

Clamp connections

Hyphae are composed of many individual cells. Often these are simple and the cross wall shows as a line across the hypha; these divisions are called septa. Sometimes however, clamp connections are present, appearing as a bulge near the septum, with the septum itself apparently ‘bent’. Clamps occur in many different situations are often difficult to find and you need to check several septa before concluding that they are absent. Check scalps and gill tramas of Inocybe species to practice locating some.

Cap cuticle

As mentioned above in ‘Structures in the flesh’, the cap cuticle can be useful in determining the genus (and occasionally species) of some fungi. There are two basic cuticle structures that you will be able to see by taking a scalp from the toadstool (see ‘Fungal microscopy techniques’ below):

• • Cellular – where the cap cuticle is composed of more or less spherical elements, arranged in a palisade across the cap e.g. Conocybe, Coprinus, Panaeolus, Psathyrella, Bolbitius

  • Filamentous – where the cap cuticle is composed of horizontally arranged filamentous hyphae e.g. Galerina, Cortinarius, Stropharia, Agaricus

As with the gill tramas there are some fungi that present intermediate forms particularly in the genus Pluteus. These structures are often best observed at a lower magnification e.g. x 100.

There are other characters within the cap cuticle that can be viewed by taking a section rather than a scalp. The scalp presents a ‘bird’s eye view’ of the structures, a section gives you a ‘side on view’.

Don’t forget that other features such as cap cystidia, veil remnants and scales might also require close examination.

Spores and basidia

Agaric and bolete spores are produced on special cells called basidia, which cover the gill (or tube) face and sometimes edge. In most genera each basidium carries 4 spores although occasionally some species will have 2 or 3 (eg some Laccaria and Entoloma species) and very occasionally (eg some Sistotrema species) 6 or 8. Each spore is raised on a projection called a sterigma (plural sterigmata) and it is usually necessary to focus up and down on the basidia to check how many spores are present. The apiculus is the name given to the short projection at one end of the spore where it was attached to the sterigma.

Spores are a very useful aid to identification. Their colour can be seen without the use of a microscope but other features can only be seen with a microscope. A magnification of at least x 400 is recommended for looking at spores.

Spore features:

• • Size – a calibrated graticule in one of the eyepieces is necessary to measure spores. It is usual to measure both the length and the width of a spore using micrometres or microns (ie 1000^th of a millimetre!) often written μ.

  • Shape – spores vary in shape from globose to ellipsoid, from limoniform (lemon shaped) to reniform (kidney shaped) or lacrymoid (tear shaped), they can be strangulated in the centre or shaped like a bullet. Most good field guides include illustrations of different spore shapes; Fungi of Switerland (Breitenbach and Kranzlin, Vols 1 – 6) have good illustrations of spore shape alongside each included species.

  • Ornamentation – range from spines to warts of various sizes as well as networks of ridges. Sometimes these stain in Melzer’s. It should be noted that many spores are completely smooth.

  • Guttules – the presence of one or more droplets within the spore

  • Loosening perispore and spore walls – in some species the outer wall of the spore (perispore) loosens and detatches itself giving the spore a characteristic outline (Galerina calyptrata, some Coprinus species). Other spores may be thick walled.

  • Germ pores – present or absent. This is an area where the wall of the spore shows a distinct thinning and where germinating hyphae will emerge. This is usually at the end of the spore opposite the apiculus (the apiculus is where the spore was attached to the sterigma)..

'Sometimes the spores can immediately place the fungus into a genus'

Make sure that you are familiar with as many of the following spore shapes as possible:

• • Russula and Lactarius spores are variously ornamented with a network of ridges, spines and warts –staining blue black in Melzer’s solution.

  • Entoloma spores are pink and angular, sometimes almost cuboid or stellate.

  • Laccaria – round or ovate spiny, white spores

  • Inocybe – many Inocybe species have dull brown nodulose spores

  • Lepiota cristata and close relatives – spores bullet shaped.

Cystidia

Cystidia are sterile cells that appear on the outer surfaces of many fungi. They are distinguished on the basis of which surface they appear on, how deeply they originate in the gill trama and also on their contents. The nomenclature varies between different texts and only those cystidia that you are most likely to come across during the early stages of microscopy are described below.

• Cheilocystidia Gill edge

• Pleurocystidia Gill face

• Caulocystidia Stem/stipe surface

• Pileocystidia Cap surface (pileus)

• Chrysocystidia Have contents that go yellow in ammonia or KOK. Useful when working with, e.g. the genera Stropharia and Hypholoma.

Cheilocystidia and pleurocystidia are found on the gills and their presence or absence are usually easy to note when you make a gill section (see ‘Fungal microscopy techniques’ below). It is also possible to decide whether or not they are thick walled and whether there are any encrustations on the top (metuloid) from a gill section but to get a clear idea of the overall shape of the cystidia it is necessary to make a ‘squash’ preparation (see ‘Fungal microscopy techniques’ below). Gathering information about the gill cystidia is particularly important for the genera Pluteus, Mycena, Melanoleuca and Inocybe and you should try and look at material from each of these to familiarise yourself with their cystidia. Macrocystidia cucumis also has very distinctive cystidia.

Caulocystidia are found on the stipes of some fungi and are of particular importance in Inocybe, Galerina and Conocybe. They are sometimes visible to the naked eye as a pruinosity and careful collection (ie don’t hold them by the stipe!) of specimens is always important to avoid damaging these characters. They can be observed under the microscope by taking a scalp but their presence or absence can often be determined in small fruit bodies by simply placing them onto a slide and looking under the lowest magnification of the compound microscope.

Useful references for micro features

Marriott J.V.R. 1994 Guides for the amateur mycologist 2. Guide to identification with a microscope. British Mycological Society

Largent D., Johnson D. & Watling R 1977. How to identify mushrooms to genus III: microscopic features. Mad River Press

Watling R. 1974 Identification of the larger fungi. Hulton Educational Publications (out of print)

Chemical tests

Please read the relevant health and safety notes before using any chemicals. Wearing disposable gloves is recommended.

• • Fe (iron) crystal or solution on the stipe of Russula species can be a useful aid to identification in this genus.

  • If you have a white spore print, a drop of Melzer’s solution onto the dry spore print will tell you whether or not the spores are inamyloid, amyloid or dextrinoid.

  • There are many other stains and chemicals that you might come across in the keys. Most are not essential for identification purposes.