1. Characteristics of the russuloid fungi
Fig. 1. One of the most important macroscopical features of the Russulales is the chalky or brittle structure of the flesh, here exemplified by Russula chloroides and Lactarius deterrimus
(Photo © Marco Floriani)
1.1. What groups of fungi compose Russulales?
At the heart of Russulales, we find 2 genera of very common mushrooms with gills: Russula and Lactarius. Both genera have for the first time been described from Europe, where they are very common and easy to recognize: both genera produce convex to funnel-shaped caps on top of a stipe which never has a ring nor volva and they all are very similar in general appearance. Mushrooms in Lactarius or Russula differ from all other gilled mushrooms in the field in that their flesh is not flexible or fibrous but breaks neatly as a piece of chalk would.
More recent inventories and more performant techniques have complicated the definition of these genera by showing that not all taxa of Russula and Lactarius comply to this classical European concept and that several groups of mushrooms with very different morphology are closely related to these gilled mushrooms. To read more about the definition of Russulales and russuloid fungi, go here.
1.2. Principal features (shared by all genera)
The characters that define the Russulales most accurately are unfortunately undetectable in the field: you will at least need to verify two staining reactions under the microscope if you want to avoid looking at molecular characters to identify a member of Russulales. Indeed, all Russulales share two important staining reactions: one for spores and another one for cystidia and other cells or hyphae with similar contents, such as lactifers.
Fig. 2. Spores of Russula globispora, showing the typical amyloid reaction of the ornamentation, which is formed in this case by conspicuous, isolated warts.
(Photo © Marco Floriani)
1.2.1. Amyloid spore ornamentation
The spores of Russulales range from globose to elliptic in shape, and their surfaces carries an ornamentation which may be missing in a small area just above the apiculus which is called the 'suprahilar spot' or 'plage' in French literature. This ornamentation varies from isolate warts or spines to linear crests to a complete reticulum of variable height. In all cases, however, this ornamentation stains bluish black when treated with Melzer's reagent. This is called the amyloid reaction. The active substance in Melzer's reagent is the iodine which reacts with some polysaccharides in the spore ornamentation to produce a very dark colour. In certain groups of Russula and Lactarius, the same black staining reaction is produced by a mass of polysaccharides deposited on the suprahilar spot, which is then also said to be amyloid.
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1.3. Secondary features (shared by most but not all genera)
1.3.1. Brittle context and sphaerocytes
If you have ever used a field guide to identify mushrooms when out in the field you probably are familiar with the fact that Russula and Lactarius are different from all the other gilled mushrooms because they possess a brittle or "chalky" context. This characteristic can easily be appreciated in the field by breaking the stipe of a Russula or Lactarius: the flesh of these genera lacks any kind of fibrous consistence, and breaks in an uneven manner (Fig. 1).
In the other gilled mushrooms, as well as boletes for example, the texture of the flesh is usually very different, much more fibrous.
But why is the flesh of Russula and Lactarius breaking in such a manner? The answer is very simple: their flesh is mainly composed of a different cell type which is absent in the other mushroom genera. In order to observe this difference you will need a microscope, but photographs 4 and 5 will allow you to understand this better. The former (fig. 4) shows the flesh of a Russula. As you can see, it is largely composed of large, almost round cells, that are called 'sphaerocytes' or 'sphaerocysts'; the second picture (fig. 5) is taken from a gilled mushroom that does not belong to Russulales. Here, the flesh entirely consists of narrow, long filaments or hyphae that are composed of elongated, cylindrical cells. The fact that the flesh is entirely made up of these long filaments makes it fibrous.
Fig. 4. Example of a tissue composed essentially of sphaerocytes as found in most species of the genus Russula.
(Photo © Giancarlo Partacini)
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Fig. 5. An example of the tissue of a mushroom in Agaricales. The flesh of most mushrooms is essentially composed of filamentous hyphae and strongly inflated cells, if present, are usually scarce, and often limited to the cap surface or veil tissue.
(Photo © Giancarlo Partacini)
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In many species, especially in cold or dry climates, the sphaerocytes are arranged in small groups or 'rosettes' as they are sometimes referred to in the literature: small islands of sphaerocyte tissue held in a filamentous matrix. (fig. 6)
The abundance of sphaerocytes varies greatly among the different genera of Russulales. We find, for example, no sphaerocytes in resupinate, polyporoid or clavarioid members in the russuloid lineage. Even among species of Lactarius and Russula there exist important differences. Among the Russulaceae of the northern hemisphere, for example, most Lactarius have very poorly developed sphaerocytes in the gill trama, whereas species of Russula usually have many sphaerocytes in between both gill surfaces (fig. 8-9). In a humid tropical climate, on the other hand, the context of most of the more fragile Russula species is entirely composed of sphaerocytes (fig. 7). It is very likely that sphaerocytes can be interpreted as a successful adaptation towards a more rapid expansion of the epigeous agaricoid fruit bodies.
Fig. 6-7. On the left, the gill trama of Russula ochroleuca; on the right, the gill trama of a member of section Pelliculariae.
(Photo © Bart Buyck)
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Fig. 8-9. Two examples of the gill trama of Russula, showing the occurrence of numerous sphaerocytes.
(Photo © Bart Buyck)
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1.3.2. Lactifers
Lactifers are latex-exuding hyphae present in some of the Russulales, but can be found also in other fungi. The latex - or "milk" as we usually refer to it - can be scarce or abundant depending on the species or the condition of the fruit bodies, and may be variously colored (red, orange, yellow, green, white…) or completely transparent. In most species, these lactifers react strongly with sulphoaldehydes (fig. 12). As far as European Lactarius are concerned, it is probably safe to say that all Lactarius exude a latex when in fresh condition and not too old (fig. 10). This is a good character to separate them in the field from eventually very similar species in Russula. In other climates, particularly in the tropics, it can be much trickier to use exudation of latex to differentiate between Russula and Lactarius. Some Lactarius do not exude a latex or only infrequently so. In other instances, some species of Russula may exude droplets on the gills (fig. 11) that could inadvertently be taken for colorless milk, or the flesh may be misleadingly soaked with moisture.
Fig. 10. Gills of Lactarius rubroviolascens, showing droplets of white milk.
(Photo © Annemieke Verbeken)
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Fig. 11. Reddish droplets observed on the gills of Russula ventricosipes
(Photo © Bart Buyck)
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Although the presence or absence of lactifers may appear to be a relatively unambiguous feature to observe under the microscope, the reality is much more complex. Typical lactifers in Lactarius and some other Russulales are very long and intensively branched, building an extensive network in the context. Moreover, lactifers end near the surface of the fruit body in the form of pseudocystidia. In literature, many Russulas are said to possess lactifers in the context of the cap, stipe and even gills, but these structures are not exactly comparable with the much more branched network of lactifers that visibly exude latex in Lactarius. Being much less branched and not ending at the surface of the fruit body in the form of pseudocystidia, this probably also explains why such hyphae in Russula never exude any latex notwithstanding their identical content. Perhaps a term like 'tramal gloeocystidium' more accurately describes these structures, which — unlike lactifers — gradually become smaller and smaller when closer to the surface of the fruit body, until they finally turn into the typically short dermatocystidia found at the surface of these same species.
Fig. 12. Part of the lactifers network, ending in the form of pseudocystidia, in the gill of a Lactarius species.
(Photo © Bart Buyck)
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Fig. 13. The long cylindrical 'lactifers' in Russula are much less branched compared to Lactarius and are perhaps better called 'tramal gloeocystidia' (or 'gloeoplera' following Clémençon, 2004).
(Photo © Bart Buyck)
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In his book 'Cytology and Plectology of the Hymenomycetes' (2004), Clémençon distinguishes between hyphae filled with water-soluble contents (hydroplera) and not water-soluble contents (heteroplera). Lactifers are in the latter category, as are gloeoplera (lactifers that do not exude any visible amount of latex). According to Clémençon's view, gloeoplera would be the correct term for what I called 'tramal gloeocystidia' in Russula, with the important difference that the distinction is not merely quantitative, but certainly also morphological. Interesting to note is finally the fact that according to Clémençon, some (mainly tropical) Lactarii possess hydroplera.
1.3.3. Oleiferous hyphae
Oleiferous hyphae are present in most Russulales. They can be easily recognized by their strongly refractive, more or less yellowish and very homogeneous contents, which are clearly different from the contents of lactifers. Oleiferous hyphae are always strongly orthochromatic in cresyl blue (lactifers often have a more or less metachromatic wall and less intense orthochromatic contents) and become at most pinkish red with sulphoaldehydes (whereas lactifers may become black, although this reaction varies strongly for different species). Usually, oleiferous hyphae are also much more irregular in outline, with distinct, occasional septa or are limited to shorter fragments of hyphae. The term 'oleiferous hypha' was first introduced in French literature and was clearly based on the oil-like aspect of these cells. Clémençon (2004), however, points out that this is merely a resemblance and that no lipids are present in these cells. He therefore, rejects the term 'oleiferous hyphae' replacing it by 'thromboplera'.
Fig. 14. Oleiferous hyphae differ from lactifers by the very dense and homogeneous contents which are much more strongly refringent and devoid of crystals (compare with picture 13).
(Photo © Bart Buyck)
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Fig. 15. In the trama of many Russula species you can also find fragments of hyphae with oleiferous contents.
(Photo © Bart Buyck)
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1.3.4. Ectomycorrhizal mode of nutrition
All Russulas and Lactarii are ectomycorrhizal, meaning they can not develop and survive without being associated to the roots of their host plants. The same is probably true for all of the hypogeous genera in Russulales, but not for some of the polyporoid and other groups in Russulales, some of which are parasitic (Bondarzewia) or saprophytic (some of the corticioid genera).
The symbiotic structures, called ectomycorrhiza, are morphologically very diverse, both in the field and under the microscope. Some examples from are shown in fig. 16.
Fig. 16. Three examples of ectomycorrhizae from the genera Russula (left, centre) and Lactarius (right).
(Photo © Bart Buyck)
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