What to observe and how to do it?
Taking spore prints
Iron sulfate (FeSO4)
Potassium hydroxide (KOH)
Tissue and structure
The general form of Russula spores varies from ellipsoid to subglobose (perfectly globose spores are probably not present). This feature has been attributed some taxonomic importance in Lactarius (cf. sect. Dictyosporini Neuhoff). In European Russula, its importance is limited to the specific level being characteristic of a number of species of very different systematic position (Amoeninae, Griseinae, Melliolentinae, Lilaceinae, ...). Among the tropical Russulae, subglobose spores are quite common (Buyck, 1989) and often very voluminous.
The terms 'ellipsoid' and 'subglobose' are often used without being precisely defined. The mean length - width ratio should, therefore, always be calculated for a given species (based on at least 20 measures) to describe the spore form. For Russula species, this average quotient ranges from 1.05 to 1.50.
Apart from a few species with exceptionally large (ex. R. adulterina) or very small spores (R. heterophylla), the variation of the spore size within European Russula is too small to be of muche importance in the identification of most species. Nevertheless, spores have always been considered one fo the most important characters of any species, so mycologists usually pay much attention to features of the spores. Among tropical Russulae, extremely large and extremely small spores are much more frequent and may be characteristic at supraspecific level (for ex. subsect. Archaeinae).
I do not want to repeat what has been said already on the expression of spore measurements and I can simply refer here to some general articles on this subject (Heinemann & Rammeloo, 1985; Krüger, 1987). Einhellinger (1985) recommends to calculate also the spore volume as it is a useful diagnostic character for certain species.
Caution is needed in the interpretation of spore dimensions of Russulales because some of the older authors (for ex.: Schaeffer, Singer) usually include the spore ornamentation in their spore dimensions.
The obturation of a spore deposit or - in case of failure - fallen spores adhering to the surface of the stipe or pileus are often considered to be indispensable in obtaining reliable data on spore dimensions. In my experience, preparation of a small piece of lamella in Melzer reagent gives quite similar results because the ornamentation of the spores enables the observer to evaluate the development of the spores correctly.
The appropriate name for this projection on the basal end of the spore has long been the subject of discussion. Usually called 'hilar appendix' in American and English literature (also 'hilar appendage', see Vellinga in Bas & al., 1988), it has received different names in French literature. Josserand (1952, 1983) retains the old term 'apiculus', which was rejected by Romagnesi and Heim because it suggests the idea of an apex, thus exactly the inverse of its position on the spore. Heim then used the term 'hilum' which he later replaced by 'appendice hilaire'. This term was adopted in Anglosaxon literature where it is still used. However, Romagnesi rejected also this term because the projection on the basal end of the spore is not the actual site of attachment or insertion (hilum) on the sterigma, but it is the structure who bears that insertion point on its distal part; he, therefore, replaced it by 'appendice hilifère' (hiliferous appendix in English) which, I agree, is more logic and more accurate.
The hiliferous appendix of the ballistospore bears a subterminal and abaxial hilum of the nodulose type (Pegler & Young, 1969) and an equally subterminal but adaxial punctum lacrymans (Hugueney, 1972). When observing spores of Russulales under the scanning electron microscope, one can often observe a globulose emanation at the punctum lacrymans, which probably corresponds to the "early enlargement stage" (McLaughlin & al., 1985) of Bullers drop (see fig. 1 a).
The taxonomic importance of the hiliferous appendix is probably negligible and most descriptions don't even mention it. In some instances, however, the hiliferous appendix may be remarquably small or large. From my personal observations, the length of the hiliferous appendix varies between 0.5 and 1.5 µm for Russula. Having studied the spores of all African Russula with the scanning electron microscope (Buyck, unpubl.), the size of the hiliferous appendix appears not to be related to the size or the form of the spore. It relates instead to the height of the spore ornamentation (fig. 1 & 2).
The area situated above the hiliferous appendix is usually indicated as 'suprahilar area' or 'suprahilar plage', Anglo-Saxon counterparts for the French 'plage supra-appendiculaire' or 'tache hilaire'. I find all these terms unsatisfactory because the hilum is on the abaxial side of the appendix, the meant area thus being rather situated above the punctum lacrymans.
I, therefore, agree with Marchand (1977) that the term 'plage', which has no other use in botanical terminology, is amply sufficient to refer to the area which alignes with the hiliferous appendix.
The degree of amyloidity of the plage is certainly a very important taxonomic feature, characterising different large divisions of the genus. The inamyloid plage is usually considered to be the oldest (primitive) condition in Russulales. My observations on African Russulae confirm this, but suggest that the amyloid plage has originated several times in the genus. It relates to the height of the spore ornamentation in the sense that a strongly amyloid plage is only found in spores with an ornamentation of at least 1.5 µm high. However, not all spores with such an ornamentation possess a distinctly amyloid plage. This phenomenon certainly deserves further attention.
The difficulties of description are most prominent with regard to spore ornamentation. The problem with Russulaceae does not result from the large range of different spore ornaments, as is generally assumed, but stems principally from the fact that a limited number of different ornaments may occur in a large number of combinations, not only within a certain species, but even on a single spore.
If precise and detailed observation of the spore ornamentation is required, this must be performed in fresh Melzer's reagent (Melzer, 1924, 1951).
Scanning electron microscopy (SEM) has provided a very easy means in obtaining good and reliable pictures of the spore ornamentation. It should always be performed for important studies, even if the result - in the case of Russulaceae - is hardly spectacular and adds only little to the observation with a good light microscope (Fig. 5). The existing variation for the basic spore ornaments, as given in fig. 3 & 4, applies only to Central African Russulae, but represents probably the existing variation for non-linear ornaments in the entire genus.
The ontogeny of the spore ornamentation in Russulales has already been studied in the past from a structural point of view, as it is an important character in the classification of the higher fungi. As I usually study the spore ornamentation of my tropical Russulae on a piece of the lamellae, it struck me only recently, that the same final ornamentation may originate in different ways depending on the species. Unfortunately, I did not make enough observations yet to go in more detail now, but this is certainly a feature which could be of considerable taxonomic interest. Several prominent specialists of the genus Russula have more of less developed their own vocabulary for the description of the spore ornamentation. Two terminologies should be mentioned here: that of Singer (introduced in 1932 and unaltered since) and that of Romagnesi (1967, now widely accepted and applied). Both systems are unsatisfactory.
The following example illustrates well the problem of terminology in mycological descriptions: Spores of Russula martinica Pegler (Pegler & Singer, 1980) were originally described as "of isolated verrucae (-0 5 pm) with no connectifs". Singer (Singer et al., 1983: 281), after examining the same material, describes the spores as "with an amylaceous ornamentation of type IV, Ilb, IV-VIII, IIIb-VIII, rarely IIIa, if with type IV, mostly with an extremely fine reticulation of lines scarcely visible in light microscopy which makes the ornamentation of type IIIa, more rarely IIIb, the warts and rod-like spinules of the ornamentation projecting (0. 2) 0.5-1.1 µm, mostly about 0.6-0.8 µm, altogether showing the usual ornamentation in Sanguineae.
When comparing my own illustration of the spores of R. martinica with the cited descriptions, it is obvious that the original description is erroneous and illustrates well the elasticity of terms like 'verruca'. The description of Singer, although rather accurate, is quite incomprehensible for outsiders by the reduction of all possible ornamentations into only a few general types being referred to by numbers.
The descriptive terminology for spore ornamentation by Romagnesi (1967) is one of the rare examples of a terminology which has transcended linguistic barriers and became almost internationally adopted. A positive aspect of Romagnesi's terminology is the separation of the ornamental units (les unités) from their arrangement on the spore surface (le dessin). However, despite its large distribution, the application of Romagnesi's terminology raises several problems. The cause for this lies probably in the definitions of the basic ornamental units, which are not primarily based on the form but on the height of the elements (one of the most variable features for a given species as Romagnesi admits himself). Furthermore, expressions such as 'more or less' (plus ou moins ) or 'almost' (presque) in many definitions add too much vagueness to the interpretation. As I extensively discussed elsewhere (Buyck, 1989: 52-62), Romagnesi himself is not always consequent in the use of his own terminology. Some examples:
The application of Romagnesi's terminology generally results in a large number of terms to characterise the ornamentation and nearly always requires additional precision, which is exactly what Romagnesi provides for every species, thereby minimising the benefit of his terminology. It is, therefore, better to describe the spore ornamentation in simple, general morphological terms and to add a good illustration of the spore ornamentation. With respect to the latter, the drawings of the spores by Imler in Romagnesi's book constitute an invaluable contribution to the knowledge of the European Russulae.
Nevertheless, the following terminology remains quite useful and accurate for spore description purposes:
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