North American Species of Crepidotus
The earliest studies of Crepidotus of any consequence were made early in the nineteenth century and were concerned, as were studies of gill fungi generally at that period, with the macroscopic features of the basidiocarps. The starting point may be said to be Systema Mycologicum by Elias Fries in 1821, p. 272, where we find Tribus XXX Crepidotus proposed under the broadly conceived genus Agaricus. In this work we find A. atrotomentosus, A. panuoides, A. vulpinus, A. depluens, and A. byssisedus, species now excluded from Crepidotus, as well as C. mollis, C. variabilis, and C. epibryus, which are still retained in Crepidotus. In his later works Fries (1836-38) removed A. atrotomentosus and A. panuoides to Paxillus and in 1874 transferred A. depluens, A. byssisedus, and A. variabilis to Claudopus, a group characterized by a vinaceous colored spore deposit.
It has been shown by Donk (1949), through his diligent searching of the older literature, that Staude (1857) was the first to ascribe generic rank to Crepidotus. Staude treated only one species under this name, C. mollis, which under our present rules of nomenclature means that it must be accepted as the type species of the genus.
Kummer (1871) in his descriptive key briefly described ten species as follows: C. variabilis, C. violaceo-fulvus, C. pezizoides, C. byssisedus, C. depluens, C. proboscideus, C. haustellaris, C. applanatus, C. mollis, and C. alveolus. As can be seen, the concept of the genus, as it was developing, was that of a group of sessile to laterally or eccentrically stipitate agarics with colored spores. The modern delimitation of species on this basis had not yet been reached, since C. violaceo-fulvus, for instance, actually has white spores.
Quelét (1872) also treated Crepidotus as a genus and transferred additional species to it, though he was either unaware of Staude's and Kummer's works, or simply disregarded them. Quelét (1886), under Series III (Phaeospori), recognized about a dozen species, but these were only briefly described. Only macroscopic features were used.
In North America, Peck (1886) was the first to provide keys and descriptions for our Crepidotus flora. In his "New York Species of Pleurotus, Claudopus, and Crepidotus" Peck described eleven species. His descriptions, following the pattern of European authors, used macroscopic features predominantly but included brief notations of spore color, shape, and size (in fractions of inches). With the exception of C. tiliophilus, the species he treated are still retained in the genus; although three (C. haerens, C. tomentosus, and C. dorsalis) are now placed in synonymy with other species. It is a point of historical interest that Peck placed Crepidotus variabilis in Claudopus.
Murrill (1917) was the next to treat the North American species, but the characters he used were about the same as those used by previous authors, with the result that his work is valuable chiefly as a compilation. Kauffman (1918) treated 15 species, including one which was new. None of these authors recognized infrageneric categories above the rank of species.
Pilát (1948) limited Crepidotus to those species with brown to yellow-brown spore deposits varying to ocher-flesh color. He recognized the following subgenera: 1) Tapinella, characterized by bilateral lamellar trama and represented by C. panuoides, a species he interprets as forming a transition to Paxillus. 2) Paxillina, similar to Tapinella but lacking bilateral gill trama. It was considered to unite Tapinella with Paxillus. 3) Gelocutis, characterized by a gelatinous cuticle on the pileus and represented by C. mollis. 4) Subgenus Sphaerocrepidotus, embracing taxa with globose, rough spores, represented by C. applanatus. 5) Naucoriopsis, in which the basidiocarp is truly stipitate even at maturity, and in which the spores are ellipsoid-reniform. 6) Stipitina, with basidiocarp stipitate, as in 5 above, but with oblong-ellipsoid spores. He assigned C. phillipsii here and considered both 5 and 6 to connect to Naucoria. 7) Geophila, embracing species growing on the soil or on debris, rarely on wood not in contact with soil. Examples of species placed here are C. fragilis, C. caspari, and C. bresadolae. 8) Nebulosi, carpophores small but solidly and firmly fleshy, and spores ellipsoid, fusiform or amygdaloid. C. lundellii and C. subverrucisporus are placed here. 9) Dochmiopus, small species with spores brown, tinted flesh color. Examples: C. variabilis, C. cesatii, and others. 10) Muscicola, small species growing among living mosses, approaching Dochmiopus, but with difficult to determine spore-color, such as C. subepibryus and others. 11) Colorati, with carpophores with reddish gill edges, as in C. cinnabarinus. 12) Pearsonomyces, with spores yellowish in H20 but becoming dark rusty in KOH, and also the basidia and some hyphae with a similar KOH reaction. Pilát regarded this group, represented by C. hibernicus, as a transition between Crepidotus and Pholiota sensu Singer. Though very elaborate, this division of Crepidotus has not found favor in the eyes of other investigators and at least for the North American species seems to lack perspective. The names were not validly published.
Singer, in his various publications (1947, 1951, 1962) used a more realistic approach when he recognized two sections for the genus: Echinosporae, those taxa with the spore wall punctate, and Crepidotus (Laevisporae), embracing species with the spore wall lacking ornamentation.
In the course of his studies of type specimens, Singer (1947) transferred a few species from other genera to Crepidotus, for example, Tremellopsis antillarum. Singer recognized a relatively large number of crepidotoid genera: Melanotus, Phaeomarasmius, Pleuroflammula, Pyrrhoglossum, Naucoria, Pleurotellus. In 1962, he took up the name Simocybe to replace Naucoria in the sense of his 1951 work. We accept a number of these genera and have included all their North American species in our keys. Under our heading, Excluded Species, we include descriptions of them as an aid to the investigator collecting crepidotoid fungi.
Pilát (1950) also published studies of types, particularly those of extra European species of Crepidotus, and presented a key based on a combination of Singer's and his own type studies. He described seven new species from various parts of the world, but none from North America. He also gave valuable notes on the exsiccati examined. Except for Pilát (1948) and Singer (1947, 1951, 1962), little has been proposed in the way of infrageneric categories for Crepidotus. A synopsis of infrageneric categories as we have developed them for the North American species follows. We consider this arrangement an approximation of natural relationships of the species.
Singer in 1947 gave the following synopsis of the infrageneric classification.
Section Echinosporae Pilát 1929
Type species: C. carpaticus
Subsec. Porpopharini Singer
Type species: C. applanatus
Subsec. Aporpini Sing.
Type species: C. cinnabarinus
Section Laevisporae Pilát 1929
Type species: C. mollis
Type species: C. albidus
Type species: C. mollis
These are validly published and are to be taken into account in any subdivision of Crepidotus into smaller taxa. Our proposed synopsis follows.
I. Subgenus Crepidotus
Type species: C. mollis (Fr.) Staude
1. Section Cinnabarini Hes. & Sm.
Type species: C. cinnabarinus Pk.
2. Section Tubariopsis Hes. & Sm.
Type species: C. pubescens Bres.
3. Section Stratosi Hes. & Sm.
Type species: C. stratosus Hes. & Sm.
4. Section Parvuli Hes. & Sm.
Type species: C. parvulus Murr.
5. Section Crepidotus
6. Section Versuti Hes. & Sm.
Type species: C. versutus (Pk.) Sacc.
II. Subgenus Sphaerula Hes. & Sm.
Type species: C. applanatus (Pers.) Kummer
7. Section Nyssicolae (Sing.) Hes. & Sm.
Type species: C. nyssicola (Murr.) Sing.
8. Section Sphaerula
a. Subsection Sphaerula
b. Subsection Colorantes Hes. & Sm.
Type species: C. confertus Hes. & Sm.
c. Subsection Fulvofibrillosi Hes. & Sm.
Type species: C. nephrodes (B. & C.) Sacc.
III. Subgenus Dochmiopus (Pat.) Pilát
Type species: C. variabilis (Pers. ex Fr.) Kummer
9. Section Cystidiosi Hes. & Sm.
Type species: C. albatus Hes. & Sm.
10. Section Fulvidi Hes. & Sm.
Type species: C. kauffmanii Hes. & Sm.
11. Section Phaseoli Hes. & Sm.
Type species: C. phaseoliformis Hes. & Sm.
12. Section Fusisporae Hes. & Sm.
Type species: C.fusisporus Hes. & Sm.
13. Section Betulae Hes. & Sm.
Type species: C. betulae Murr.
14. Section Dochmiopus (Pat.) Pilát
15. Section Crepidotellae Hes. & Sm.
Type species: C. submollis Murr.
The basidiocarp of Crepidotus species, and those in closely related genera, is reduced to the extent that the stipe varies from underdeveloped to absent. In this respect, the group of genera around and including Crepidotus is similar to the pleurotoid members of the Tricholomataceae. A survey of the structural features of the basidiocarp is presented in the following paragraphs in accordance with currently accepted procedure in agaric taxonomy. This survey is divided into two parts, one dealing with macroscopic characters and the other with microscopic details. The importance we attach to individual features is expressed by the emphasis given them in the formal descriptions, but a summary of our data pertaining to the various characters should aid the reader in developing perspective.
The Pileus. The size, shape, color, and ornamentation of the pileus are among the classical features of the genus and are still important in the delimitation of species in the so-called modern approach. The pileus is relatively small, ranging in breadth from 2 mm to about 8 cm, but with most species falling in a range of 10-30 mm. In the majority of species the pileus is attached laterally to the substratum, with or without a lateral basal extension. This habit of growth results in the shape of the pileus being fan-shaped, semi-orbicular, reniform, or spathulate, depending to some extent on how much the cap is tapered to the point of attachment. The basidiocarps, of course, grow on stumps, logs, fallen branches, twigs-in fact, on woody debris generally, and occasionally on mosses or on herbaceous debris. They rarely occur on the soil. If the basidiocarp is growing on the under side of a log, it may be resupinate at first, but by maturity an upper pileus surface is usually evident at least along the margin. Although the habit is usually scattered to gregarious, there are a few species in which the basidiocarps are produced in an imbricate fashion. It must be kept clearly in mind that the subiculum, or hyphae on the substratum around the basidiocarp, are to be regarded as part of the mycelium, and for this reason may be (and do in some species) reveal differences not shown by hyphal elements arising directly from the pileus.
Pileus color. Our observations lead us to conclude that color is basic in the delimitation of species. There are several aspects of pileus color, such as whether the pigment is dissolved in the cell sap, or whether it is incrusted on the hyphal walls or actually in the walls. In addition to these possibilities, the position of the pigmented hyphae in the fruiting body is important. Pigment changes from youth to old age are also important. In many species the pileus and gills are white and become colored only as the spores mature. This change in color is more than just a reflection of the brown spores through a moist cap. As in Agaricus, it is due in part at least to accessory pigments which develop as the pigment becomes evident in the spores. Thus, white species may become buff to tan as the spores mature and, when dried, the pileus may be dingy buff to alutaceous or darker brown. From the color of the pileus even in well-dried exsiccati one can only guess at the pileus color in the fresh state; hence notes on the color when the specimens were fresh are essential to correct identification.
The pileus may be white or whitish, but the surface fibrils or hairs may be colored. In such species the pigment in the colored hyphae is often encrusted on the hyphal walls of the cuticle hyphae or those of the epicutis. Such colors are most pronounced on the young caps and become more dilute in age as the pigmented hyphae become separated. The pigments associated with spore maturation may influence the ground color of these species at maturity just as they do in the others already discussed.
In other species the surface of the pileus may be colored (yellow, buff, gray, drab, reddish, or avellaneous), and white or colored surface hyphae may be present. The taxonomic characters of importance as regards pileus color are the combinations which one finds. For instance, the ground color of the pileus may be orange and that of the epicuticular hyphae dark brown; thus the combination is striking. Or, the pileus may be gray and the epicuticular elements white. In the following taxonomic treatment we have used these features in the delimitation of sections and subsections, and it should be understood that these combinations are basic at the level of species.
The epicuticular hyphae, in addition to showing differences in color from species to species also show important differences in distribution, size, and arrangement. At one extreme they may appear to be absent (under a hand lens), and such pilei are termed glabrous. Actually, some (not all) apparently glabrous pilei may show scattered to dense epicuticular hyphae when viewed in sections of the cap under the microscope. The details of these hyphae are treated in the discussion of microscopic features.
The pileus may be moist and hygrophanous or dry and fibrillose. However, we have used the hygrophanous character sparingly because it is easy to misinterpret it, and, in particular, notes by inexperienced collectors may lead an investigator astray. The correlated feature of a translucent-striate pileus margin is also deceptive as far as most collectors are concerned, and notes on it are often unreliable, especially if the margin is indicated as not striate. Basically, however, these are valuable characters in the recognition of species. The hygrophanous nature of the pileus is essentially that of a water-soaked condition; and as moisture escapes, the color changes markedly, and the flesh which then becomes opaque does not allow the gills to show as lines (striations) through it. A pileus may be hygrophanous and have colored or hyaline (white) epicuticular hyphae, or, except for the mycelial tomentum at the point of attachment, such a pileus may be glabrous.
In a few species, a cuticular layer of gelatinous hyphae causes the surface to feel viscid to slippery. It is important to note this condition in fresh material, although in the study of herbarium specimens the presence of gelatinous hyphae can be readily ascertained. This character is often misinterpreted by professional and amateur mycologists alike, to the point that one soon finds himself falling into the habit of not giving credence to the collector's notes. This statement applies to all genera of gill fungi. The basic feature involved is a chemical difference between the hyphae producing the slippery feel and those beneath that layer, and it is this difference which is of taxonomic significance.
Context. The features of the context are not as contrasting as in many genera, but nevertheless some are important. The taste of the raw flesh in some species is bitter to disagreeable and, as in other genera, this is important at the species level. The odor in most Crepidoti is mild or bland and hence of little taxonomic account. Likewise, little assistance can be obtained from the slight differences in color of the flesh which are shown by most species.
Lamellae. The characters involved here are the usual ones: color when young and when mature, spacing, breadth, and whether the edges are crenulate, fimbriate, or even. The color of the young gills, in species in which they are something other than merely white to grayish, is caused by pigment dissolved in the cell sap of the basidioles or the tramal hyphae. This pigmentation is independent of the color changes and pigment development associated with spore maturation. At maturity, obviously, the gill color reflects that of the spores, but one must not assume they are identical; the color of the spore deposit should be obtained not only as a check but for its own inherent taxonomic value.
The spacing of the gills is comparatively constant for a species and hence is of some value at the species level. Spacing varies with the species from crowded, close, subdistant to distant, as for other agaric genera. A comparison of the species illustrated will give one an idea of the degree of difference involved between each category. We found that counting the number of gills which reach the base and the number of tiers of lamellulae does not give an accurate index for these differences, because the size of the fruiting body varies so greatly. We have not tried to arrive at a mathematical basis for distinguishing between crowded, close, etc., although this could probably be done. The width of the gills is a feature of some importance, but is easily misinterpreted if one lacks a knowledge of the way Crepidoti develop. Large numbers of fruiting bodies often occur on a log in a single fruiting and are likely to be all in the same stage of development. The gills in many species are narrow at first and develop their typical width only after expansion of the cap is complete. If one finds a log with mature basidiocarps of species A, the gills may be broad (3-4 mm), whereas on the next log younger basidiocarps may bear narrow gills (2-2.5 mm broad), and one might think he was dealing with two species. Since spores soon develop (before the cap is fully expanded), this feature cannot be used as an index to basidiocarp maturity. However, in regard to the use of both gill width and spacing, we believe they must be correlated with other characters to have much taxonomic value in species recognition.
Stipe. As previously mentioned, the stipe is not as a rule a conspicuous feature of Crepidotus species. In C. haustellaris, however, it is a well-formed, narrow structure persistent throughout the life of the basidiocarp and hence an important taxonomic feature. In C. stipitatus, the stipe is much like that of C. haustellaris, but is smaller in relation to the size of the cap and more like the pseudostipe of many species. The pseudostipe is merely a rudimentary stipe 2-3 mm long and about 1 mm wide which is found on the underside of the pileus near the point of attachment to the substratum. It usually disappears by the time the basidiocarp has matured. In species in which the pseudostipe is present, the pileus is laterally attached to the substratum at maturity. Although of minor importance as a taxonomic feature, the pseudostipe may have ontogenic significance which indicates a phylogenetic connection from normally stipitate ancestors. Usually when Crepidotus is truly stipitate, the pileus is marginate around the base (or, as explained in some books, the pileus is marginate behind). This is a distinctive feature.
In the current study we at first gave equal attention to the morphology of the spores, basidia, cystidia, gill trama, pileus trama, cuticle, and the presence or absence of clamp connections. As experience was gained, however, it appeared that limited taxonomic use could be made of the characteristics of the basidia, and of the trama of the pileus and gills. In contrast, we found that great reliance could be placed on spore size, shape, and ornamentation, on the morphology of the cheilocystidia, and also the pleurocystidia, if such were present, and on the presence or absence of clamp connections.
Clamp Connections. The presence or absence of clamp connections on the hyphae of the basidiocarp is now an empirically much used character in the taxonomy of Basidiomycetes generally, but it still needs to be evaluated. We have not been able to investigate this character in detail in Crepidotus but have used it empirically, as other investigators have, as an important feature mainly because it is a positive character; clamps are either present or absent. Before any ultimate value can be assigned to the character, we need to culture all the species of Crepidotus to see whether clamps are present on the mycelium when they are absent from the hyphae of the basidiocarp. Because of what is not yet known about this character we would not use it as an important generic character.
It is imperative, however, in using our infrageneric classification to determine at the outset whether clamps are present or absent on the hyphae of the basidiocarp. The technique we use is simple: tangential sections of the pileus are cut from near the outer edge and mounted in 2% KOH. Clamps are then searched for on the hyphae of the cuticle, context, gill trama, and base of the basidia. Clamps may be readily observed on epicuticular hyphae, at the base of pileocystidia, or on the repent hyphae themselves. This is due in most species to the fact that these hyphae or hyphal elements are not greatly inflated. Hence, there is no distortion of the clamp following cell division. The hyphae of the context frequently become quite inflated after cell division takes place, with the result that clamps, though originally present at the cross-walls, are nearly to completely obliterated by subsequent changes in the cell. Although clamps are typically present at the base of basidia in clamp-bearing species, the manner of hyphal branching which gives rise to the hymenium is such that it is often difficult to decide when one is dealing with a clamp or a young branch. In addition, observations on these hyphae and their branches are difficult because of the compact arrangement of the components of the region. Hence, the presence or absence of clamps in a Crepidotus, as used here, means that the character was determined from an examination of the cuticular hyphae. One misleading fact enters into the determination of this character state. The hyphae around the base of the fruiting body may, in age, extend over the surface of the basal area and become actually a part of the basidiocarp. Clamps may be present on such hyphae, but not on those making up the original basidiocarp. The extent to which this situation prevails needs further investigation. One should not understand from what has been said that when clamps occur on the cuticular hyphae they are found one at each septum. If abundant, most septa will show them, but there are species in which they are relatively scattered or very rare. The reasons for this pattern of occurrence need to be ascertained. The size of the clamp may be different from species to species, and some are of the type called medallion clamps, i.e., large and looping. Crepidotus appears to be a favorable genus for a study of clamp connections per se. Pending results from such an investigation, we have followed modern usage of this character in Crepidotus. We recognize 1 subgenus Crepidotus for those species lacking clamps, as indicated above, and divide the subgenus into six sections.
Spores. As in other genera of agarics, spore morphology in Crepidotus furnishes constant characters generally regarded as basic to an understanding of speciation. Our observations have verified those of previous authors in this respect. In Crepidotus, as might be expected, certain spore features that occur in other brown-spored genera are present, and some features have developed in a manner more or less characteristic of the genus. Spore shape and ornamentation are most important, and spore size, ranking third, is used mainly at the species level. Although important, the color of the spore deposit is not critically known for enough of the species, but this in no way reflects on its possible intrinsic value as a taxonomic feature. Using these features in our classification we find that the clamp-bearing species may be grouped as follows: (1) those with globose to subglobose spores grouped in Sphaerula; (2) those with ellipsoid, ovoid, or rarely lanceolate spores are placed in Crepidotellus; (3) the fusoid-spored species are placed in sections Fusisporae and Tubariopsis.
The ornamentation of the spore is a more basic feature than the presence or absence of clamps, but in Crepidotus it is often very indistinct. A study of the spore wall in Crepidotus under the electron microscope is needed to furnish a better understanding of this character in the genus. As we have used spore ornamentation, the surface is described as either smooth or ornamented as seen under a 1.3 NA oil immersion lens. Under the heading of ornamentation, all types of irregularities of the spore surface are included, as well as pores or plugs extending through the wall. It is a feature of a number of Crepidoti that there are minute canals extending through the spore wall which, where they reach the surface, produce a minute spot or opening which is seen as a discontinuity of the surface. Such spores are said to be punctate. If the canal is filled with a solid substance, this may project slightly beyond the surrounding spore surface as a short rod. Such spores are termed echinulate. This type of ornamentation is not common in Crepidotus. In a few species, the ornamentation is caused by a wrinkling of the outer wall. Such spores are described as rugulose, and if the outer wall remnants are gathered into discrete, approximately isodiametric particles or warts, the spore is said to be verrucose. The spores of Crepidotus lack a germ pore, although at times an indistinct thin spot (callus) may be present. Features of the spore ornamentation in Crepidotus are best observed with a 1Ox eye piece and a 1.3 NA oil immersion lens.
Spore-deposit color is in the yellow to earth-brown color range, with many species having deposits "clay color" to a duller yellow-brown. In some the deposit is yellowish to pinkish buff to pale ochre buff. In others, it is a dingy earth brown, and in some, near "Sayal brown," (a pale cinnamon). Borderline species for the genus may have somewhat flesh tinted spore deposits. Spore-deposit color, as far as our data indicate, is a very constant and important feature. A difficulty is, however, that a good spore deposit is not always readily obtained, since the fruiting bodies are small and often wither quickly. Thus, the problem of getting deposits of comparable density the practical use of this character in identification work. It is essential, of course, to get some indication of the color in order to identify the genus.
Spore size is a routine feature in Crepidotus, as in other agarics, and our observations have shed no new light on the pattern of variation within species or its use in delimiting species. The range in spore size for the genus, about 4-10 µ in length, is one of the narrowest for any agaric genus. Because the ornamentation of Crepidotus spores is not prominent, as it is in many species of Lactarius, our measurements always include the ornamentation, if any is present. Spore shape, in our estimation, is a more important feature than spore size. It is a very reliable character at the species level and serves well as an aid in circumscribing infrageneric taxa. One must be sure to view the spores in profile view as seen on the sterigmata as well as in the front or back views. Crepidotus spores fall in the category of "thick walled." At least the wall is fairly rigid, but in most species it is scarcely thick enough to measure.
Basidia. From species to species, basidia vary some in size, but with the evidence at hand we feel that the differences are of little value taxonomically. The variation in a single cap is often very great. It is important to note the size in descriptions, however, since the information may be useful to some investigators, such as cytologists engaged in studies of the basidium per se. It is, of course, important to note whether basidia are two-spored or four-spored, and whether both kinds occur on one basidiocarp. This may affect spore size, as spores on two-spored basidia tend to be larger than on four-spored, even in the same species. Also, the feature of two-sporedness, at least in some agarics, is very constant and can be used as a convenient character for identification. Its taxonomic value, however, rests on whether or not it can be correlated with more basic features. We made a preliminary attempt to count chromosomes in a number of species, but the few figures available and the small size of the chromosomes were such that we decided to make this the subject of a later special study.
Cystidia. Scattered among the basidia on the sides, on the edges of the gills, or on the pileus, cystidia may be present. In Singer (1962: 660) the statement is made under Crepidotus: "cystidia none on the sides of the lamellae but always present on the edges (cheilocystidia)." Pilát (1948) also states that in Crepidotus cystidia are absent on the sides of the lamellae. Our studies contradict these statements. We have found cheilocystidia to be present in all species except C. albidus and C. aquosus. We have found pleurocystidia in a number of species and have used their presence to help define infrageneric taxa (see Figs. 44, 51, 54, 67, 70, 72, 112, 117, 121, 125, and others). We have known for over twenty years that this situation existed, but have never published on it. In Crepidotus these structures are not always as conspicuous as in many genera of agarics, nevertheless they are present. Our use of this feature is in line with present day treatments in other genera. In Crepidotus, as in Tricholomopsis, the morphological differences between the cystidia of the various species is not great and hence not of major taxonomic significance.
Cheilocystidia. These structures are more diverse in form than the pleurocystidia, as the illustrations (see drawings) readily show. They furnish a number of the characters on which species are distinguished. One of the features well shown in Crepidotus and not common in other genera is the crooked to contorted condition, which at first glance one might suspect of being an abnormality (see especially Figs. 109, 120, 153, 154, 165, 184, 196). The development of secondary septa in the cheilocystidia of a few species has been noted by us in the recognition of a number of varieties (Figs. 80, 81, 95). It is interesting that secondary septa may also develop in other genera, such as Rhodophyllus. This is not a surprising feature in Hymenomycetes; actually the formation of such septa in basidia is known in some groups, as the Clavariaceae. Some cheilocystidia, which are nothing more than prolongations of tramal hyphae, of course, would be expected to be septate or only the end cell is described as a cystidium.
Gill Trama. We found little that was helpful here in recognition of infrageneric taxa. In most of the species we have examined, the hyphae are hyaline, smooth walled, and in subparallel arrangement extending roughly from the pileus context toward the gill edge. In some species, however, the hyphae are intricately interwoven (extend in all directions), and rarely have we found them truly parallel. Also, the compactness or looseness is a difficult feature to evaluate or use, because generally the hyphae are compactly arranged at first and more loosely arranged in age. Typically, the hyphal cells themselves are not inflated at first, but may become somewhat inflated in age. In a few instances considerable enlargement may take place, but the difficulties of using these features were considered to outweigh any practical taxonomic value they might have.
Pileus Trama. In a majority of the species, it can be seen in a tangential section of the pileus that the context hyphae are interwoven. In a lesser number there is a tendency toward a radial, and more or less parallel, arrangement. In the latter type of structure, the trama in tangential section appears cellular because one is viewing the cut ends of hyphae. In such species a section cut radially will show the filamentous structure of the context. From our observations the pileus trama is composed of a single hyphal system in most species; a few also show a system of laticiferous hyphae. In a small number of species, layering is evident in the trama, and it is on this feature that section Stratosi is based. In it the inner half or less is composed of densely interwoven hyphae, whereas the layer exterior to this is composed of loose tangled hyphae. Except for this feature there is not much of taxonomic value in the organization of the pileus context. On the other hand, the organization of the pileus cuticle was found to show differences of great value in the delimitation and grouping of species.
The Cuticle. The characters of the dermal layers of the basidiocarp have come into prominence in the so-called modern approach to agaric taxonomy. This is well illustrated in Singer's Agaricales in Modern Taxonomy. Our review of these features for Crepidotus species has confirmed their value in taxonomy, but it should be pointed out that the diversity of features found in some genera is not present in Crepidotus. In fact, in some Crepidoti the dermal region is scarcely distinct in structure from that of the context. In the following discussion we proceed from this simple condition to the more complex types.
Homogenous Pileus: In certain species the surface zone consists of hyphae interwoven as in the context (or radially arranged) and not differentiated from context hyphae, except perhaps for their slightly more compact arrangement. We do not believe that this condition indicates a primitive condition from an evolutionary point of view. It could just as easily come about by reduction from a more complex type. In order to determine the truly primitive as contrasted to highly evolved species, all features of a species need to be considered.
The Simple Cutis: In species with a glabrous pileus there is usually some differentiation between the hyphae of the context and the hyphae forming the cutis. A tangential section of the pileus reveals that the cuticular hyphae are more compactly arranged than in the context, often more regular in arrangement, and that the hyphae forming the layer are narrower than the context hyphae and have a tendency to greater encrustation of pigment on the walls and-or intracellular pigment. The thickness of the layer and the degree of differentiation vary with the species. In such a layer one may find a few undifferentiated hyphal ends either erect or somewhat decumbent. If many of these are present, the surface of the cap appears pruinose when fresh and moist.
The Trichodermium: This type of covering is a further development of the hyphae which cause the pruina. If they elongate and become septate as well as densely arranged, we have a type of covering termed a trichodermium. The hyphae composing it are basically upright, but if they elongate much, they become matted into a loose tangled mass. To observe the true nature of such a layer, the younger caps should be sectioned and observed. The elements of the trichodermium may themselves show features of taxonomic value. These elements may become coiled or contorted (Figs. 175, 192, 194, 205) in a characteristic manner, or only the terminal cell of an element may show distinctive features. If the elements projecting from the pileus surface are single cells and they show some differentiation, by inflation at the apex or midportion, or the apex narrowing to a point, they are termed pileocystidia. In a given species, pileocystidia often resemble the cheilocystidia but vary through a wider range in most characters. Pileocystidia are often best observed near the margin of the cap. In a number of species, fascicles of hyphae extend out from the pileus or from appressed fibrils which are continuous at first but become broken up as the pileus expands. These hyphae may show pigment incrustations arranged in various patterns over the exterior. This pattern of pigmentation in correlation with other features is valuable in species circumscription.
The Gelatinous Cuticle: This type is distinctive, and is based on a slight chemical difference in the hyphal walls in the cuticle as compared to those of the context. If a section is made of the pileus of a species in the C. mollis complex, or of some species in section Betulae, and mounted in 2% KOH, a clear translucent (or "glassy") zone about 50-350 µ thick can readily be seen under the low power. The hyphal walls in this zone are mucilaginous, at least over the exterior layer, and this zone (it is seldom clearly defined) expands greatly when mounted in water or KOH. Thus, the moist pileus surface having such a layer has a viscid, slippery, or slimy feel, and if the layer is thick, the cap is rubbery in texture. In a species with a gelatinous cuticle, the surface, as it dries out, usually takes on a shiny appearance, as if it had been varnished. However, sections mounted in KOH will clearly show the gelatinous nature of the hyphae from such a cap. In C. uber, gelatinous hyphae are present not only in the cuticle but in the pileus context as well, with the result that in C. uber the gelatinous cuticle lacks a clear boundary. With reference to surface hyphae resting on a gelatinous zone and not themselves being gelatinous, one of several conditions prevails, depending on the species: (1) the surface may bear erect, colorless hyphae arranged in more or less of a turf, and with the terminal cells cystidioid, as in C. alabamensis; (2) the hyphae are colorless as above, but the terminal cell is not differentiated from the other hyphal cells, as in C. maximus and C. betulae; (3) the surface bears conspicuously brown hyphae, at least some (not necessarily all) of which are incrusted (Fig. 50). At times, the terminal element may be cystidioid, and often these brown hyphae are in tufts or clusters (scales), as in the C. mollis complex and others; (4) the surface hyphae are brown but are not incrusted, and may be intermixed with colorless hyphae, as in C. fraxinicola. In some pilei, the surface may seem to be glabrous, but a section of the cap examined under the microscope will reveal scattered to clustered, brown, incrusted hyphae. Hence, the pileus should be examined under a hand lens before deciding whether or not it is glabrous.
Pileocystidia (Examples: Figs. 69, 75, 83, 94, 108, 116). These structures may be defined as single cells projecting from the pileus surface and in some way differentiated from a simple hyphal tip. Commonly they are clavate, fusoid-ventricose, or capitate, and, in other genera of agarics, other shapes are known. They may originate as outgrowths from the cuticular hyphae, but at times can be traced to connective hyphae in the context. Not infrequently the pileocystidia of a species resemble the cheilocystidia. In a few species the pileocystidia are relatively short, are of uniform height, stand erect, and are close enough together to form a palisade. It is questionable whether the differentiated end-cell of a trichodermial hypha can be properly called a pileocystidium. In summary, it may be stated that the characters of the pileus cutis along with the epicuticular elements do indeed furnish important characters for the recognition and grouping of species in Crepidotus.
Up to this time no extensive systematic work has been published to determine what chemically caused color changes or other reactions may be significant for Crepidotus species. During the summer of 1963, Smith made tests with KOH and FeSO4 on the species available, with encouraging results. Future studies should emphasize this approach. It is only in this manner that an evaluation of chemical characters in the taxonomy of the genus can be made.
In so far as possible during the course of this study, we examined the specimens collected in the fresh state and also studied them microscopically. They were then dried and later restudied. For the latter study, a wedge-shaped piece was cut from a pileus, dipped in alcohol (70-95%) to drive out the air, softened for a few minutes in water, then placed between two pieces of pith, and sectioned so as to give cross sections of the gills that are tangential to the pileus. These sections were then mounted in 2% KOH. At one's discretion, one may add to the mount a drop of basic phloxine, or better still, Congo red.
Dried specimens, as these have accumulated in herbaria, represent a motley arrangement of materials without adequate notes and only someone's guess as to what the species might be. As a result of the present study, we are more aware of this situation than other workers have been. In view of this state of affairs, some recommendations to future investigators are pertinent. Since basidiocarps of Crepidotus are typically small and in texture rather thin, the drying technique using Silica Gel, published by Hoseney (1963), is strongly recommended. Also, notes on odor, taste, colors of all parts, and color changes from handling, as well as any changes resulting from the application of chemicals, should be taken before the specimens are dried. There is no longer any excuse for placing collections in an herbarium and using up valuable storage space when the features of the specimens are not properly recorded. This does not mean that all the previously collected material should be discarded, but it is time to bring up to date our requirements for an acceptable herbarium specimen in the fleshy fungi.
In all, we have examined and recorded observations on about one thousand collections, including sixty-three type collections. We have had about one hundred ten collections from the Southern Appalachians, and some seven hundred from the Middle West, the Rocky Mountains, and the Pacific Coast states. Some collections from Massachusetts and Maine, from Florida, and from the Southwest have also been available to us. We have cited the collections by number. All of Smith's collections are on deposit in the Herbarium at the University of Michigan, Ann Arbor, and all of Hesler's are in the Herbarium of the University of Tennessee at Knoxville. The location for each type studied is given under its respective species. For other collections, as previously stated, the herbaria are indicated by the abbreviations recommended in Index Herbariorum Part I 1959.
Basidiocarps of most species occur on wood or very close to logs and stumps, and are particularly abundant on slash, especially the smaller branches and debris. Though much more abundant on the wood of hardwoods than on conifer wood, certain species may almost cover the decaying tops of fallen conifers, down to branches not more than 5 mm thick. Few species are truly clustered, although at times on large pieces of wood they may fruit in imbricate masses. In some there is considerable amount of mycelium around the base of the basidiocarp, but the amount of humidity appears to influence the degree of its development. There is not much specificity as to substratum in this genus, though aspen (Populus), basswood (Tilia), and maple (species of Acer), in our experience, are the most favorable substrata.
The study of the geographical distribution of Crepidoti is still in its early stages. We hope that the species concepts presented here will serve to advance this phase of our knowledge of the genus. It is eminently clear that the North American species will probably outnumber the European species by a ratio of two to one. This paucity of species in the European flora could very well be caused in part by the European habit of picking up from the forest floor branches for firewood, thus "cleaning up" the forest and removing the most favorable habitats for Crepidotus.
That an extreme reduction in the Crepidotus basidiocarp has resulted in species lacking lamellae, and with basidiocarps more or less cup-shaped, now seems to be generally admitted. Singer (1963: 656) keys some of these genera out in his family Crepidotaceae. The implications of this view in regard to the ancestors of Crepidotus are obvious. The ancestors must be sought among the true gill fungi.
As yet we do not accept the Crepidotaceae of Singer nor his ideas of phylogeny. However, we do not care to propose any counter system with suggested finality. To us the significant features of evolution in the group have not been brought into clear focus. Hence in the following account, we content ourselves with pointing out trends and possibilities.
First, it is obvious that a number of white-spored agarics have gone over to the "Pleurotoid habit" as have, to some extent, almost all species of fungi inhabiting dead wood. Even if we limit ourselves to the fleshy fungi, it is obvious that colored spores have originated "de novo" in a number of Pleurotoid groups; so there is no reason to think that yellow, yellow-brown to earth-brown or cinnamon spores necessarily originated from stipitate agarics of the same spore color. True, this could possibly happen, and in a few instances most assuredly has happened. But yellow to yellow-brown spored species could have arisen as easily from white spored species as have those with lilac spores (Pleurotus ostreatus) or pink spores (Phyllotopsis). One must also keep clearly in mind that it has been shown in culture that white spores (colorless) can originate (have originated) as a mutant from a purple-brown spored species. Thus, species of Crepidotus could just as easily give rise to species of Pleurotus (sensu lato). We have found evidence of such a change occurring in nature in the genus Inocybe (unpublished data of A. H. Smith). We do not believe that Pleurotellus as a genus distinct from Crepidotus will stand the test of close scrutiny, and regard these pale-spored species as a connecting pathway to the small species of Pleurotus, many of which have globose spores. In effect, differences in the color of the spore deposit in these reduced forms may not mean as much as they do in some other groups. We regard it as more comparable to the situation as found in Russula where the color of the spore deposit varies from white to orange.
The second point we would make is that the punctate-ornamented spore of Crepidotus is indicative of an evolutionary line within the genus. Hence resemblances to scattered species in other groups with somewhat similar spores may simply be more cases of parallel development. This point needs to be studied in great detail, particularly with the aid of electron microscopy. The smooth spored species could have easily come, at least in part, from Ramicola (Naucoria centuncula group), as C. haustellaris almost certainly did.
A third point is that a critical study of Crepidotus and Pleurotus should be made to circumscribe stirpes, some of which will almost surely be intermediate between the two genera. When this is done, the pattern of differentiation of these groups will give us a better basis for organizing larger phylogenetic groups in what is assuredly, in our estimation, a polyphyletic aggregation of species. As a genus, however, Crepidotus does and probably will continue to serve a useful place in agaric classification. We consider it most unlikely that any supposed connections to gasteromycetes or to boletes have the slightest validity here.
As for connections to Rhodophyllus, all possibilities need to be considered. Rhodophyllus, in our estimation, can be just as logically derived from Collybia as any other group, especially the lignicolous species with pinkish spore deposits. Pleurotoid Rhodophylli, then, can logically be regarded as reduced forms. This idea fits well into the general pattern of reduction as applied to Crepidotus. But one can still separate the angular, red-spored Pleurotoid species from true Crepidoti with reddish spores by spore outline. In other words, a reddish tint to the spore deposit in species we have recognized here as Crepidotus does not necessarily have any phylogenetic significance in connecting these Crepidoti to Rhodophyllus. It is just as logically regarded as a pigment trend within Crepidotus, just as we have a red trend in spore deposit color in Coprinus, Psathyrella, and Hebeloma.