Classification of Tremellales | Fungi

In this article we will discuss about the classification of tremellales.

Family Dacrymycetaceae of Tremellales:

All members of this family are saprobes being almost confined to dead wood, with or without bark. The basidiocarps are mostly gelatinous or waxy, drying down to a thin sheet or horny mass. They vary from thin, broadly effused sheets to cushion-like, cupulate or pileate structure, or clavarioid with smooth or wrinkled surface, coralloid or spathulate upright forms.

The basidiocarps are often bright yellow or orange. The major pigment responsible for the yellow or orange colour characteristic of these fungi is beta-carotene. Pigment formation is dependent on light, but once the pigment is formed remains stable even after the organism is subjected to continuous darkness.

The basidium consists of a cylindrical base—hypobasidium terminally on septate hypha, is elongated and somewhat thicker than the parent hypha.

The hypobasidium forks into two long arms, the epibasidia, each of which terminates into a pointed sterigma on which a sausage-shaped basidiospore is obliquely perched. This is the so-called tuning fork type of basidium.

Genus Dacryopinax(=Guepinia) of Tremellales:

This genus is widely distributed throughout the tropical zones of the world. Most common species is D. spathularia. Species of this genus are commonly found growing on the decaying stumps of wood, in shady and moist environments. The mature specimens can easily be identified by their stipitate, spathulate and petaloid form and the charac­teristic unilateral inferior hymenium.

Basidiocarps are at first discoid, becoming stipitate; pileus obliquely cupulate, spathulate, petaloid, occasionally lobed or somewhat morchelloid (Fig. 277A, B). Stipe is covered with a layer of cylindrical, thick-walled hyphae. Hymenium is unilateral, inferior, smooth forming gill-like folds or occasionally lobed (Fig. 277 O).

Cortex is stiffly gelatinous to cartilaginous homogeneous composed of fibrous thick-walled hyphae with bulbctas septa.

The basidiocarps are erumpent, usually gelatinous when wet, drying to horny consistency but soon reviving when moistened. Pigmentation is variable, yellow and orange predominating, but on drying the colour changes to apricot yellow.

The basidia are intermingled with sterile hyphae in the hymenium (Fig. 277 O). They start as binucleate terminal cells, soon thicken than the rest of the hyphae (Fig. 277C). They are first long cylindrical or somewhat clavate. The two nuclei unite and then divide once stichobasidially, and then a second time in the same direction (Fig. 277D to G).

The branched apex of the basidium becomes lobed to produce two epi-basidia of the same diameter and often of almost the same length as the hypobasidium. Each epibasidium is terminated by a sterigma. The upper two nuclei migrate into the two epibasidia and then through the sterigma at the tip of each into the basidiospores (Fig. 277H).

The two nuclei remaining in the hypobasidium degenerate (Fig. 277 I). The mature basidium takes a tuning fork appearance (Fig. 277J).

Basidiospores are very pale yellow under the hand lens, elliptical to kidney-shaped, uni- to tri-septate (Fig. 277K to N), germinating by conidia. Spore print is light ochraceous buff.

Note:

Dacryopinax, a name substituted for the more familiar but pre-occupied name Guepinia by G. W. Martin (1948).

Some Indian species of Tremellales:

Dacryopinax spathularia (Sch.) Martin; D. ramosa Currey.

Series Hymenomycetes:

The fungi included in the Hymenomycetes are with basidiocarp gymnocarpous or hemiangiocarpous. The basidia are often arranged in a palisade-like fashion to form a hymenium which is fully exposed at maturity, in contrast with the Gasteromycetes where the basidiocarp is angiocarpous. Interspersed among the basidia are sterile struc­tures, usually the paraphyses and sometimes other specialized sterile structures of taxonomic importance.

Basidia of all ages may be found in the hymenium. The basidia vary greatly in shape in the Hymenomycetes. They may be cylindrical, tapering at the very base and rounded at the apex,, or clavate or almost globose. The basidia are aseptate and may be bi-, or tetra-sterigmate. The basidiospores are placed asymmetri­cally on the sterigmata and are discharged violently by the fluid drop mechanism.

The basidiocarps show a very great variability of size and complexity of structure. The simplest form of basidiocarp is a thin crust-like, smooth layer spread over the surface of the substratum with the hymenium fully exposed—resupinate (Fig. 278A).

The basidiocarps developed on the surface of the substratum may have the tendency to have the edges pulled away from the substratum to form—effused-reflexed or effuso-reflexed forms (Fig. 278B). This tendency persists until shelf-like—reflexed or dimidiate (Fig. 278G & D) basidiocarps with little or no resupinate portion are produced.

These reflexed structures vary widely in form (Fig. 278E & F) and may be narrowed toward the point of attachment often producing a distinct stalk-like structure, known as stipe which bears the rest of the tissue containing hymenium—the pileus.

The stipe may be attached eccentrically at the edge or centrally to the under surface of the pileus. All these forms of basidiocarp are known as stipitate (Fig. 278G). The stipe may be missing and the basidiocarp may then be shell-like (conchate) or fan-shaped (flabellate). Other forms are: cupular (cyphelloid) and club-like or coralloid (clavarioid).

Associated with the development of the basidiocarps, the orientation of the hymenium also changes making growth all-round or on two sides— amphigenous; or one-sided—unilateral with a tendency that the hymenium faces toward inferior, especially in the effused-reflexed, reflexed and stipitate forms.

Along with the change in form of the basidiocarps and the orientation of the hymenium, the hymenial surface also undergoes changes to become smooth, wrinkled or toothed, porous, or lamellate. The consistency of the basidiocarp may be fleshy, woody, corky, leathery or papery. So also in colour, the basidiocarp may be white or shades of grey or bright- coloured or almost black, Basidiocarp may be short-lived or may persist for many years.

In the taxonomic studies of the Hymenomycetes importance is given to the hyphal analysis of the basidiocarps. In some of the Hymenomycetes, particularly the Aphyllo- phorales (Polyporales), the hyphae of the tertiary mycelium may be differentiated into three different hyphal types combined in three different hyphal systems.

Hyphal types are- generative hyphae which are branched, thin-walled, usually sep­tate with or without clamp-connections, they give rise to other types of hyphae and the hymenial layer; skeletal hyphae which are branched or unbranched, thick-walled, aseptate, straight or slightly flexuous with thin-walled apices; and binding hyphae which are much branched, thick-walled, aseptate, interwoven, narrow, often coralloid, they bind the generative and skeletal hyphae together (Fig. 279).

The skeletal and binding hyphae build a framework. The generative hyphae produce mediate hyphae which are sparingly branched, thick-walled, aseptate.

The mediate hyphae ultimately give rise to skeletal and binding hyphae, and to the hymenium. These hyphal types are combined into monomitic, dimitic or trimitic hyphal systems. The hyphal system of a basidiocarp is monomitic when it is made of only one kind of hyphae—generative hyphae.

This is generally encountered in most soft and fleshy basidiocarps. Again the hyphal system is known as dimitic when a basidiocarp is composed of two kinds of hyphae—generative and skeletal hyphae. Whereas, it is trimitic having hyphae of three kinds—generative, skeletal and binding hyphae.

Hard and tough basidiocarps may be monomitic with the generative hyphae becoming thick-wailed, dimitic with thick-walled skeletals, or trimitic. Basidiocarps of every species of fungus have a well-defined constant construction, which is maintained regardless of changes in the external morphology of the basidiocarps due to environmental conditions.

Hence the importance is given to the hyphal system. An important distinction between the Agaricales and Aphyllophorales is that the basidiocarps of Agaricales are fleshy being usually composed of thin-walled hyphae which inflate. In contrast, the basidiocarps of many Aphyllophorales may be monomitic, dimitic, or trimitic.

In many Agaricales the developing basidiocarp is surrounded by one or more veils, but these are not present in the Aphyllophorales. A further distinction is that in the Agaricales the nuclear spindles in the basidia are transverse (chiastobasidial) whilst in the Aphyllophorales forms with longitudinally oriented spindles (sticho- basidial) occur.