Part 28

About 2,000 species of Lichens have been described. If we disregard the Basidiolichenes, which will be considered on page 176, the remaining Lichens (Ascolichenes) may be divided into the two following orders according to the structure of the fruit-bodies:--

Order 1. =Pyrenolichenes.= The ascocarps (apothecia) are spherical or flask-shaped, as in the Pyrenomycetes, more rarely linear (Graphis).

According to the nature of the thallus, these Lichens may be divided into:--

a. Thallus homoiomerous, but not gelatinous, branching according to the mode of growth of the Algæ: Ephebe (Fig. 133), with Algæ of the genus Stigonema.

b. Thallus homoiomerous, gelatinous: Lichina.

c. Thallus heteromerous, crustaceous: Verrucaria, Pyrenula; Graphis (Fig. 139), which may be considered as Hysteriaceæ with gonidia; several species of Graphis are common on bark.

d. Thallus heteromerous, foliaceous: Endocarpon.

e. Thallus heteromerous, fruticose: Sphærophorus.

Order 2. =Discolichenes.= These, as in the Discomycetes, have open apothecia, which, as a rule, are cupular, more rarely hemispherical (Cladonia).

According to the nature of the thallus, these Lichens may be divided into:--

a. Thallus homoiomerous, but not gelatinous, branching according to the mode of growth of the Algæ: Cœnogonium.

b. Thallus homoiomerous, gelatinous: Collema (Fig. 132), with Algæ of the genus Nostoc; Leptogium.

c. Thallus heteromerous, crustaceous: Pertusaria (Fig. 140), Lecidea, with apothecia open from the beginning; Lecanora, with apothecia, which in the beginning are closed, later on open, but with a rim formed by the thallus (Fig. 138); Bæomyces, whose apothecia are borne on a stem formed by the thallus.

d. Thallus heteromerous, foliaceous: Parmelia (P. saxatilis; P. parietina, Wall-Lichen, Fig. 141, is yellow, very frequent on tree-stems, stone-walls, tiles); Physcia (P. ciliaris, frequent on tree-stems); Sticta (S. pulmonacea, Lung-Lichen, on tree-stems); Peltigera, especially on the Moss among trees; Umbilicaria, on rocks.

e. Thallus heteromerous, fruticose: Cetraria (C. islandica), “Iceland Moss,” with an olive-brown, flat, furrowed, fringed thallus, on heaths; C. nivalis, white, in the Polar regions; Evernia, Ramalina, Usnea (U. barbata, Beard-Lichen, Fig. 143); Roccella, Stereocaulon, Cladonia, of which the genus C. rangiferina, Reindeer-Moss (Fig. 142) is important; Cladonia has two kinds of thallus, one scaly and leaf-like, the other erect, which bears the apothecia and may be fruticose (Fig. 142), or cupular (Fig. 144); they grow in soil in forests and on heaths.

Sub-Class 2. =Basidiomycetes.=

This sub-class embraces the most highly developed Fungi, with large “fruit-bodies,” which in ordinary language we shortly term Funguses, Toadstools, or Mushrooms.

They have no sporangia, but reproduce only by means of basidiospores, conidia, chlamydospores and oidia. The chief characteristic of this sub-class is the basidium (Fig. 145), i.e. the conidiophore, which has a distinctive form, and bears a definite number (generally 4) of characteristically shaped conidia (basidiospores, Fig. 145 c, d, e).

The summit of each basidium is produced generally into four conical points (sterigmata, Fig. 145 b), from each of which a basidiospore is abstricted. The basidia may be classified into three principal groups, each of which accompanies a distinctive conidiophore: 1, the long, filamentous, transversely divided basidia, with lateral sterigmata and spores, found in the Uredinaceæ (Figs. 146 D, 153), Auriculariaceæ (Fig. 160 B), and Pilacraceæ; 2, the spherical, longitudinally divided basidia of the Tremellaceæ (Figs. 160 C d; 161 iii. iv.); and 3, the ovoid, or cylindrical, undivided basidia of the Autobasidiomycetes (Figs. 145, 163, etc.); the two last have apical sterigmata and spores.

The first two groups are the septate basidia (protobasidia), of the Protobasidiomycetes; while the unseptate basidia (autobasidia) of the Autobasidiomycetes are the third group. On the formation of the basidiospores, the nucleus of the basidium divides into four nuclei, each of which is transferred to a spore.

In addition to the basidia, simple conidiophores are also found. In the Protobasidiomycetes, the simple conidia are very generally found as accessory methods of reproduction in conjunction with the basidiospores; but less frequently in the Autobasidiomycetes, e.g. among the Dacryomycetes, Tomentellaceæ, Heterobasidion annosum.

The simple conidiophores vary in size, and in the number and shape of the conidia; they, however, resemble the basidia, and are doubtless an early stage in the development of the definitely formed basidia.

Finally, well-defined chlamydospores, formed in various ways, appear in the Basidiomycetes as supplementary reproductive bodies (compare p. 90). Among the Protobasidiomycetes, chlamydospores are at present only found among the Uredinaceæ, but in various forms; in the majority of families of the Autobasidiomycetes oidia frequently occur (Fig. 162), but genuine chlamydospores seldom.

In the same species several of the known forms of reproduction may be distinguished.

The mycelium is generally composed of white, branched strands, consisting of numerous felted hyphæ; in some, sclerotia are found.--The great majority are saprophytes; some (particularly all the Uredinaceæ), are parasites.

DIVISIONS OF THE BASIDIOMYCETES.

Series 1. PROTOBASIDIOMYCETES: partly gymnocarpic, partly angiocarpic. „ 2. AUTOBASIDIOMYCETES. Family 1. DACRYOMYCETES: gymnocarpic. „ 2. HYMENOMYCETES: partly gymnocarpic, partly hemiangiocarpic. „ 3. PHALLOIDEÆ: hemiangiocarpic. „ 4. GASTEROMYCETES: angiocarpic. Appended. BASIDIOLICHENES: Lichen-forming basidiomycetes.

Series I. =Protobasidiomycetes.=