Order. Helvellaceæ. Spathulea is yellow and club-shaped, and forms “fairy rings” in woods.—Geoglossum (Earth-tongue) projects above the ground as a black tongue, or as a club-shaped body. Several species are found in meadows and on heaths.—Helvella has a stalk, bearing an irregularly folded head, on the external surface of which is the hypothecium.—Morchella (Morell, Fig. [130]), the stalk bears on its summit the conical or spherical head, the external surface of which is reticulate and bears the asci.—Mitrula. Verpa.
Fig. 130.—Morchella esculenta: a an entire specimen, about one half natural size; b longitudinal section through the head.
Appendix to the Ascomycetes:
Family 7. Ascolichenes (Lichen-forming Ascomycetes).
The Lichens were formerly classed among the Thallophyta as a group quite distinct from the Algæ and Fungi. Investigations during the last twenty-five years, however, have conclusively proved that the Lichens are Fungi which reproduce in the same manner as the Ascomycetes, or, more rarely, the Basidiomycetes, and have entered into a peculiar symbiotic relation with Algæ, especially the Cyanophyceæ and Protococcoideæ, with which they associate, and without which they would be unable to exist. The Fungus forms the largest portion of the Lichen, enclosing the Alga with which it may be said to be commensal. The Fungus especially produces reproductive bodies and absorbs the inorganic nourishment through the rhizoids, whilst the Alga supplies it with the organic materials. In consequence of this the Lichens, in contradistinction to other Fungi, need light for the development of their nutritive organs, and are therefore, in any case internally, of a more or less greenish colour. The form and condition of the thallus is unusual among the Fungi, and they can grow upon rocks and in other places where no dead organic matter, such as would be required by other Fungi, is obtainable.
Fig. 131.—Transverse section through the thallus of Sticta fuliginosa (× 500): r-r rhizoid-strands, which arise from the under side; g-g gonidial layer; m medullary layer; o upper, u lower cortex.
Two cellular forms are therefore to be found in each Lichen:
1. The cells which belong to the Fungus. These are generally septate, branched hyphæ without any trace of chlorophyll. In the thallus of the majority of Lichens there may be found a medullary layer (Fig. [131] m) of loosely-woven hyphæ, between which there are large air chambers; and an external layer (cortex) (Fig. [131] o, u) formed of closely-woven hyphæ without any intercellular spaces. In some Lichens (Collemaceæ) the hyphæ wind about in the thallus, being equally distributed throughout, without forming any decided strata. These Lichens moreover become gelatinous when exposed to moisture (Fig. [132]), on account of the swelling of the walls of the Algæ. The hyphæ contain protoplasm with drops of oil, but never starch; their walls easily swell when exposed to damp after having been dried, and in some (e.g. Cetraria islandica) they become gelatinous when cooked. Certain strata of hyphæ become blue on treatment with iodine alone, from which it is inferred that the wall is allied, in its chemical nature, to starch.