Lichens

by Annie Lorrain Smith

(6) The hyphae from the ascogonial cells spread out in a complex layer at the base of the hymenium, and send up branches which form the asci, either, as in most Ascomycetes, from the penultimate cell of the fertile branch, or from the last cell, as in Sphyridium (Baeomyces rufus)[575] and in Baeomyces roseus. The same variation has been observed in fungi—in a species of Peziza[577], in which it is the end-cell of the branch that becomes the mother-cell of the ascus; but this deviation from the normal is evidently of rare occurrence either in lichens or fungi.

d. Hypothecium and Paraphyses. The hypothecium is the layer of hyphae that subtends the hymenium, and is formed from the complex of hyphae that envelope the first stages of the carpogonia. It is vegetative in origin and distinct from the generative system.

In lichens belonging to the Collemaceae, the paraphyses rise from the branching of the carpogonial stalk-cell immediately below the ascogonium[578], but have no plasma connection with it. They are thus comparable in origin with the paraphyses of many Discomycetes.

In several genera in which the algal constituents are blue-green, such as Stictina, Pannaria, Nephroma, Ricasolia and Peltigera, Sturgis[579] found that reproduction was apogamous and also that asci and paraphyses originated from the same cell-system: a tuft of paraphyses arose from the basal cell of the ascus, or an ascus from the basal cell of a paraphysis. These results are at variance with those of most other workers, but the figures drawn by Sturgis seem to be clear and convincing.

Again in Usnea barbata, as described by Nienburg[580], the ascogonial cells, after the disappearance of the trichogyne, branch profusely not only upwards towards the cortex but also downwards and to each side. The upward branches give rise normally to the asci, the lower branches produce the subhymenium and later the paraphyses, and the two systems are thus genetically connected, though they remain distinct from each other, and asci are never formed from the lower cells.

In most heteromerous lichens, however, the origin of the paraphyses is exclusively vegetative: they arise as branches from the primordial complex that forms the covering hyphae of the ascogonium both above and below. Schwendener[581] had already pointed out the difference in origin between the two constituents of the hymenium in one of his earlier studies on the development of the apothecium, and this view has been repeatedly confirmed by recent workers, except by Wahlberg[582] who has insisted that they rise from the same cells as the asci, a statement disproved by Baur[583]. The paraphyses originate not only from the covering hyphae, but from vegetative cells in close connection with the primordium. In this mode of development, lichens diverge from fungi, but even in these a vegetative origin for the paraphyses has been pointed out in Lachnea scutellata[584] where they branch from the hyphae lying round the ascogonium.

There is no general rule for the order of development. In Lecanora subfusca Baur[583] found that vertical filaments had reached the surface by the time the trichogyne was formed, and their pointed brown tips gave a ready clue to the position of the carpogonia. In Lecidea enteroleuca[585] they show their characteristic form and arrangement before there is any trace of ascus formation. In Solorina[585] they are well formed before the ascogenous hyphae appear. In other lichens such as Placodium saxicolum[586], Peltigera rufescens[587] and P. malacea[587] the two systems—paraphyses and ascogonium—grow simultaneously, though in P. horizontalis the ascogonium has disappeared by the time the paraphyses are formed. In the genus Nephroma, in Physcia stellaris and in Xanthorina parietina the paraphyses are also late in making their appearance.

In most instances, the paraphyses push their way up between the cortical cells which gradually become absorbed, or they may stop short of the surface as in Nephromium tomentosum[587]. The overlying layer of cortical cells in that case dies off gradually and in time disappears. Such an apothecium is said to be “at first veiled.” Later formed paraphyses at the circumference of the apothecium form the parathecium, which is thus continuous with the hypothecium.

e. Variations in apothecial Development. Lichens are among the least stereotyped of plants: instances of variation have been noted in several genera.

aa. Parmeliae. A somewhat complicated course of development has been traced by Baur[588] in Parmelia acetabulum. In that lichen the group of three to six carpogonia do not lie free in the gonidial tissue, but originate nearer the surface ([Fig. 96]) and are surrounded from the first by a tissue connected with, and resembling the tissue of the cortex. In the several ascogonia, there are more cells and more spirals than in Collema or in Physcia, and all of them are somewhat confusedly intertwined. The trichogynes are composed of three to five cells and project 10 to 15µ above the surface. When further development begins, the ascogonial cells branch out and form a primary darker layer or hypothecium above which extends the subhymenium, a light-coloured band of loosely woven hyphae. Branches from the ascogonial hyphae at a later stage push their way up through this tissue and form above it a second plexus of hyphae—the base of the hymenium. Baur considers this a very advanced type of apothecium; he found it also present in Parmelia saxatilis, though, in that species, the further growth of the first ascogonial layer was more rapid and the secondary plexus and hymenium were formed earlier in the life of the apothecium. He has also stated that a similar development occurs in other genera such as Usnea, though Nienburg’s[589] work scarcely confirms that view.