Fig. 112. Free spermogonia in spinous cilia of Cetraria islandica Ach. A, part of frond; B, cilia. × 10.
D. Structure
a. Origin and Growth. The spermogonia (or pycnidia) of lichens when mature are more or less hollow structures provided with a distinct wall or “perithecium,” sometimes only one cell thick and then not easily demonstrable, as in Physcia speciosa, Opegrapha vulgata, Pyrenula nitida, etc. More generally the “perithecium” is composed of a layer of several cells with stoutish walls which are sometimes colourless, but usually some shade of yellow to dark-brown, with a darker ostiole. The latter, a small slit or pore, arises by the breaking down of some of the cells at the apex. After the expulsion of the spermatia, a new tissue is formed which completely blocks up the empty spermogonium. In filamentous lichens such as Usnea a dangerous local weakening of the thallus is thus avoided.
Spermogonia originate from hyphae in or near the gonidial zone. The earliest stages have not been seen, but Möller[714] noted as the first recognizable appearance or primordium of the “pycnidia” in cultures of Calicium trachelinum a ball or coil of delicate yellowish-coloured hyphae. At a more advanced stage the sporophores (or spermatiophores) could be traced as outgrowths from the peripheral hyphae, directed in palisade formation towards the centre of the hyphal coil about 20-30 µ long and very slender and colourless. They begin to bud off spermatia almost immediately, as it has been found that these are present in abundance while the developing spermogonium is still wholly immersed in the thallus. Meanwhile there is gradually formed on the outside a layer of plectenchyma which forms the wall. Additional spermatiophores arise from the wall tissue and push their way inwards between the ranks of the first formed series. The spermogonium slowly enlarges and stretches and as the spermatiophores do not grow any longer a central hollow arises which becomes packed with spermatia (or spores) before the ostiole is open.
A somewhat similar process of development is described by Sturgis[715] in the spermogonia of Ricasolia amplissima, in which species the primordium arises by a profuse branching of the medullary hyphae in certain areas close to the gonidial zone. The cells of these branching hyphae are filled with oily matter and gradually they build up a dense, somewhat cylindrical body which narrows above to a neck-like form. The growth is upwards through the gonidial layer, and the structure widens to a more spherical outline. It finally reaches the outer cortex when some of the apical cell membranes are absorbed and a minute pore is formed. The central part becomes hollow, also by absorption, and the space thus left is lined and almost filled with multicellular branches of the hyphae forming the wall; from the cells of this new tissue the spermatia are abstricted.
b. Forms and types of Spermatiophores. The variations in form of the fertile hyphae in the spermogonium were first pointed out by Nylander[716] who described them as sterigmata[717]. He considered the differences in branching, etc. as of high diagnostic value, dividing them into two groups: simple “sterigmata” (or spermatiophores), with non-septate hyphae, and arthrosterigmata, with jointed or septate hyphae.
Simple “sterigmata” comprise those in which the spore or spermatium is borne at the end of a secondary branch or sterigma, the latter having arisen from a cell of the upright spermatiophore or from a simple basal cell. The arthrosterigmata consist of “short cells almost as broad as they are long, much pressed together, and appearing almost agglutinate especially toward the base; they fill almost the whole cavity of the spermogonium.” The arthrosterigmata may grow out into the centre of the cavity as a single cell-row, as a loose branching network, or, as in Endocarpon, they may form a tissue filling the whole interior. Each cell of this tissue that borders on a cavity may bud off a spermatium either directly or from the end of a short process.
Fig. 113 A. Types of lichen “sporophores” and pycnidiospores. 1, Peltigera rufescens Hoffm. × 910; 2,Lecidea (Psora) testacea Ach. × 1200; 3,Cladonia cariosa Spreng. × 1000; 4, Pyrenula nitida Ach. × 1130; 5, Parmelia tristis Nyl. × 700; 6, Lobaria pulmonaria Hoffm. × 1000 (after Glück).
The most important contributions on the subject of spermogonia in recent years are those of Glück[718] and Steiner[719]. Glück, who insisted on the “pycnidial” non-sexual character of the organs, recognized eight types of “sporophores” differing in the complexity of their branching or in the form of the “spores” ([Fig. 113 A]):