_c._ COLOUR OF SPERMOGONIA. Though usually the ostiole is visible as a darker point than the surrounding tissue, spermogonia are often difficult to locate unless the thallus is first wetted, when they become visible to slight magnification. They appear as black points in many _Parmeliae_, _Physciae_, _Roccellae_, etc., though even in these cases they are often brown when moistened. They are distinctly brown in some _Cladoniae_, in _Nephromium_, and in some _Physciae_; orange-red or yellow in _Placodium_ and concolorous with the thallus in _Usnea_, _Ramalina_, _Stereocaulon_, etc. [Illustration: 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.