They tend to congregate on, and indeed are practically restricted to the better lighted portions of the thallus. On the fronds of foliose forms, they appear, for instance, on the swollen pustules of Umbilicaria pustulata, while in Lobaria pulmonaria, they are mostly lodged in the ridges that surround the depressions in the thallus. In Parmelia conspersa, Urceolaria (Diploschistes) scruposa and some others, they occasionally invade the margins of the apothecium or even the apothecial disc as in Lichina. Forssell[712] found that a spermogonium had developed among cells of Gloeocapsa that covered the disc of a spent apothecium of Pyrenopsis haematopis.
In fruticose lichens such as Usnea, Ramalina, etc. they occur near the apex of the fronds, and in Cladonia they occupy the tips of the ascyphous podetia or the margins of the scyphi. In some Cladoniae, however, spermogonia are produced on the basal squamules, more rarely on the squamules that clothe the podetia.
b. Form and Size. Spermogonia are specifically constant in form, the same type being found on the same lichen species all over the globe. The larger number are entirely immersed and are ovoid or roundish ([Fig. 111] A) or occasionally somewhat flattened bodies (Nephromium laevigatum), or again, but more rarely, they are irregular in outline with an infolding of the walls that gives the interior a chambered form ([Fig. 111] B) (Lichina pygmaea); but all of these are only visible as minute points on the thallus.
Fig. 111. Immersed spermogonia. A, globose in Parmelia acetabulum Dub. × 600; B, with infolded walls in Lecidea (Psora) testacea Ach. × 144 (after Glück).
A second series, also immersed, are borne in small protuberances of the thallus. These very prominent forms are rarely found in crustaceous lichens, but they are characteristic of such well-known species as Ramalina fraxinea, Xanthoria parietina, Ricasolia amplissima, Baeomyces roseus, etc. Other spermogonia project slightly above the level of the thallus, as in Cladonia papillaria and Lecidea lurida; while in a few instances they are practically free, these last strikingly exemplified in Cetraria islandica where they occupy the small projections or cilia ([Fig. 112]) that fringe the margins of the lobes; they are free also in most species of Cladonia.
In size they vary from such minute bodies as those in Parmelia exasperata which measure 25-35 µ in diam., up to nearly 1 mm. in Lobaria laetevirens. As a rule, they range from about 150 µ to 400 µ across the widest part, and are generally rather longer than broad. They open above by a small slit or pore called the ostiole about 20 µ to 100 µ wide which is frequently dark in colour. In one instance, in Icmadophila aeruginosa, Nienburg[713] has described a spermogonium with a wide opening, the spermatiophores being massed in palisade formation along the bottom of a cup-like structure.
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.
