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. [Illustration: 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.