_a._ SCATTERED SOREDIA. The simplest example of soredial formation may be seen on the bark of trees or on palings when the green coating of algal cells is gradually assuming a greyish hue caused by the invasion of hyphal lichenoid growth. This condition is generally referred to as “leprose” and has even been classified as a distinct genus, _Lepra_ or _Lepraria_. Somewhat similar soredial growth is also associated with many species of _Cladonia_, the turfy soil in the neighbourhood of the upright podetia being often powdered with white granules. Such soredia are especially abundant in that genus, so much so, that Meyer[486], Krabbe[487] and others have maintained that the spores take little part in the propagation of species. The under side of the primary thallus, but more frequently the upright podetia, are often covered with a coating of soredia, either finely furfuraceous, or of larger growth and coarsely granular, the size of the soredia depending on the number of gonidia enclosed in each “head.” Soredia are only occasionally present on the apothecial margins: the rather swollen rims in _Lobaria scrobiculata_ are sometimes powdery-grey, and Bitter[488] has observed soredia, or rather soralia, on the apothecial margins of _Parmelia vittata_; they are very rare, however, and are probably to be explained by excess of moisture in the surroundings. _b._ ISIDIAL SOREDIA. In a few lichens soredia arise by the breaking down of the cortex at the tips of the thalline outgrowths termed “isidia.” In _Parmelia verruculifera_, for instance, where the coralloid isidia grow in closely packed groups or warts, the upper part of the isidium frequently becomes soredial. In that lichen the younger parts of the upper cortex bear hairs or trichomes, and the individual soredia are also adorned with hairs. The somewhat short warted isidia of _P. subaurifera_ may become entirely sorediose, and in _P. farinacea_ the whole thallus is covered with isidia transformed into soralia. The transformation is constant and is a distinct specific character. Bitter[488] considers that it proves that no sharp distinction exists between isidia and soralia, at least in their initial stages. [Illustration: Fig. 82. _Usnea barbata_ Web. Longitudinal section of filament and base of “soredial” branch × 40 (after Schwendener).]