[Illustration: Fig. 8. _Physcia parietina_ De Not. Vertical section of thallus obtained by synthetic culture × 130 (after Bonnier).]
Some years later Bonnier[198] made an interesting series of synthetic cultures between the spores of lichens germinated in carefully sterilized conditions, and algae taken from the open (Figs. 7 and 8). Separate control cultures of spores and algae were carried on at the same time, with the result that in one case lichen hyphae alone, in the other algae were produced. The various lichen spores with which he experimented were sown in association with the following algae:
Pure synthetic cultures of _Physcia_ (_Xanthoria_) _parietina_ were begun in August 1884 on fragments of bark. In October 1886 the thallus was several centimetres in diameter, and some of the lobes were fruited.
_Physcia stellaris_ was also grown on bark; in one case both thallus and apothecia were developed.
_Parmelia acetabulum_, another corticolous species, formed only a minute thallus about 5 mm. in diameter, but entirely identical with normally growing specimens.
_Lecanora_ (_Rinodina_) _sophodes_, sown on rock in 1883, reached in 1886 a diameter of 13 mm. with fully developed apothecia.
_Lecanora ferruginea_ and _L. subfusca_ after three years’ culture formed sterile thalli only.
_Lecanora coilocarpa_ in four years, and _L. caesio-rufa_ in three years formed very small thalli without fructification.
_Opegrapha vulgata_ in two years had developed thallus and apothecia. The control culture of the spores formed, as in nature, a considerable felt of mycelium in the interstices of the bark, but no pycnidia or apothecia.
_Graphis elegans._ Only the beginning of a differentiated thallus was obtained with this species.
_Verrucaria muralis_ (?)[199] gave in less than a year a completely developed thallus.
Bonnier also attempted cultures with species of _Collema_ and _Ephebe_, but was unsuccessful in inducing the formation of a lichen plant.
Reference has already been made to the minute green cells which were originally described by Nylander[200] as occurring in the perithecia of a few Pyrenolichens as free gonidia, _i.e._ unentangled with lichen hyphae. Fuisting[201] found them in the perithecium of _Polyblastia_ (_Staurothele_) _catalepta_ at a very early stage of its development when the perithecial tissues were newly differentiated from those of the surrounding thallus. The gonidia enclosed in the perithecium differed in no wise from those of the thallus: they had become mechanically enclosed in the new tissue; and while those in the outer compact layers died off, those in the centre of the structure, where a hollow space arises, were subject to very active division, becoming smaller in the process and finally filling the cavity. Winter’s[202] researches on similar lichens confirmed Fuisting’s conclusions: he described them as similar to the thalline gonidia but lighter in colour and of smaller size, measuring frequently only 2·3 µ in diameter, though this size increased to about 7 µ when cultivated outside the perithecium.