In a recent study of a crustaceous sandstone lichen, “_Caloplaca pyracea_,” Claassen[217] has sought to prove a case of pure parasitism. The rock was at first covered with the green cells of _Cystococcus_ sp. Later there appeared greyish-white patches on the green, representing the invasion of the lichen fungus. These patches increased centrifugally, leaving in time a bare patch in the centre of growth which was again colonized by the green alga. The lichen fruited abundantly, but wherever it encroached the green cells were more or less destroyed. The true explanation seems to be that the green cells were absorbed into the lichen thallus, though enough of them persisted to start new colonies on any bare piece of the stone. In the same way large patches of _Trentepohlia aurea_ have been observed to be gradually invaded by the dark coloured hyphae of _Coenogonium ebeneum_. In time the whole of the alga is absorbed and nothing is to be seen but the dark felted lichen. The free alga as such disappears, but it is hardly correct to describe the process as one of destruction. This algal genus _Trentepohlia_ (_Chroolepus_) forms the gonidia of the Graphideae, Roccelleae, etc. It is a filamentous aerial alga which increases by apical growth. In the Graphideae, many of which grow on trees beneath the outer bark (hypophloeodal), the association between the two symbionts may be of the simplest character, but was considered by Frank[218] to be of an advanced type. According to his observations and to those of Lindau[219], the fungal hyphae penetrate first between the cells of the periderm. The alga, frequently _Trentepohlia umbrina_, tends to grow down into any cracks of the surface. It goes more deeply in when preceded by the hyphae. In some species both organisms maintain their independent growth, though each shows increased vigour when it comes into contact with the other. In some instances the cells of the alga are clasped by the fungus which causes the disintegration of the filament. The cells lose their bright yellow or reddish colour and are changed in appearance to greenish lichen gonidia; but no penetration by haustoria has ever been observed in _Trentepohlia_. Bachmann’s[220] study of a similar gonidium in a calcicolous species of _Opegrapha_ confirms Frank’s results. The algae had pierced not only between the looser lime granules but also through a crystal of calcium carbonate, and occupied nests scooped out in the rock by means of acid formed and excreted by their filaments. When association took place with the fungus, the algal cells were more restricted to a gonidial zone; but some of the cells, having been pushed aside by the hyphae, had started new centres of gonidia. On contact with the hyphae there was a tendency to bud out in a yeast-like growth.