[Illustration: Fig. 33. _Prasiola parietina_ Wille × 500 (after West).] B. CHANGES INDUCED IN THE ALGA _a_. MYXOPHYCEAE. Though, as a general rule, the alga is less affected by its altered life-conditions than the fungus, yet in many instances it becomes considerably modified in appearance. In species of the genus _Pyrenopsis_—small gelatinous lichens—the alga is a _Gloeocapsa_ very similar to _G. magma_. In the open it forms small colonies of blue-green cells surrounded by a gelatinous sheath which is coloured red with gloeocapsin. As a gonidium lying towards or on the outside of the granules composing the thallus, the red sheath of the cells is practically unchanged, so that the resemblance to _Gloeocapsa_ is unmistakable. In the inner parts of the thallus, the colonies are somewhat broken up by the hyphae and the sheaths are not only less evident but much more faintly coloured. In _Synalissa_, a minute shrubby lichen which has the same algal constituent, the tissue of the thallus is more highly evolved, and in it the red colour can barely be seen and then only towards the outside; at the centre it disappears entirely. The long chaplets of _Nostoc_ cells persist almost unchanged in the thallus of the Collemaceae, but in heteromerous genera such as _Pannaria_ and _Peltigera_ they are broken up, or they are coiled together and packed into restricted areas or zones. The altered alga has been frequently described as _Polycoccus punctiformis_. A similar modification occurs in many cephalodia, so that the true affinity of the alga, in most instances, can only be ascertained after free cultivation. Bornet[290] has described in _Coccocarpia molybdaea_ the change that the alga _Scytonema_ undergoes as the thallus develops: in very young fronds the filaments of _Scytonema_ are unchanged and are merely enclosed between layers of hyphae. At a later stage, with increase of the thallus in thickness, the algal filaments are broken up, their covering sheath disappears, and the cells become rounded and isolated. _Petractis_ (_Gyalecta_) _exanthematica_ has also a _Scytonema_ as gonidium, and equally exact observations have been made by Fünfstück[291] on the way it is transformed by symbiosis: with the exception of a very thin superficial layer, the thallus is immersed in the rock and is permeated by the alga to its lowest limits, 3 to 4 mm. below the surface, _Petractis_ being a homoiomerous lichen. The _Scytonema_ trichomes embedded in the rock become narrower, and the sheath, which in the epilithic part of the thallus is 4µ wide, disappears almost entirely. The green colour of the cells fades and septation is less frequent and less regular. The filaments in that condition are very like oil-hyphae and can only be distinguished as algal by staining reagents such as alkanna. They never seem to be in contact with the fungal elements: there is no visible appearance of parasitism nor even of consortism.