In certain cases where two kinds of algae were supposed to be present in some lichens, it has been proved that one species only is represented, the difference in their form being caused by mechanical pressure of the surrounding hyphae, as in _Endocarpon_ and _Staurothele_ where the hymenial gonidia are cylindrical in form and much smaller than those of the thallus. They were on this account classified by Stahl[296] under a separate algal genus, _Stichococcus_, but they are now known to be growth forms of _Protococcus_, the alga that is normally present in the thallus. Similar variations were found by Neubner[297] in the gonidia of the Caliciaceae, but, by culture experiments with the gonidia apart from the hyphae, he succeeded in demonstrating transition forms in all stages between the “_Pleurococcus_” cells and those of “_Stichococcus_,” though the characters acquired by the latter are transmitted to following generations. The transformation from spherical to cylindrical algal cells had been also noted by Krabbe[298] in the young podetia of some species of _Cladonia_, the change in form being due to the continued pressure in one direction of the parallel hyphae. Isolated algal cells have been observed within the cortex of various lichens. They are carried thither by the hyphae from the gonidial zone in the process of cortical formation, but they soon die off as in that position they are deprived of a sufficiency of air and of moisture. Forssell[299] found _Xanthocapsa_ cells embedded in the hymenium of _Omphalaria Heppii_. They were similar to those of the thallus, but they were not associated with hyphae and had undergone less change than the thalline algae. C. CONSTANCY OF ALGAL CONSTITUENTS