_b._ CHLOROPHYCEAE. As a rule the green-celled gonidium such as _Protococcus_ is not changed in form though the colour may be less vivid, but in certain lichens there do occur modifications in its appearance. In _Micarea_ (_Biatorina_) _prasina_, Hedlund[292] noted that the gonidium was a minute alga possessing a gelatinous sheath similar to that of a _Gloeocapsa_. He isolated the alga, made artificial cultures and found that, in the altered conditions, it gradually increased in size, threw off the gelatinous sheath and developed into normal _Protococcus_ cells, measuring 7 to 10µ in diameter. The gelatinous sheath was thus proved to be merely a biological variation, probably of value to the lichen owing to its capacity to imbibe and retain moisture. Zukal[293] also made cultures of this alga, but wrongly concluded it was a _Gloeocystis_. Moebius[294] has described the transformation from algae to lichen gonidia in a species epiphytic on Orchids in Porto Rico. He had observed that most of the leaves were inhabited by a membranaceous alga, _Phyllactidium_, and that constantly associated with it were small scraps of a lichen thallus containing isolated globose gonidia. The cells of the alga, under the influence of the invading fungus, were, in this case, formed into isolated round bodies which divided into four, each daughter-cell becoming surrounded by a membrane and being capable, in turn, of further division. Frank[295] followed the change from a free alga to a gonidium in _Chroolepus_ (_Trentepohlia_) _umbrinum_, as shown in the hypophloeodal thalli of the Graphideae. The alga itself is frequent on beech bark, where it forms wide-spreading brownish-red incrustations consisting of short chains occasionally branched. The individual cells have thick laminated membranes and vary in width from 20 µ to 37 µ. The free alga constantly tends to penetrate below the cortical layers of the tree on which it grows, and the immersed cells become not only longer and of a thinner texture, but the characteristic red colour so entirely disappears, that the growing penetrating apical cell may be light green or almost colourless. As a lichen gonidium the alga undergoes even more drastic changes: the red oily granules gradually vanish and the cells become chlorophyll-green or, if any retain a bright colour, they are orange or yellow. The branching of the chains is more regular, the cells more elongate and narrower; usually they are about 13 to 21 µ long and 8 µ wide, or even less. Deeper down in the periderm, the chains become disintegrated into separate units. Another notable alteration takes place in the cell-membrane which becomes thin and delicate. It has, however, been observed that if these algal cells reach the surface, owing to peeling of the bark, etc., they resume the appearance of a normal _Trentepohlia_.