62 CONSTITUENTS OF THE LICHEN THALLUS 



Moebius^ 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 con- 

 taining 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^ followed the change from a free alga to a gonidium in Chroolepus 

 {Trentepohlia) umbrimun, 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 fx to 37/a. 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 under- 

 goes 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 2i/t long and 8/i 

 wide, or even less. Deeper down in the periderm, the chains become dis- 

 integrated 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. 



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 sur- 

 rounding 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^ under a separate algal genus, 

 Stichococcus, but they are now known to be growth forms of Proiococcus the 

 alga that is normally present in the thallus. Similar variations were found 

 by Neubner' 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 " P leurococcus " cells and those of 

 " Stichococcus," though the characters acquired by the latter are transmitted 

 to following generations. The transformation from spherical to cylindrical 



1 Moebius 1888. ^ pjank 1876, p. 158. ^ Stahl 1877. " Neubner 1893. 



