. Lichens. Lichens. 48 CONSTITUENTS OF THE LICHEN THALLUS no further algae were encountered. The cell-walls of the swollen hyphae and their branches had begun to thicken and to become united to form a kind of cellular tissue or ''paraplectenchyma\" At a later date, about a month. Fig. 15. Synthetic culture of Physcia parietina spores and ProCococcus viridis five days after germination, s, lichen-spore; a, septate fila- ments; b-, clasping filaments;'f, searching filaments, x 500 (after Bonnier). after the sowing of the spores, there was a definite cellular cortex formed over the thallus.
. Lichens. Lichens. 48 CONSTITUENTS OF THE LICHEN THALLUS no further algae were encountered. The cell-walls of the swollen hyphae and their branches had begun to thicken and to become united to form a kind of cellular tissue or ''paraplectenchyma\" At a later date, about a month. Fig. 15. Synthetic culture of Physcia parietina spores and ProCococcus viridis five days after germination, s, lichen-spore; a, septate fila- ments; b-, clasping filaments;'f, searching filaments, x 500 (after Bonnier). after the sowing of the spores, there was a definite cellular cortex formed over the thallus. The hyphal cells are uninucleate, though in the medulla they may be 1-2-nucleate. The hyphae in close contact with the gonidia remain thin-walled, and termed by Wainio^ ";' They furnish the growing elements of the lichen either apical or intercalary. In most genera the organs of fructification take rise from them, or in their immediate neighbourhood, and isidia and soredia also originate from these gonidial hyphae. As the filaments pass from the gonidial zone to other layers, the cell- walls become thicker with a consequent reduction of the cell-lumen, very noticeable in the pith, but carried to its furthest extent in the "decomposed" cortex where the cells in the degenerate tissue often become reduced to dis- connected streaks indicating the cell-lumen, and the outer cortical layer is merely a continuous mass of mucilage. All lichen tissues arise from the branching and septation of the hyphae, the septa always forming at right angles to the long axis of the filaments! There is no instance of longitudinal cell-division except in the spores of certain genera {Collema, Urceolaria, Polyblasiia, etc.). The branching of the hypha is dichotomous or lateral, and very irregular. Frequent septation and coherent growth result in the formation of plectenchyma. ^ Term coined by Lindau (1899) to describe the pseudo-cellular tissue of Hchens and fun
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