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 1 ." At a later date, about a month 



Fig. 15. Synthetic culture of Physcia parictina spores and Protococcus 

 viridis five days after germination, s, lichen-spore ; a, septate fila- 

 ments ; b, clasping filaments; c, 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 i-2-nucleate. 



The hyphae in close contact with the gonidia remain thin-walled, and 

 have been termed by Wainio 2 "meristematic." 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, Polyblaslia, etc.). The branching of the 

 hypha is dichotomous or lateral, and very irregular. Frequent septation and 

 coherent growth result in the formation of plectenchyma. 



1 Term coined by Lrndau (1899) to describe the pseudo-cellular tissue of lichens and fungi now 

 referred to as "plectenchyma." 2 Wainio 1897. 



