64 CONSTITUENTS OF THE LICHEN THALLUS 



Peltidea section of the genus. In this connection, however, we must bear in 

 mind Forssell's view that it is the Archilichens that are the more primitive'. 

 The alien blue-green algae with their gelatinous sheaths are adapted to 

 the absorption and retention of moisture, and, in this way, they doubtless 

 render important service to the lichens that harbour them in cephalodia. 



D. Displacement of Algae within the Thallus 



a. Normal displacement. Lindau^ has contrasted the advancing 

 apical growth of the creeping alga Trentepohlia with the stationary condition 

 of the unicellular species that multiply by repeated division or by sporulation, 

 and thus form more or less dense zones and groups of gonidia in most 

 lichens. The fungus in the latter case pushes its way among the algae and 

 breaks up the compact masses by a shoving movement, thus letting in light 

 and air. The growing hypha usually applies itself closely round an algal 

 cell, and secondary branches arise which in time encircle it in a network of 

 short cells. In the thallus of Variolaria^ the hyphae from the lower tissues, 

 termed push-hyphae by Nienburg", push their way into the algal groups and 

 filaments composed of short cells come to He closely round the individual 

 gonidia. Continued growth is centrifugal, and the algae are carried outward 

 with the extension of the hyphae (Fig. 12). Cell-division is more active at the 

 periphery, that being the area of vigorous growth, and the algal cells a«@5in 

 consequence, generally smaller in that region than those further back, the 

 latter having entered more or less into a resting condition, or, as is more 

 probable, these smaller cells are apknospores not fully mature. 



b. Local displacement. Specimens of Parmelia physodes were found 

 several times by Bitter, the grey-green surface of which was marbled with 

 whitish lines, caused by the absence of gonidia under these lighter-coloured 

 areas. The thallus was otherwise healthy as was manifested by the freely 

 fruiting condition : no explanation of the phenomenon was forthcoming. 

 Bitter compared the condition with the appearance of lighter areas on the 

 thallus of Parmelia obscurata. 



Something of the same nature was observed on the thallus of a Peltigera 

 collected by F. T. Brooks near Cambridge. The marking took the form of 

 a series of concentric circles, starting from several centres. The darker lines 

 were found on examination to contain the normal blue-green algal zone, 

 while the colour had faded from the lighter parts. The cause of the difference 

 in colouration was not apparent. 



' See Chap. VII. ^ Lindau 1895. ' Darbishire 1897. * Nienburg 1917. 



