STRATOSE THALLUS 



93 



under surface or are confined to special areas. RosendahP has described 

 their development in the brown species of Parmeliae: the under cortex in 

 these lichens is formed of a cellular plectenchyma with thickish walls ; the 

 rootlets arise by the outgrowth of several neighbouring cells from some slight 

 elevation near the edge of the thallus. Branching and interlacing of these 

 growing rhizinal hyphae follow, the outermost frequently spreading outwards 

 at right angles to the axis, and forming a cellular cortex. The apex of the 

 rhizoid is generally an enlarged tuft of loose hyphae involved in mucilage 

 (Fig. 53), a provision for securing firmer cohesion to the support; or the 

 tipsspread out as a kind of sucker. Not unfrequently neighbouring "rootlets" 

 are connected by mucilage at the tips, or by outgrowths of their hyphae, 

 and a rather large hold-fast sheath is formed. 



In species of Peltigera (Fig. 54) the rhizinae are confined to the veins 

 or ridges (Fig. 55); they are thickish at the base, and are generally rather 



^^i*^'-^ 





Fig. 54. Peltigera cmiina DC. (S. H., Photo.]. 



Fig- 55- Peltigera canina DC. 

 Under surface with veins and 

 rhizoids (after Reinke). 



long and straggling. Meyer^ states that the central hyphae are stoutish 

 and much entangled owing to the branching and frequent anastomosis of 

 one hypha with another; the peripheral terminal branches are thinner-walled 

 and free. These rhizinae vary in colour from white in Peltigera canina to 

 brown or black in other species. Most species of Peltigera spread over grass 

 or mosses, to which they cling by these long loose "rootlets." 



Lichen rhizinae, distinguished by Reinke^ as "aerial rhizinae," are more 



' Rosendahl 1907. 



2 Meyer igoi. 



3 Reinke 18 



186. 



