RADIATE THALLUS 



109 



afforded by the lenticels. The sheath hyphae are continuous with the medul- 

 lary hyphae of the frond, and gonidia are frequently enclosed in the tissues; 

 the sheath spreads to some extent over the surface of the bark, and round 

 the base of the fronds, thus rendering the attachment of the lichen to the 

 tree doubly secure. 



Avaong Ramalinae, the development of the base was followed by Brandt^ 

 in one species, R. Landroensis, an arboreal lichen from S. Tyrol. A rosette- 

 like sheath was formed consisting solely of strands of thick-walled hyphae 

 which spread over the bark. There were no gonidia included in the tissue. 



A different type of attachment was found by Lilian Porter'-' in corti- 

 colous Ramalinae — R. fraxinea, R. fastigiata, and R. pollinaria. The lichens 

 were anchored to the tree by strands of closely compacted hyphae longi- 

 tudinally arranged and continuous with the cortical hyphae. These enter 

 the periderm of the tree by cracks or lenticels, and by wedge action cause 

 extensive splitting. The strands may also spread horizontally and give rise 

 to new plants. The living tissues of the tree were thus penetrated and 

 injured, and there was evidence that hypertrophied tissue was formed and 

 caused erosion of the wood. 



Several Ramalinae — R. siliquosa, R. Ciirnowii, etc. — grow on rocks, 

 often in extremely exposed situations, in isolated tufts or in crowded swards 

 (Fig. 65). The separate tufts are not unfrequently connected at the base by 



Fig. 65. Rajnalina siliqttosa A. L. Sm., on rocks, reduced {M. P., Photo.). 



' Brandt 1906. 



2 Porter 1916. 



