208 Proceedings of the Royal Irish Academy. 



phenomenon occurs in other species where one thallus lobe is closely pressed 

 to the surface of another or has not become completely separated from the 

 parent lobe. 



Iu tig. 4. a section of P. a mliata, the gonidial layer is still 



evident below the covering lobe, showing that the upper lobe was in process 

 of growing out along the surface of the lower one. rather than that the lower 

 one represents the younger growth. 



It is these connecting masses of tissue which unite the thallus into the 

 coii.; . >wth which is one of its charach - The mider-surface is not 



only papillose but also corrugated, even when moist, the ridges being out- 

 growth- of the lower cortex with a core of medullary tissue. 



•".. / 1 1.. i. 



The black under- '.as rhizines reaching to the edge. To the naked 



eye the lower cortex ; an aim th surface, but on examination 



it i? found to be rug. even papillose. Ii stripped From the thallus.it 



i dotted appearam ,■ and a brown i olour. In transverse section it is seen 

 to consist of a matted felt of Bhort hyphal endings at right angles to the 

 surface, th-- transition from the medullary tissue with its horizontally directed 

 hyphae I _ irp. 



The rhizines are very numerous and well developed, and are clearly com- 

 posed of more or less branched strands of hyphae which inn together from 

 the lowei I and medulli -. forming in the young state a papilla 



' whi b in length and thickness till it reaches the Bubstratum. Here it 



inds, the apical hyphae epi - more or less radially to form an out- 



. th which, when torn from its support, app< cup or disc dig. 5). 



wtlis continue until a complete layer i- formed covering the 

 substratum. This layer would presumably be included among the hypothal- 

 line structures of Zukal (Zahlbruckner), 1 and is wi gether by lichen 



detritus and exci _ - nixed bark-cells, and all the usual 



: such la;. 

 In fig. 6, the irk of alder is shown. The apical 



hyp: have united to form not only a hypothalline layer, 



but also an eroding, hapten! mas.> from which mycelial strands run in all 

 directions. 



Iu tig. 7 a single lhizine is shown, having covered and filled up an 

 interspace in the bark of larch. 



a •> shows the normal action of the rhizines in forming a hypothalline 



1 Zahlbruckner, lw 



