BY JOHN SHIKLEY, D.SC. Gl 



the outer layer in solid rhizinte, and the outer and inner 

 layers in hollow rhizinse. Like the hyphal threads of the 

 medulla which adjoin them, the cells of the lower cortex 

 are arrainged parallel to the surface, except in the neigh- 

 bourhood of a rhizina, where they turn parallel to its 

 walls. 



The cellules of the lower cortex are equal in size to 

 those of the upper layer in lichens of the first and second 

 divisions, but in the third type, exemplified in P. tinctorutn 

 and P. Jaxa, the cells of the lower layer are larger, and 

 possess thicker walls than those of the upper rind. 



Where the cell-wall is thinner than usual, and less 

 provided with the dense brown colour, as in the lower 

 cortex of P. tiliacea, the cellules are seen to be uninuc- 

 leate OT binucleate. 



In P. perlata the two cortices are of equal thickness, 

 but the cellules of the upper layer are oblique to the 

 surface, while these of the lower layer are parallel to the 

 substratum. Parmelia iiierforata differs from all others 

 of its genus in having the cellules of both the lower and 

 upper cortices set at right angles to the horizontal surface. 

 The long axis of the cell lumen is the guide in determin- 

 inp- the direction of the cellules. 



3. The Hypha;. 



These are best studied in ParmeJia placorhodioides 

 (Plate v., fig 8), in which they are the largest of the 

 genus, rea.ching .002 in transverse diameter. As a rule 

 they lie in the thallus longitudinally, but, as before stated, 

 they turn at right angles to the medulla in the algal or 

 gonidial layer, and reach the upper surface in various 

 ways as described in the last subdivision. Each hypha 

 is multinucleate, and, by long staining in Delafield's hae- 

 matoxylin, the nuclei may be made visilDle in many sipecies. 

 The medullary hyphae of P. pJacorlwdioides and P. mun- 

 data are also remarkable for presenting an apparentlv 

 cellular appearance. This is due to their containing oval 

 masses of protoplasm, revealed through the gelatinised 

 walls." Sometimes these oval masses are larger than 

 usual, and bulge out the hyphal wall. These larger 

 masses are single or occasionally in pairs and recall in 

 their position and relative size the heiterocysts of Nostoc 

 and its allies. It was probably through being misled 

 by these appearances that Minks and his school combated 

 the theory of the dual nature of lichens, and strove to 

 prove that the gonidia were produced from the hyphal 



