60 THE THALLUS OF THE GENUS PAEMELIA^ 



and P . saxatilis, in which the cells are abstricted to form 

 minute necklaces standing at right angles to the upper 

 surface. The second as exemplified in F . mundata and F. 

 oUvacea, in which the cellular ends of the hyphas strike 

 the surface of the thallus obliquely, and the third in which 

 the chains of cells are parallel to the surface, as in F. 

 tinctorum. 



The first series, v/ith cells at right angles tO' the sur- 

 face, has a cortex made up of larger units than those of the 

 two following series. It is usually a mass of oval-obloaig 

 cellsi, .005-. 006 mm. long, with a small lumen, not more 

 than .0015 mm. in transverse diameter, and with very 

 thick wallsi. The structure of the cortex in such lichens as 

 P. limhata and P. saxatilis reminds one of the formation 

 of similar tissues in the brown algse Hormosira and Fucus, 

 but in these the lumen of each cell ii larger in proportion 

 to the wall. 



Although not belonging to Parmeliacese, this type of 

 cortex can best be studied in Thamnolia verniicniaris, Sw., 

 in which the hyphge of the medulla are loose, parallel and 

 horizontal, on each side of the central cavity, but in the 

 algal layer they turn oiutwards at right angles^, surrounding 

 the gonidia, and are continued beyond them to form, the 

 cortex. The same arrangeiment holds good for Farmelias 

 of the P. limhata type, but the hyphse are not as .discrete 

 and easy; to foillow as in Thamnolia, nor are they as wholly 

 parallel and longitudinal in the centre. In P. limhata the 

 upper rind is 5-6 cells in thickness. 



The second series, typically shown in P. mundata 

 and P. olivacea, has still smaller cells, biit with thinner 

 walls, these have 3-5 cells to the chain in each hyphal 

 termination, and average .004 mm. in length. The obliq- 

 uity of each thread is affected by curves and depressions 

 of the siurface, but, as a rule, the oblique threads are 

 raised at their outward terminals towards the apex of the 

 thallus. 



The third type is best stixdied in P. tinctorum. In 

 this lichen the hjrphse ramify in and around the small 

 segregated masse© of gonidia, and above and outside these 

 run parallel to the surface. In dividing into necklaces of 

 cells there is no enlargement b}? means of thickened walls 

 as in the two preceding species. This form gives the most 

 delicate cortex to be found in the family. 



(B.) The lower cortex is continued into and stxpplies 

 the lining hyphse for the rhizinae. These rhizinal thread? 

 are usually iTndivided into cellules, except those forming 



