62 THE THALLUS OF THE GENUS PARHELIA, 



threads. This appearance is figured by Willey in his 

 "Introduction to the Study of Lichens," p. 60, Plate II., 

 as micro-gonidia. 



In the algal (gonidial) layer the hyphse put out fine 

 branches which adhere to^ the coat of the alga, joining it 

 likei the stalk joins a cherry, and from this stalk branches 

 of much less diameter than the medullary hyphee sur- 

 round each algal cell. These branches have the same neck- 

 lace appearance, froim a division into^ cellules, as has the 

 cortical weft. When, in sectioning, a gonidium has been 

 displaced, a cavity lined with these smaller hyphae is 

 disolosed. 



In illustrations of thalline structures, small gonidia are 

 often shown sicattered through the gonidial layer. These 

 supposed gonidia are usually the cut ends of hyphae, 

 which, by an optical illusion, are made to^ appear, when 

 viewed end on, as if they were of greater diameter than 

 the hyphae of which they are transverse sections. 



Although, as shown above, the hyphae branch re- 

 peatedly in surrounding the Cystococcus cells, they divide 

 very sparingly in the central or medullary layer. The 

 branches in any given lichen usually fc-rk at about the 

 same angle, varying from 45 deg.-60 deg. in P. placor- 

 ])(:(] i aid en to 75 cleg. -90 deg. in P. mundata. The hyphfe 

 of the former branch in the central portion of the thallus 

 very sparingly, in the latter rather more freely. 



4. The A]g(v, or Gonidia. 



With an exception to be given later, the algal layer 

 in Parmeliae is confined to the stratum immediately be- 

 neath or within the upper cortex. It has already been 

 stated that the medullary hyphae turn frcrn their parallel 

 longitudinal course in the centre of the thallus, and pass 

 at right angles to their former direction to penetrate 

 between the gonidia, and form a close cellular weft 

 around them. This weft is then continued to the surface 

 to form the upper cortex. The algae lie loosely among 

 these cortical threads at their inner extremity, or are 

 ordinarily gathered into small groups probably derived 

 from a common ancestral cell. 



In siome species, as P. iiJacnrliodioides, the algal 

 cells below the apothecia are few and colourless, or nearly 

 so, and look as if they werei exhausted ct dying; but in 

 these situations a second algal layer is produced near the 

 lovi^er cortex, of vigorous cells well supplied with proto- 



