PARMELIA 473 



B. LICHENS 

 PARMELIA PARIETINA (Yellow Lichen) 



I. This species is very commonly to be found growing 

 on tree-trunks, walls, roofs, &c. : note its foliaceous 

 tballus, which has no definite mode of branching, and 

 is closely applied to the substratum. It is of a bright 

 olive-yellow colour on the upper surface, but white 

 underneath : note the processes from the under surface 

 (rhizines) by which it is attached to the substratum : 

 also its brittle character when dry : soak it in water, 

 and it will become soft but leathery. 



On the upper surface observe the more or less 

 flattened or cup-shaped apothecia, which are of darker 

 colour than the rest of the thallus, and attain a 

 diameter of about one-eighth of an inch : note that 

 those near the margin of the thallus are smaller than 

 those nearer the centre. 



II. From material which has been kept dry, select a 

 small pa,rt of a thallus where no apothecia are present : 

 soak it for a short time in alcohol, and embed in 

 paraffin : cut as thin sections as possible : having allowed 

 them to swell as much as they will in water, mount 

 them in glycerine, and examine under a low power : 

 observe 



