GENERAL CHARACTERS OF BRITISH LICHENS. 39 



nally, of a white,, cottony, spongy, medullary layer. The 

 assistance of the microscope will enable us to determine 

 that the cortical tissue consists of a closely aggregated 

 series of thick-walled, typically spherical cells, but which 

 have assumed various irregularities of shape from pressure 

 and abortion ; that the gonidic layer is composed of a series 

 of globular cells in a very loose state of aggregation, both 

 with each other and with the tissues with which they are in 

 relation, and which contain a greater or less quantity of 

 bright-green chlorophyll, either in the form of distinct gra- 

 nules, or of an amorphous, semi-fluid matter; and that the 

 medullary layer is composed of a somewhat loose network of 

 brandling tubes or filaments delicate or thick-walled, simple 

 or marked by internal septa, which indicate their constitu- 

 ent cells, and which may be empty or filled with a transpa- 

 rent gummy matter : the interstices of this medullary net- 

 work contain air. A large-celled, thick-walled, coarse me- 

 dullary tissue may be easily studied in the Peltigerea. 

 Bundles of these filaments are frequently developed down- 

 wards in the form of the rhizincB or jixurm already alluded 

 to. The whole tissues of the Lichen-thallus are thus seen 

 to consist of modifications of two forms of simple cell,— the 

 spherical or rounded, which by pressure becomes hexagonal, 



