THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. II. 



New York, Octobek, 1S81. 



No. 10. 



An Introduction to the Study 

 of Lichens. 



BY REV. W. JOHNSON. 



( Continued from page lyi. ) 



The stratified cellular layers of the 

 Lichen proper are called : — i. The 

 cortical layer (i and 4, Fig. 2,2))- This 

 is the outer covering of the thallus ; 

 and is a somewhat tough, transparent, 

 cellular membrane. In Peltigera ho- 



rizontalis, we found it to be three or 

 four cells deep ; these cells in the ho- 

 rizontal thallus, are generally angular 

 from lateral pressure ; but in fruticu- 

 lose forms, as in Ramalina, they are 

 tubular and elongate. 2. The gonidial 

 layer (2, Fig. 2,2,). This lies immedia- 

 tely beneath the cortical stratum, and 

 is composed of globular, free, bright 

 green, yellowish, or bluish-green cells. 

 The gonidia, in some species, instead 

 of being simple cells filled with green 



Fig. 31. 



granular matter, are found to consist 

 of clusters of two, three, or more 

 roundish granules, with no distinct 

 cell-membrane. These are termed, 

 granular gonidia. 3. The medullary 

 layer (3, Fig. 33). This is a mass of 

 cylindrical, interlacing, articulate, co- 

 lorless cells ; which are so woven 

 together, as often to make a milk- 

 white or flesh-colored spongy mass. 

 They enclose the gonidia on the un- 

 der side of the thallus, and constitute 

 the base of the plant. These three 

 cellular layers are quite distinct from 



each other ; so clearly, that in some 

 of the foliaceous Lichens, they may 

 be recognised by the naked eye ; yet, 

 notwithstanding their distinctness, 

 they perfectly cohere and unitedly 

 fulfill the vegetative functions of the 

 plant. Intermediate between all the 

 cells of the thallus, as well as the 

 germinal organs, is a transparent, gum- 

 my substance called lichenine, which 

 binds the whole together, and gives 

 elasticity and comparative toughness 

 to the several parts of the thallus when 

 moist. This gummy matter, evident- 



