166 



POPULAR HISTORY OP LICHENS. 



system of Lichens ; they are here very large and distinct. 

 The cortical tissue, which is properly confined to the supe- 

 rior surface, consists of a series of large thick- walled cells, 

 originally or typically globular, but w 7 hich present great 

 irregularity in form on account of pressure in a state of 

 close aggregation; they frequently exhibit an irregularly 

 hexagonal form, giving the cortical tissue the character of 

 a honeycomb network, or they are polyhedral. The lower 

 surface is unprotected by a distinct epidermic tissue, but 

 is traversed by venules, which, along with the vertical pro- 

 cesses or fixurse, which they send off at intervals, are com- 

 posed of the medullary filaments, or of filaments similar 

 thereto. These medullary filaments are broad and branch- 

 ing, and are composed of a series of comparatively short, 

 broad, somewhat cylindrical cells, — with walls greatly thick- 

 ened, apparently from internal deposits, — which give them a 

 jointed appearance. The thecse are elongated, narrow, and 

 eight- spored; the spores are closely packed spirally, and 

 are fusiform and much elongated, pale yellow, quadrilocular 

 or triseptate, sometimes curved in various degrees in the 

 old state. The young spore is a simple fusiform cell, full 

 of a grumous or finely granular protoplasm ; this becomes 

 separated into distinct portions, in the same manner as in 



