THE LICHENS 



371 



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Fig. 357. — Pehigera canino. Vertical section of thallus showing colourless cortex at top, 

 below which is the dark gonidial layer and below this the medulla, passing downwards 

 into the loose hypothallus. 



somewhat irregular. The cortical tissue is made up of living cells, and though 

 they have lost the power of division they can grow in size and thereby increase 

 the surface of the I-ichen as a whole. The cortex is directly continuous 

 with the hyphae of the central tissue, which is called the medulla, and with 

 the lower surface, which is termed the hypothallus. 



The hyphae of the medulla run obliquely in a longitudinal direction, 

 that is, they radiate from the vein towards the margin of the thallus, which 

 they meet at right angles. Between the hyphae of the medulla are large 

 intercellular spaces, whilst the hyphae themselves have thick walls. The 

 medulla becomes very thick in the region of the vein, but may be very thin 

 at the margin. 



Betv/een the cortex and the medulla is the gonidial layer, which is 

 composed of a mixture of fungal and algal cells, which in this species consist 

 of the Blue-green Alga, Nostoc punctijorme, arranged in groups embedded 

 in gelatinous sheaths. 



There is no lower cortex ; the hypothallus, which forms the under tissue, 

 consisting of loosely packed cells derived from short and much twisted hyphae. 

 The cortex and the hypothallus are connected together through the medulla 

 by long, straight hyphae, which play an important part in the mechanical 

 support of the thallus and continue throughout its life. Growth takes place 

 at the margin, where cortex, medulla and hypothallus soon become differ- 

 entiated. Attached to the lower and thicker part of the medulla are small 



