RADIATE THALLUS loi 



E. prunastri, the second species of the genus, is more distinctly upright in 

 habit, with a penetrating basal hold-fast and upright strap-shaped branching 

 fronds, light-greyish green on the ''upper" surface and white on the other 

 (Fig- 59)- The internal structure is sub-radiate; both cortices are "decom- 

 posed"; the gonidial zone consists of somewhat loose groups of algae, very 

 constant below the "upper" surface, with an occasional group in the pith 

 near to the lower cortex in positions that are more exposed to light. There 

 is also a tendency for the gonidial zone to pass round the margin and spread 

 some way along the under side. The medulla is of loose arachnoid texture 

 and the whole plant is very limp when moist. It grows on trees, often in 

 dense clusters. 



3. FRUTICOSE AND f ILAMENTOUS THALLUS 



A. General Structure of Thallus 



The conditions of strain and tension in the upright plant are entirely 

 different from those in the decumbent thallus, and to meet the new require- 

 ments, new adaptations of structure are provided either in the cortex or in 

 the medulla. 



Cortical Structures. With the exception of the distinctly plec- 

 tenchymatous cortex, all the other types already described recur in fruticose 

 lichens; in various ways they have been modified to provide not only covering 

 but support to the fronds. 



a. The fastigiate cortex. This reaches its highest development in 

 Roccella in which the branched hyphal tips, slightly clavate and thick-walled, 

 lie closely packed in palisade formation at right angles to the main axis 

 (Fig. 45). They afford not only bending power, but give great consistency 

 to the fronds. The cortex is further strengthened in R. fuciformis^ by the 

 compact arrangement of the medullary hyphae that run parallel with the 

 surface, and among which occur single thick-walled filaments. The plant 

 grows on maritime rocks in very exposed situations; and the narrow strap- 

 shaped fronds, as stated above, may attain a length of 30 cm., though usually 

 they are from 10 to 18 cm. in height. The same type of cortex, but less 

 highly differentiated, affords a certain amount of stiffness to the cylindrical 

 much weaker fronds of Thamnolia. 



b. The fibrous cortex. This type is found in a number of lichens with 

 long filamentous hanging fronds. It consists of parallel hyphae, rarely septate 

 and rarely branched, but frequently anastomosing and with strongly thick- 

 ened "sclerotic" walls. Such a cortex is the only strengthening element in 

 Alectoria, and it affords great toughness and flexibility to the thong-like 



' Darbishive 1898. 



