74 MORPHOLOGY 



marked by subsequent cracks due to unequal growth. The areolation caused 

 by primary growth conditions tends to become gradually less obvious or to 

 disappear altogether. 



Secondary areolation is due to unequal intercalary growth of the 

 otherwise continuous thallus 1 . A more active increase of any minute portions 

 provokes a tension or straining of the cortex between the swollen areas 

 and the surrounding more sluggish tissues ; the surface layers give 

 way and chinks arise, a condition described by older lichenologists as 

 "rimose-diffract" or sometimes as "rhagadiose." The thallus is generally 

 thicker, more broken and granular in the older central parts of the lichen. 

 Towards the circumference, where the tissue is thinner and growth more 

 equal, the chinks are less evident. Sometimes the more vigorously growing 

 areolae may extend over those immediately adjoining, in which case the 

 covered portions become brown and their gonidia gradually disappear. 



Strongly marked intersecting lines, similar to those round the margin 

 of the thallus, are formed when hypothalli that have themselves started 

 from different Centres touch each other. A large continuous patch of 

 crustaceous thallus may thus be composed of many individuals (Fig. 41). 



Fig. 41. Rhizocarpon geographicum BC. on boulder, reduced (M.P., Photo.}. 



b. ENDOLITHIC LICHENS. In many species, only the lower hyphae 

 penetrate the substratum either of rock or soil. In a few, more especially 

 those growing on limestone, the greater part or even the whole of the vege- 

 tative thallus and sometimes also the fruits are, to some extent, immersed 



1 Malinowski 1911. 



