78 MORPHOLOGY 



b. Hypophloeodal Lichens. These immersed lichens are compar- 

 able with the endolithic species of the rock formations, as their thallus is 

 almost entirely developed under the outer bark of the tree. They are recog- 

 nizable, even in the absence of any fructification, by the somewhat shining 

 brownish, white or olive-green patches that indicate the underlying lichen. 

 This type of thallus occurs in widely separated families and genera, Lecidea, 

 Lecanora, etc., but it is most constant in Graphideae and in those Pyreno- 

 lichens of which the algal symbiont belongs to the genus Trentepohlia. 

 The development of these lichens is of peculiar interest as it has been 

 proved that though both symbionts are embedded in the corky tissues, the 

 hyphae arrive there first, and, at some later stage, are followed by the 

 gonidia. There is therefore no question of the alga being a "captured 

 slave" or "unwilling mate." 



Frank^ made a thorough study of several subcortical forms. He found 

 that m Artkonia radiata, the first outwardly visible indication of the presence 

 of the lichen on ash bark was a greenish spot quite distinct from the 

 normal dull-grey colour of the periderm. Usually the spots are round in 

 outline, but they tend to become ellipsoid in a horizontal direction, being 

 influenced by the growth in thickness of the tree. At this early stage only 

 hyphae are present; Bornet^ as well as Frank described the outer periderm 

 cells as penetrated and crammed with the colourless slender filaments. 

 Lindau", in a more recent work, disputes that statement: he found that the 

 hyphae invariably grew between the dead cork cells, splitting them up and 

 disintegrating the bark, but never piercing the membranes. The purely 

 prothallic condition, as a weft of closely entangled hyphae, may last, Frank 

 considers, for a long period in an almost quiescent condition — possibly for 

 several years — before the gonidia arrive. 



It is always difficult to observe the entrance of the gonidia but they 

 seem to spread first under the second or third layers of the periderm. With 

 care it is possible to trace a filament of Trentepohlia from the surface down- 

 wards, and to see that the foremost cell is really the growing and advancing 

 apex of the creeping alga. Both symbionts show increased vigour when 

 they encounter each other: the thallus at once develops in extent and in 

 depth, and, ultimately, reproductive bodies are formed. In some species the 

 apothecia or perithecia alone emerge above the bark, in others the outer 

 peridermal cells are thrown off, and the thallus thus becomes superficial to 

 some extent as a white scurfy or furfuraceous crust. 



The change from a hypophloeodal to a partly epiphloeodal condition 

 depends largely on the nature of the bark. Frank' found that Lecanora 

 pallida remained for a long time immersed when growing on the thick 

 rugged bark of oak trunks. When well lighted, or on trees with a thin 



1 Frank 1876. ^ Bornet 1873, p. 81. ^ Lindau 1895. 



