DISEASES OF LICHENS 269 



the upper cortex may be rolled back and eventually torn away ; the gonidial 

 layer is exposed and transformed into soredia which are swept away by the 

 wind and rain, till finally only traces of the lower cortex are left. 



Zukal' has instanced, as a case of diseased condition observed by him, 

 the undue thickening of the cortex in Pertusaria communis whereby the 

 formation of the fruiting bodies is inhibited and even vegetative development 

 is rendered impossible. There arrives finally a stage when splitting takes 

 place and the whole thallus breaks down and disappears. As a rule however 

 there need be no limit to the age of the lichen plant. There is no vital 

 point or area in the thallus ; injury of one part leaves the rest unhurt, and 

 any fragment in growing condition, if it combines both symbionts, can carry 

 on the life of the plant, the constant renewal of gonidia preventing either 

 decay or death. Barring accidents many lichens might exist as long as the 

 world endures. 



G. Harmful Effect of Lichens 



One lichen only, Strigida complanata, a tropical species, has been proved 

 to be truly and constantly parasitic. It grows on the surface of thick leathery 

 leaves such as those of Camellia'-, etc. and the alga and fungus both penetrate 

 the epidermis and burrow beneath the cuticle and outer cells, causing them 

 to become brown. It undoubtedly injures the leaves. 



Friedrich' has given an isolated instance of the hold-fast hyphae of Usnea 

 piercing through the cortex to the living tissue of the host, and not only 

 destroying the middle lamella by absorption, but entering the cells. The 

 Usnea plant was characterized by exceptionally vigorous growth. Practically 

 all corticolous lichens are epiphytic and the injury they cause is of an acci- 

 dental nature Crustaceous species on the outer bark occupy the dead 

 cortical layers and seem to be entirely harmless*. The larger foliose and 

 fruticose forms are not so innocuous: by their abundant enveloping growth 

 they hinder the entrance of air and moisture, and thus impede the life of 

 the higher plant. Gleditsch', one of the earliest writers on Forestry, first 

 indicated the possibly harmful effect of lichens especially on young trees 

 and " in addition," he says, " they serve as cover for large numbers of small 

 insects which are hurtful in many ways to the trees." Lindau" pointed out 

 the damage done to pine-needles by Xanthoria parietina which grew round 

 them like a cuff and probably choked the stomata, the leaves so clothed being 

 mostly withered. Dufrenoy' states that he found the hyphae of a Parmelia 

 entering a pine-needle by the stomata, and that the starch disappeared from 

 the neighbouring parenchyma the cells of which tended to disintegrate. 



It is no uncommon sight to see neglected fruit trees with their branches 

 crowded with various lichens, Evernia prunastri, Ramalina farinacea, etc. 

 Such lichens often find the lenticels a convenient opening for their hold-fasts 



' Zukal 1896, p. 255. ^ Cunningham 1879. ^ Friedrich 1906, p. 401. ■* See p. 78. 



^ Gleditsch 1775, p. 31. " Lindau 1895, p. 53. ' Dufrenoy 1881. 



