GALL-FORMATION 271 



irritation excite the algal zone and cortex to more active growth, so that an 

 extensive tangential development takesplace. The small spiders mayexercise 

 the same power; evidently the larger holes were formed by them. 



Later Zopf added to gall-deformed plants Ramalina scopulorum var. in- 

 crassata and R. cuspidata var. crassa. He found in the hollow swollen fronds 

 abundant evidence of mites, but whether identical with those that attacked 

 R. Kullensis could not be determined. These two Ramalinae are maritime 

 species; they are morphologically identical, as are also the deformed varieties, 

 and the presence of mites, excreta, etc., are plainly visible in our British 

 specimens. 



Bouly de Lesdain 1 found evidence of mite action in Ramalina farinacea 

 collected from Pinus sylvestris on the dunes near Dunkirk. The cortex 

 had been eaten off either by mites or by a small mollusc (Pupa muscorum) 

 and the fronds had collapsed to a more or less convex compact mass. 

 Somewhat similar deformations, though less pronounced, were observed in 

 other Ramalinae. 



In Cladonia sylvatica and also in Cl. rangiformis Lesdain has indicated 

 ff. abortiva Harm, as evidently the result of insect attack. In both cases the 

 tips of the podetia are swollen, brown, bent and shrivelled. 



One of the most curious and constant effects, also worked out by Lesdain, 

 occurs in Physcia hispida ,(fh. stellaris var. tenella\ In that lichen the 

 gonidia at the tips of the fronds are scooped out and eaten by mites, so 

 that the upper cortex becomes separated from the lower part of the thallus. 

 As the hyphae of the cortex continue to develop, an arched hood is formed 

 of a whitish shell-like appearance and powdery inside. Sometimes the 

 mites penetrate at one point only, at other times the attack is at several 

 places which may ultimately coalesce into one large cavity. In a crustaceous 

 species, Caloplaca (Placodium) citrina he found constant evidence of the 

 disturbing effect of the small creatures, which by their action caused the 

 areolae of the thallus to grow into minute adherent squamules. A patho- 

 logical variety, which he calls var. sorediosa, is distinguished by the presence 

 of cup-like hollows which are scooped out by Acarinae and are filled by 

 yellowish soredia. In another form, var. maritima, the margins of the areolae, 

 occasionally the whole surface, become powdery with a citrine yellow 

 efflorescence as a result of their nibbling. 



Zukal 2 adds to the deformations due to organic agents, the hypertrophies 

 and abnormalities caused by climatic conditions. He finds such irregularities 

 of structure more especially.developed in countries with a very limited rain- 

 fall, as in certain districts of Chili, Australia and Africa, where changes in 

 cortex and rhizoids and proliferations of the thallus testify to the disturbance 

 of normal development. 



1 Lesdain 1910. 2 Zukal 1896, p. 258. 



