H. Gall-Formation
Several instances of gall-formation to a limited extent have been already noted as caused by parasitic fungi or lichens. Greater abnormality of development is induced in a few species by the presence of minute animals, mites, wood-lice, etc. Zopf[977] noted these deformations of the thallus in specimens of Ramalina Kullensis collected on the coasts of Sweden. The fronds were frequently swollen in a sausage-like manner, and branching was hindered or altogether prevented; apothecia were rarely formed, though pycnidia were abundant. Here and there, on the swollen portions of the thallus, small holes could be detected and other larger openings of elliptical outline, about 1-1-1/2 mm. in diameter, the margins of which had a nibbled appearance. Three types of small articulated animals were found within the openings: species of mites, spiders and wood-lice. Mites were the most constant and were more or less abundant in all the deformations; frequently a minute Diplopodon belonging to the genus Polyxenus was also met with.
Zopf came to the conclusion that the gall-formation was mainly due to the mites: they eat out the medulla and possibly through some chemical irritation excite the algal zone and cortex to more active growth, so that an extensive tangential development takes place. The small spiders may exercise the same power; evidently the larger holes were formed by them.
Later Zopf added to gall-deformed plants Ramalina scopulorum var. incrassata 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[978] 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 (Ph. 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 pathological 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[979] 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 rainfall, 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.