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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 scopnlorum van 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 (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 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.