Waite was of opinion that the lichens did considerable damage to the trees, but it has been held by others that in very cold climates they may provide protection against severe frost. Instances of damage are however asserted by Bouly de Lesdain[976]. The bark of willows he found was a favourite habitat of numerous lichens: certain species, such as _Xanthoria parietina_, completely surrounded the branches, closing the stomata; others, such as _Physcia ascendens_, by the mechanical strain of the rhizoids, first wet and then dry, gradually loosened the outer bark and gave entry to fungi which completed the work of destruction. 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_.