Friedrich[968] 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 accidental nature. Crustaceous species on the outer bark occupy the dead cortical layers and seem to be entirely harmless[969]. 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[970], 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[971] 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[972] 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 and exercise a smothering effect on the trees. Lilian Porter[973] distinctly states that _Ramalinae_ by their penetrating bases damage the tissues of the trees. The presence of lichens is however generally due to unhealthy conditions already at work. Friedrich[974] reported of a forest which he examined, in which the atmospheric moisture was very high, with the soil water scarce, that those trees that were best supplied with soil water were free from lichens, while those with little water at the base bore dead branches which gave foothold to a rich growth of the epiphytes. Experiments to free fruit trees from their coating of lichens were made by Waite[975]. With a whitewash brush he painted over the infested branches with solutions of Bordeaux mixture of varying strength, and found that this solution, commonly in use as a fungicide, was entirely successful. The trees were washed down about the middle of March, and some three weeks later the lichens were all dead, the fruticose and foliose forms had changed in colour to a yellowish or brownish tint and were drooping and shrivelled.