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The long-mooted question, whether lichens injure the trees on 

 which they live, is not to be answered, says Fries, by mere denial. 

 He does not consider it further, nor do I know of any thing of im- 

 portance relating to it beside a chapter in Hagen's Hist. Lich. Pruss. 

 J782 ; a few pages by Hoffmann, 1786; part of one in Luy ken's Dis- 

 sertation, compiled, it would seem, from Hagen; and a brief paper 

 by F. V. Merat, in the Transactions of the French Agricultural 

 Society, 1837. The three first-named writers think that lichens do 

 not injure trees, and the last that they do ; but altogether they have 

 contributed very little to our knowledge in the matter. Hagen 

 considers the whole thing an aspersion upon lichens, and defends 

 them most laboriously. Mr. Merat, on the other hand, is persuaded 

 that lichens are mischievous plants, and after showing them up to 

 the best of his ability, he gravely gives us recipes for their extirpa- 

 tion. On the whole it does not seem at all safe to deny the destruc- 

 tive power of these plants generally on the trunks (and dead wood) 

 which so many species naturally and normally inhabit. And some 

 years since (Lich. N. Eng., in Bost. Jour. 1841, p. 458) I ventured to 

 express this opinion without qualification, and to suggest the proba- 

 bility of a law determining their action in this respect in nature. 

 Soils and other circumstances affecting the health of trees are so 

 various and uncertain, that in the midst of what seems to indicate 

 power of indefinite duration, we find constantly symptoms of disease 

 and decay. Unhealthy young trees do not long survive after their 

 epidermis is well covered with lichens, and in this stage it seems to 

 me quite futile to remove the latter. But in older trunks the con- 

 nection of these plants with any morbid condition of the tree is often 

 very obscure, as indeed we might expect it to be. It is possible 

 that an unhealthy state of the trunk, whether from soil or other cir- 

 cumstances, affords certain favorable conditions for the life and 

 growth of the lichens which occupy it, and accelerate its death : or 

 it may be that the lichens of themselves induce disease and accom- 

 plish dissolution ; more quickly and visibly in a young tree, and more 

 slowly and obscurely in an old. However it be, we assert generally, 

 again, a probable connection between the life and growth of liche- 

 nose vegetation on trees, and the death of the latter; and infer 

 thence the probability of a law determining in this way the action 

 of lichens on living vegetable matter. The universality and fre- 

 quency of lichens wherever are earth, and air, and light, and time 

 is given, and their singular power of assimilating the substance of 

 the matrix (in corticoline species the very epidermis of the tree 

 fills often the place of a lichenose cortical layer; and on calcareous 

 rocks we find the crustaceous thallus and the calcareous matrix 

 together grown into a subcalcareous mass) must not be forgotten. 

 And this leads us to the consideration of the action of lichens on 

 inorganic natural bodies, and their precedence of vegetation as 

 Protophytes, whereby they afford, also, the first beginnings of 

 humus for after successions of higher and higher vegetable devel- 

 opment With regard to the first clause of the last sentence, I am 

 unprepared to add any thing to the mere indication of the question, 

 further than that these plants seem sometimes to aid in the disinte- 

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