von Schrenk — Dissemination of Usnea barbata. 197 



On walking through a forest of Conifers covered by Usnea 

 one is struck by the fact that those portions of the trees 

 where the lichen grows are either dead or dying. The tempt- 

 ation is a 'strong one to hold the lichen responsible for their 

 death. The effect of lichens on trees has often been dis- 

 cussed. Lindau* mentions a number of species which seem 

 to injure leaves and branches. " Xanthoria parietina grows 

 around the leaves of pines like a cuff, and since the leaves are 

 almost always dead, I am inclined to lay the greatest blame 

 on the lichen, for it suffocated the leaves by covering the 

 stomata." Evernia Prunaslri attaches itself to lenticels on 

 young twigs and blocks them. In general Lindau maintains 

 that some lichens may injure trees by closing their air channels, 

 but more often the appearance of lichens on trees is a sign 

 that the latter are not healthy. Waite,f Waugh % and others 

 have discussed the effect of licheus on fruit trees. Ward § 

 speaks of the shading effect of a tropical leaf lichen. But 

 all these lichens are attached firmly to their substratum, so 

 facts bearing upon their effect are not pertinent as far as Usnea 

 is concerned. If the latter does any harm, it must be because 

 it covers organs and deprives them of air and light. When 

 large masses of this lichen become firmly attached to a branch 

 they completely surround the leaves and hide them (PI. 

 XVI). When this happens in spring, as it frequently must, 

 it is easily seen that leaves covered as are those in the figure, 

 cannot assimilate and grow. The result is that they die 

 and fall off. The terminal bud shares the same fate, 

 and the next year that branch bears no leaves. If one 

 takes away the mass of Usnea in August and September, 

 the leaves covered by the lichen will almost always 

 drop off. It is very noticeable that trees near the 

 coast have more lichens on the sides toward the prevail- 



* Lindau. Lichenologische Untersuchungen. Heft 1:53. 1895. 



t Waite, M. B. Experiments with Fungicides in the removal of lichens 

 from pear trees. Journal of Mycology 7 : 264-68. 



% Waugh, F. A. 11th Kept. Vermont Exp. Station. —Also Meehan's 

 Monthly 8: 173. 1898. 



§ Ward, H. Marshall. On the Structure, developmeDt and life history 

 of a tropical Epiphyllous Lichen, Strigula complanata. Trans. Linn. Soc. 

 London, ii. Bot. 2: 87. 1884. 



