270 BIONOMICS 



and excercise a smothering effect on the trees. Lilian Porter 1 distinctly 

 states that Ramalinae by their penetrating bases damage the tissues of the 

 trees. The presence of lichens is however generally due to unhealthy con- 

 ditions already at work. Friedrich 2 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 8 . 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. 



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 4 . 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 ascendcns, by the mechanical strain of the rhizoids, first wet and then 

 dry, gradually loosened the outer bark and gave entry to fungi which com- 

 pleted 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 develop- 

 ment is induced in a few species by the presence of minute animals, mites, 

 wood-lice, etc. Zopf 8 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 

 \-\\ 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 



1 Porter 1917. " Friedrich 1906. 3 Waite 1893. * Lesdain 1912. 5 Zopf 1907. 



