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2 7o
BIONOMICS
and excercise a smothering effect on the trees. Lilian
1 Porter 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.
2
Friedrich 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
With a whitewash brush he painted over the infested branches
by Waite 3 .
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 wer.e drooping and shrivelled.
to the
Waite was of opinion that the lichens did considerable damage
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 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 5 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.
2 Friedrich 1906. Waite 1893.
4 Lesdain 1912.
5
Zopf 1907.