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358
ECOLOGY
globulifera formed a continuous crust on the trees as much as 120 x 90 cm.
Lecanora tartarea seemed to thrive exceptionally well when subject to
driving mists and rains from mountain or moorland, and was in these circumstances
frequently the dominant epiphyte. Bruce Fink 1
also observed
in his ecological excursions that the number of species and individuals was
greater near lakes or rivers.
Though a fair number of lichens are adapted to life wholly or partly
under water, land forms are mostly xerophytic in structure, and die off if
submerged for any length of time. The Peltigerae are perhaps the most
hydrophilous of purely land species. Many Alpine or Polar forms are
covered with snow for long periods. In the extreme north it affords more
or less protection; and Kihlman 2 and others have remarked on the scarcity
of lichens in localities denuded of the snow mantle and exposed to severe
winter cold. On the other hand lichens on the high Alpine summits that are
covered with snow the greater part of the year suffer, according to Nilson 3
,
from the excessive moisture and the deprivation of light. Foliose and
fruticose forms were, he found, dwarfed in size; the crustaceous species had
a very thin thallus and in all of them the colour was impure. Gyrophorae
seemed to be most affected : folds and outgrowths of the thallus were formed
and the internal tissues were partly disintegrated. Lichens on the blocks
of the glacier moraines which are subject to inundations of ice-cold water
after the snow has melted, were unhealthy looking, poorly developed and
often sterile, though able to persist in a barren state. Lindsay 4 noted as
a result of such conditions on Cladoniae not only sterility but also de-.
formity both of vegetative and reproductive organs ; discolouration and
mottling of the thallus and an increased development of squamules of the
primary thallus and on the podetia.
c. WIND. Horizontal crustaceous or foliose lichens are not liable to
direct injury by wind as their close adherence to the substratum sufficiently
shelters them. It is only when the wind carries with it any considerable
quantity of sand that the tree or rock surfaces are swept bare and prevented
from ever harbouring any vegetation, and also, as has been already noted,
the terrible winds round the poles are fatal to lichens exposed to the
blasts unless they are provided with a special protective cortex. After
crustaceous forms, species of Cetraria, Stereocaulon and Cladonia are best
fitted for weathering wind storms: the tufted 5 cushion-like growth adopted
by these lichens gives them mutual protection, not only against wind, but
against superincumbent masses of snow. Kihlman 2 has given us a vivid
account of wind action in the Tundra region. He noted numerous hollows
completely scooped
out down to the sand : in these sheltered nooks he
1 2 Fink 1894.
Kihlman 1890.
4
Lindsay 1869.
6 Sattler 1914.
3 Nilson 1907.