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E. IMRE FRIEDMANN AND MARGALITH GALUN
and Lecideaceae, but not Acarosporaceae. The spectrum of the species
is, however, a different one.
Weber (1962) in discussing the flora of the Southwestern United States
says that this flora " . . . consists of widespread circumpolar species particularly localized in the Rocky Mountains; a significant number belong
to a desert-steppe element disjunctly distributed in the isolated semiarid
regions of both hemispheres; and a small percentage are pan-subtropical
in distribution."
It should be pointed out that the information on lichens in arid zones
and desert regions is scarce and scattered. Furthermore, a great number
of inadequately researched taxa have received recognition, causing chaos
in the nomenclature. Therefore, it is difficult to be conclusive in regard
to the distribution patterns, and it is premature to expect appropriate comparison to be made with the distributional pattern of the higher plants.
D. MORPHOLOGY
Morphological adaptation to xeric conditions protects lichens from
desiccation and against the excessive radiation of direct sunlight. One relevant example is the reduction of the evaporating surface. Most of the
heteromerous desert lichens belong to the crustose type, which have the
ventral side of the thallus closely attached to the substrate and only the
upper surface exposed to atmospheric factors. Another common feature
in desert lichens is a thickened cortex (Vogel, 1955) and various amorphous exterior layers, such as a powdery cover (the pruina), thick necrotic
or gelatinous layers covering the living tissue underneath (Galun, 1963).
For example, the reddish squamules of Lecidea decipiens are heavily
pruinose in the deserts, but naked in moderate regions. Buellia canescens,
which grows in Sweden, has no amorphous upper layer, whereas the same
species growing in the Negev Desert is covered by a thick superficial amorphous layer. Although this sort of protection is only of limited duration,
it does prolong the daytime photosynthetic activity by slowing down the
decrease in water content.
Also considered a xeromorphic character is the bright or pale color of
the thalli of many desert lichens, which presumably reduces heat ray absorption. Even if the basic color of the cortex is dark, it is in many cases
concealed by a flowery or powdery whitish cover (Galun, 1963). The
members of the Northern Chilean Buellietum albulae lichen association
are covered by a fine layer of sand and dust, which according to Follmann
(1965b) reduces the amount of light transmitted to the algal layer.
Vogel (1955) describes the inverted structure of the thallus of a Buellia
species, which frequently settles on quartz in the South African desert.
E. IMRE FRIEDMANN AND MARGALITH GALUN
and Lecideaceae, but not Acarosporaceae. The spectrum of the species
is, however, a different one.
Weber (1962) in discussing the flora of the Southwestern United States
says that this flora " . . . consists of widespread circumpolar species particularly localized in the Rocky Mountains; a significant number belong
to a desert-steppe element disjunctly distributed in the isolated semiarid
regions of both hemispheres; and a small percentage are pan-subtropical
in distribution."
It should be pointed out that the information on lichens in arid zones
and desert regions is scarce and scattered. Furthermore, a great number
of inadequately researched taxa have received recognition, causing chaos
in the nomenclature. Therefore, it is difficult to be conclusive in regard
to the distribution patterns, and it is premature to expect appropriate comparison to be made with the distributional pattern of the higher plants.
D. MORPHOLOGY
Morphological adaptation to xeric conditions protects lichens from
desiccation and against the excessive radiation of direct sunlight. One relevant example is the reduction of the evaporating surface. Most of the
heteromerous desert lichens belong to the crustose type, which have the
ventral side of the thallus closely attached to the substrate and only the
upper surface exposed to atmospheric factors. Another common feature
in desert lichens is a thickened cortex (Vogel, 1955) and various amorphous exterior layers, such as a powdery cover (the pruina), thick necrotic
or gelatinous layers covering the living tissue underneath (Galun, 1963).
For example, the reddish squamules of Lecidea decipiens are heavily
pruinose in the deserts, but naked in moderate regions. Buellia canescens,
which grows in Sweden, has no amorphous upper layer, whereas the same
species growing in the Negev Desert is covered by a thick superficial amorphous layer. Although this sort of protection is only of limited duration,
it does prolong the daytime photosynthetic activity by slowing down the
decrease in water content.
Also considered a xeromorphic character is the bright or pale color of
the thalli of many desert lichens, which presumably reduces heat ray absorption. Even if the basic color of the cortex is dark, it is in many cases
concealed by a flowery or powdery whitish cover (Galun, 1963). The
members of the Northern Chilean Buellietum albulae lichen association
are covered by a fine layer of sand and dust, which according to Follmann
(1965b) reduces the amount of light transmitted to the algal layer.
Vogel (1955) describes the inverted structure of the thallus of a Buellia
species, which frequently settles on quartz in the South African desert.