196
E. IMRE FRIEDMANN AND MARGALITH GALUN
a lichen from xeric environment in the Sinai Desert, and in G. mesopotamica Stein, a species growing under moderate climatic conditions in
the Golan Heights, the fungus-alga relationship was found to be similar
and thus apparently unaffected by environmental factors.
E. LICHEN-SUBSTRATE RELATIONSHIP
There has been a general belief that lichens play a significant role in
rock weathering and breakdown. Thus, Krumbein (1969) claims that in
humid climates the lichens usually protect rocks from weathering, whereas
in arid climates they are one of the most effective agents in rock decay
and deterioration. He finds a correlation between the quantity of microbial
population, including fruticose, crustose, and endolithic lichens, algae,
fungi, and bacteria, and the rate of the weathering process. Placodial soil
lichens of arid regions, subjected to high winds and extreme temperature
fluctuations, develop extraordinarily strong and relatively long strands of
rhizines or rhizoidal hyphae (Vogel, 1955; Poelt and Baumgärtner, 1964)
for a better and more stable attachment. Although penetrating deeply into
the substrate, these rhizines have apparently no role in the water relation
of the lichens (Poelt and Baumgärtner, 1964).
F. PHYSIOLOGY
Water absorption and water loss are both entirely physical processes
in lichens. Furthermore, lichens possess no mechanism for water conservation. Their water content is, therefore, subject to fluctuations governed by
environmental conditions. In conditions of severe drought the water content may be as low as 5% of the dry weight of the lichen (Lange et al.,
1968), while after water is supplied for even short periods, either as
vapor or in liquid form, their water content may rise considerably, often
to above 100% of the dry weight.
Some understanding of the water-dependent metabolic processes, such
as photosynthesis and respiration, which they can maintain in barren habitats unsuitable for most other kinds of plants, may be gained from the
investigations of Lange and his collaborators (Lange and Bertsch, 1965;
Lange et ai, 1968, 1970a,b; Lange, 1969a,b). Lange, using elaborate techniques, measured C0 2 gas exchange simulating the natural conditions of
a "true" desert—the Israeli Negev Desert. The small amount of annual
rainfall (less than 100 mm) in this region is restricted to a few days during
the short winter season. Throughout the rest of the year, dew is the main
source of moisture for a positive balance of carbon dioxide assimilation,
as shown in Fig. 17 for Ramalina macijormis. When there is no dew, the
E. IMRE FRIEDMANN AND MARGALITH GALUN
a lichen from xeric environment in the Sinai Desert, and in G. mesopotamica Stein, a species growing under moderate climatic conditions in
the Golan Heights, the fungus-alga relationship was found to be similar
and thus apparently unaffected by environmental factors.
E. LICHEN-SUBSTRATE RELATIONSHIP
There has been a general belief that lichens play a significant role in
rock weathering and breakdown. Thus, Krumbein (1969) claims that in
humid climates the lichens usually protect rocks from weathering, whereas
in arid climates they are one of the most effective agents in rock decay
and deterioration. He finds a correlation between the quantity of microbial
population, including fruticose, crustose, and endolithic lichens, algae,
fungi, and bacteria, and the rate of the weathering process. Placodial soil
lichens of arid regions, subjected to high winds and extreme temperature
fluctuations, develop extraordinarily strong and relatively long strands of
rhizines or rhizoidal hyphae (Vogel, 1955; Poelt and Baumgärtner, 1964)
for a better and more stable attachment. Although penetrating deeply into
the substrate, these rhizines have apparently no role in the water relation
of the lichens (Poelt and Baumgärtner, 1964).
F. PHYSIOLOGY
Water absorption and water loss are both entirely physical processes
in lichens. Furthermore, lichens possess no mechanism for water conservation. Their water content is, therefore, subject to fluctuations governed by
environmental conditions. In conditions of severe drought the water content may be as low as 5% of the dry weight of the lichen (Lange et al.,
1968), while after water is supplied for even short periods, either as
vapor or in liquid form, their water content may rise considerably, often
to above 100% of the dry weight.
Some understanding of the water-dependent metabolic processes, such
as photosynthesis and respiration, which they can maintain in barren habitats unsuitable for most other kinds of plants, may be gained from the
investigations of Lange and his collaborators (Lange and Bertsch, 1965;
Lange et ai, 1968, 1970a,b; Lange, 1969a,b). Lange, using elaborate techniques, measured C0 2 gas exchange simulating the natural conditions of
a "true" desert—the Israeli Negev Desert. The small amount of annual
rainfall (less than 100 mm) in this region is restricted to a few days during
the short winter season. Throughout the rest of the year, dew is the main
source of moisture for a positive balance of carbon dioxide assimilation,
as shown in Fig. 17 for Ramalina macijormis. When there is no dew, the