IV. DESERT ALGAE, LICHENS, AND FUNGI
179
measures about 0.1-2.5 mm. As far as known to date, these algae are
members of the blue-green order Chroocaccales, and probably belong
to the genus Gloeocapsa. They seem to form monospecific populations in the rock although they may be accompanied by bacteria. Scanning
electron microscopic studies have shown that the algal cells adhere to the
surfaces of calcite (Fig. 10) or quartz crystals (Figs. 11 and 12) of which
the rock is composed, or they fill the microscopic cavities of the amorphous
limestone (Fig. 13).
Since the light-colored rocks are translucent to some extent, it is obvious
that a steep and decreasing gradient of light intensity exists in the upper
layer of the rock. Light intensity is an important factor in the vertical
distribution of algae in the rock, and the upper and generally sharp borderline of the endolithic algal zone may be determined by the maximum
tolerable level of irradiation, while the lower level probably indicates the
minimum threshold of useful light. Indeed, in the upper level of the algal
zone, cells show morphological characteristics of adaptation to high light
intensities (thick, layered, dark-pigmented sheaths, brownish cell contents)
while cells in the lower levels are typical shadow forms with thin, unpigmented sheaths and vivid green cell contents.
The porous rock with its imperforate surface layer represents a system
eminently suitable to trap and to retain moisture and to ensure balanced
temperature conditions in a hot desert macroclimate. Available water, be
it from dew or rain, is imbibed in the porous rock by capillary force. The
dense surface crust is pervious to water, yet as air temperature rises during
daytime, it retards evaporation. Thus a slowly evaporating water reservoir
is retained in the porous rock, maintaining a comparatively high level of
moisture during the daytime.
The low heat conductivity of the porous rock, together with high reflectance owing to its light color, prevents a sudden temperature rise which
might result from high insolation. The cooling effect of the protracted
daytime evaporation from the rock further reduces wide temperature
fluctuation.
D. WATER SOURCE
Water is obviously a pivotal problem for all desert cryptogams. Rain
is scarce, and its uneven distribution further reduces its usefulness and
availability to organisms. However, macrometeorological data such as rainfall do not necessarily give a correct picture of the situation prevailing
in the microscopic ecological niches where microorganisms live. Vogel
(1955), Schwabe (1960a), Friedmann et al (1967), and Friedmann
(1971) point out that the main water source of desert algae is probably
179
measures about 0.1-2.5 mm. As far as known to date, these algae are
members of the blue-green order Chroocaccales, and probably belong
to the genus Gloeocapsa. They seem to form monospecific populations in the rock although they may be accompanied by bacteria. Scanning
electron microscopic studies have shown that the algal cells adhere to the
surfaces of calcite (Fig. 10) or quartz crystals (Figs. 11 and 12) of which
the rock is composed, or they fill the microscopic cavities of the amorphous
limestone (Fig. 13).
Since the light-colored rocks are translucent to some extent, it is obvious
that a steep and decreasing gradient of light intensity exists in the upper
layer of the rock. Light intensity is an important factor in the vertical
distribution of algae in the rock, and the upper and generally sharp borderline of the endolithic algal zone may be determined by the maximum
tolerable level of irradiation, while the lower level probably indicates the
minimum threshold of useful light. Indeed, in the upper level of the algal
zone, cells show morphological characteristics of adaptation to high light
intensities (thick, layered, dark-pigmented sheaths, brownish cell contents)
while cells in the lower levels are typical shadow forms with thin, unpigmented sheaths and vivid green cell contents.
The porous rock with its imperforate surface layer represents a system
eminently suitable to trap and to retain moisture and to ensure balanced
temperature conditions in a hot desert macroclimate. Available water, be
it from dew or rain, is imbibed in the porous rock by capillary force. The
dense surface crust is pervious to water, yet as air temperature rises during
daytime, it retards evaporation. Thus a slowly evaporating water reservoir
is retained in the porous rock, maintaining a comparatively high level of
moisture during the daytime.
The low heat conductivity of the porous rock, together with high reflectance owing to its light color, prevents a sudden temperature rise which
might result from high insolation. The cooling effect of the protracted
daytime evaporation from the rock further reduces wide temperature
fluctuation.
D. WATER SOURCE
Water is obviously a pivotal problem for all desert cryptogams. Rain
is scarce, and its uneven distribution further reduces its usefulness and
availability to organisms. However, macrometeorological data such as rainfall do not necessarily give a correct picture of the situation prevailing
in the microscopic ecological niches where microorganisms live. Vogel
(1955), Schwabe (1960a), Friedmann et al (1967), and Friedmann
(1971) point out that the main water source of desert algae is probably