264
H. WALTER AND E. STADELMANN
A . POIKILOHYDRIC XEROPHYTES
Poikilohydric cryptogams are the only vegetation (500-600 kg dry
weight per ha) of the Takyr plains in Central Asia. In that area clay soils
are inundated for a short period during the spring. A variety of 43 species
of Cyanophyceae (mostly Phormidium, Microcoleus, and others), 49
Chlorophyceae, Flagellatae, Diatoms, and Xanthophyta were found
in a takyr in the northern part of the Kara kum Desert (Turkmenistan).
For a takyr at the foothills of the Kopet-Dag 95 Cyanophyceae, 54 Chlorophyceae, two Xanthophyceae, 33 Diatoms and two Euglenaceae (total
186 algal species) are reported (Rodin, 1963, p. 255). Higher plains
which are never inundated have lichens (Diploschistes albissimus, Squamarna lentigera, and others ). These lichens absorb water from the exceedingly humid air during the night. Their C0 2 assimilation is positive during
the morning hours (cf. also Ramalina maciforma in the Negev Desert,
Lange, 1969, p. 6).
Desert plants called window algae grow on the lower side of transparent
quartz stones (Tchan and Beadle, 1955, p. 101; Vogel, 1955, p. 52ίϊ).
Light rains occur frequently and dew runs along the surface of the stones
to provide the underlaying algae with moisture and to protect them against
evaporation. Earth lichens with thalli a few millimeters long are able to
utilize dew (Vogel, 1955, p. 115). Window algae and lichens were recorded in the foggy Namib Desert but are not found further inland where
no fog develops, although summer rains may occur there. Window algae
also occur in other arid regions (Walter, 1964, p. 387).
Xeromorphic mosses (e.g., certain Ricciaceae) with partly subterranean
thalli, which are found in many arid regions, have not been as yet studied
ecologically. Lower plants of the desert, in general, have hardly been investigated in this regard.
Poikilohydric pteridophyta have air-dry leaves during drought periods,
but they regain turgidity and green color within only 15-30 minutes after
the first rain. These ferns are widely distributed through the arid regions
of South Africa and America but do not grow in extreme deserts. The
\p 8 values of their leaves during the rainy period are high (Table XI),
but still lower than for most winter annuals. This difference is generally
found and is characteristic for the pteridophytes, which do not have a very
efficient water-conducting system.
The leaves of most of these fern species roll and dry when the dry season
begins and the available soil moisture supply decreases. Loss of turgidity
occurs rapidly and half-dried and incompletely rolled fern leaves rarely
are seen. The rolling mechanism of these leaves (Walter and Bauer, 1937)
and the photosynthesis and respiration of the poikilohydric ferns before
H. WALTER AND E. STADELMANN
A . POIKILOHYDRIC XEROPHYTES
Poikilohydric cryptogams are the only vegetation (500-600 kg dry
weight per ha) of the Takyr plains in Central Asia. In that area clay soils
are inundated for a short period during the spring. A variety of 43 species
of Cyanophyceae (mostly Phormidium, Microcoleus, and others), 49
Chlorophyceae, Flagellatae, Diatoms, and Xanthophyta were found
in a takyr in the northern part of the Kara kum Desert (Turkmenistan).
For a takyr at the foothills of the Kopet-Dag 95 Cyanophyceae, 54 Chlorophyceae, two Xanthophyceae, 33 Diatoms and two Euglenaceae (total
186 algal species) are reported (Rodin, 1963, p. 255). Higher plains
which are never inundated have lichens (Diploschistes albissimus, Squamarna lentigera, and others ). These lichens absorb water from the exceedingly humid air during the night. Their C0 2 assimilation is positive during
the morning hours (cf. also Ramalina maciforma in the Negev Desert,
Lange, 1969, p. 6).
Desert plants called window algae grow on the lower side of transparent
quartz stones (Tchan and Beadle, 1955, p. 101; Vogel, 1955, p. 52ίϊ).
Light rains occur frequently and dew runs along the surface of the stones
to provide the underlaying algae with moisture and to protect them against
evaporation. Earth lichens with thalli a few millimeters long are able to
utilize dew (Vogel, 1955, p. 115). Window algae and lichens were recorded in the foggy Namib Desert but are not found further inland where
no fog develops, although summer rains may occur there. Window algae
also occur in other arid regions (Walter, 1964, p. 387).
Xeromorphic mosses (e.g., certain Ricciaceae) with partly subterranean
thalli, which are found in many arid regions, have not been as yet studied
ecologically. Lower plants of the desert, in general, have hardly been investigated in this regard.
Poikilohydric pteridophyta have air-dry leaves during drought periods,
but they regain turgidity and green color within only 15-30 minutes after
the first rain. These ferns are widely distributed through the arid regions
of South Africa and America but do not grow in extreme deserts. The
\p 8 values of their leaves during the rainy period are high (Table XI),
but still lower than for most winter annuals. This difference is generally
found and is characteristic for the pteridophytes, which do not have a very
efficient water-conducting system.
The leaves of most of these fern species roll and dry when the dry season
begins and the available soil moisture supply decreases. Loss of turgidity
occurs rapidly and half-dried and incompletely rolled fern leaves rarely
are seen. The rolling mechanism of these leaves (Walter and Bauer, 1937)
and the photosynthesis and respiration of the poikilohydric ferns before