268
H. WALTER AND E. STADELMANN
acteristic of the driest habitats in Arizona. In Argentina, it occurs at the
foot of the eastern slope of the Andes in a narrow desert region 2000 km
long. The leaves of this plant are small, hard, but without sclerenchymatous tissue and very drought resistant. They have a low degree of hydrostability but mostly remain on the branches during drought.
Sclerophyllous species of this subgroup are also found in extreme
deserts, but with increasing drought their habitats are restricted to gulleys
and valleys (contracted vegetation, see Section III), i.e., habitats which
provide limited water supply from the soil even during extended drought
periods. Ground water is not required; a certain amount of soil moisture
is sufficient for survival of these species in such valleys.
In the xeromorphic leaves of sclerophyllous shrubs water stress conditions may cause a water potential ψ which is considerably lower than the
osmotic potential ψ 8 (Kreeb, 1960, p. 278; 1961, p. 484; Önal, 1962,
p. 42ff). Such low ψ develops when twigs of these plants were left in the
laboratory for about 8 days (e.g., Buxus sempervirens leaves exhibit
\f/ s = —65 atm, and ψ = —110 atm). The viability of the twigs was
proven by photosynthesis and testing for the light compensation point.
Buxus sempervirens twigs, after being subjected to such water stress,
reached the light compensation point at several hundred lux indicating
that the water stress had not yet caused a permanent damage to the cells.
The excess of φ over φ 8 results in a negative turgor pressure which is
caused by the thickened and relatively rigid cell walls when higher water
deficits develop. In contrast to sclerophyllous xerophytes, mesophytes do
not develop a negative wall pressure (Önal, 1962, p. 52).
The ecophysiological importance of a negative wall pressure has been
discussed for some time, since it was thought possibly to contribute considerably to the water potential of the cell during drought periods and to
promote a better water supply. However, under natural conditions during
drought periods negative wall pressure of leaf cells was never found to
be higher than 5-10 atm in plants of the arid zones of Egypt and Spain.
Therefore negative wall pressure does not contribute a significant portion
of the total water potential (Kreeb, 1961, p. 486; 1963, p. 456) and does
not appear to be of ecological importance.
D. STENOHYDRIC XEROPHYTES
Plants belonging to this scarcely studied subgroup completely close
their stomata when the water balance is only slightly disturbed, so that
almost no decrease of φ 8 is observed. These species therefore are hydrostable. However, stomatal closure leads to cessation of gas exchange and
therefore of photosynthesis, which in turn causes a starvation condition in
H. WALTER AND E. STADELMANN
acteristic of the driest habitats in Arizona. In Argentina, it occurs at the
foot of the eastern slope of the Andes in a narrow desert region 2000 km
long. The leaves of this plant are small, hard, but without sclerenchymatous tissue and very drought resistant. They have a low degree of hydrostability but mostly remain on the branches during drought.
Sclerophyllous species of this subgroup are also found in extreme
deserts, but with increasing drought their habitats are restricted to gulleys
and valleys (contracted vegetation, see Section III), i.e., habitats which
provide limited water supply from the soil even during extended drought
periods. Ground water is not required; a certain amount of soil moisture
is sufficient for survival of these species in such valleys.
In the xeromorphic leaves of sclerophyllous shrubs water stress conditions may cause a water potential ψ which is considerably lower than the
osmotic potential ψ 8 (Kreeb, 1960, p. 278; 1961, p. 484; Önal, 1962,
p. 42ff). Such low ψ develops when twigs of these plants were left in the
laboratory for about 8 days (e.g., Buxus sempervirens leaves exhibit
\f/ s = —65 atm, and ψ = —110 atm). The viability of the twigs was
proven by photosynthesis and testing for the light compensation point.
Buxus sempervirens twigs, after being subjected to such water stress,
reached the light compensation point at several hundred lux indicating
that the water stress had not yet caused a permanent damage to the cells.
The excess of φ over φ 8 results in a negative turgor pressure which is
caused by the thickened and relatively rigid cell walls when higher water
deficits develop. In contrast to sclerophyllous xerophytes, mesophytes do
not develop a negative wall pressure (Önal, 1962, p. 52).
The ecophysiological importance of a negative wall pressure has been
discussed for some time, since it was thought possibly to contribute considerably to the water potential of the cell during drought periods and to
promote a better water supply. However, under natural conditions during
drought periods negative wall pressure of leaf cells was never found to
be higher than 5-10 atm in plants of the arid zones of Egypt and Spain.
Therefore negative wall pressure does not contribute a significant portion
of the total water potential (Kreeb, 1961, p. 486; 1963, p. 456) and does
not appear to be of ecological importance.
D. STENOHYDRIC XEROPHYTES
Plants belonging to this scarcely studied subgroup completely close
their stomata when the water balance is only slightly disturbed, so that
almost no decrease of φ 8 is observed. These species therefore are hydrostable. However, stomatal closure leads to cessation of gas exchange and
therefore of photosynthesis, which in turn causes a starvation condition in