266
H. WALTER AND E. STADELMANN
fresh leaves and those of homoiohydric angiosperms. The fresh leaves become damaged after prolonged exposure to a temperature of 50°C (Ziegler
and Vieweg, 1970, p. 106).
B. MALACOPHYLLOUS XEROPHYTES
Species belonging to this group are highly characteristic of steppes and
prairies with relatively short drought periods and extended periods of readily available water during spring and early summer. Maximov (1923) mistakenly attempted to extend the result of his studies of this subgroup to
all xerophytes.
The malacophyllous xerophytes restrict their transpiration only very
slightly at the beginning of a drought period; hence large water deficits
result. The leaves are able to withstand this water deficit to a certain
degree. The rapid decrease of the φ 8 values indicates that these species are
hydrolabile. The great difference between ψ 8 opt and ψ 8 min characterizes
these species as euryhydric plants. Species of Cistus; Phlomis, Lavendula,
Thymus, Rosmarinus, and other genera of the Mediterranean region belong to the malacophyllous xerophytes. From Arizona, Encelia farinosa,
Artemisia tridentata, and Gaertneria {Franseria) deltoïdes (a composite
with φ 8 opt around —20 atm and ψ 8 min below —52.5 atm) are malacophyllous species. Many genera from Labiatae, Compositae, and Cistaceae
found in arid regions belong to this subgroup of xerophytes.
The reaction of a typical malacophyllous xerophyte to an extended
drought period was discussed for Encelia farinosa (see Section ΧΙΙΙ,Α).
In the Mediterranean region, these plants drop almost all their leaves during the summer drought and retain only the small terminal buds. Transpiration of these buds is very small, so that a minimal water uptake from
the soil is sufficient to keep the water balance at the level adequate for
survival. The leaves of the malacophyllous xerophytes of the steppes usually remain exceedingly wilted but recover after rain. The ability to withstand large water deficits was considered by Maximov (1923) as the most
significant feature of xerophytes. This is true for the malacophyllous xerophytes but is incorrect for the next subgroup.
C. SCLEROPHYLLOUS AND APHYLLOUS XEROPHYTES
Sclerophyllous shrubs are found in all arid areas with a Mediterranean
climate, as in the Mediterranean region, Southern California, in Central
Chile, in the Cape region, and Southwestern and Southern Australia, where
Proteaceae are the most frequent representatives of this group. Eucalyptus
may also be classified within this subgroup. However, its leaves are rather
H. WALTER AND E. STADELMANN
fresh leaves and those of homoiohydric angiosperms. The fresh leaves become damaged after prolonged exposure to a temperature of 50°C (Ziegler
and Vieweg, 1970, p. 106).
B. MALACOPHYLLOUS XEROPHYTES
Species belonging to this group are highly characteristic of steppes and
prairies with relatively short drought periods and extended periods of readily available water during spring and early summer. Maximov (1923) mistakenly attempted to extend the result of his studies of this subgroup to
all xerophytes.
The malacophyllous xerophytes restrict their transpiration only very
slightly at the beginning of a drought period; hence large water deficits
result. The leaves are able to withstand this water deficit to a certain
degree. The rapid decrease of the φ 8 values indicates that these species are
hydrolabile. The great difference between ψ 8 opt and ψ 8 min characterizes
these species as euryhydric plants. Species of Cistus; Phlomis, Lavendula,
Thymus, Rosmarinus, and other genera of the Mediterranean region belong to the malacophyllous xerophytes. From Arizona, Encelia farinosa,
Artemisia tridentata, and Gaertneria {Franseria) deltoïdes (a composite
with φ 8 opt around —20 atm and ψ 8 min below —52.5 atm) are malacophyllous species. Many genera from Labiatae, Compositae, and Cistaceae
found in arid regions belong to this subgroup of xerophytes.
The reaction of a typical malacophyllous xerophyte to an extended
drought period was discussed for Encelia farinosa (see Section ΧΙΙΙ,Α).
In the Mediterranean region, these plants drop almost all their leaves during the summer drought and retain only the small terminal buds. Transpiration of these buds is very small, so that a minimal water uptake from
the soil is sufficient to keep the water balance at the level adequate for
survival. The leaves of the malacophyllous xerophytes of the steppes usually remain exceedingly wilted but recover after rain. The ability to withstand large water deficits was considered by Maximov (1923) as the most
significant feature of xerophytes. This is true for the malacophyllous xerophytes but is incorrect for the next subgroup.
C. SCLEROPHYLLOUS AND APHYLLOUS XEROPHYTES
Sclerophyllous shrubs are found in all arid areas with a Mediterranean
climate, as in the Mediterranean region, Southern California, in Central
Chile, in the Cape region, and Southwestern and Southern Australia, where
Proteaceae are the most frequent representatives of this group. Eucalyptus
may also be classified within this subgroup. However, its leaves are rather