V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
267
coriaceous and not strengthened with sclerenchyma. Aphyllous species are
Spartium junceum and many Genista species of the Mediterranean region.
Other aphyllous rod shrubs often have leaves reduced to very small scales,
or stems transformed into thorny phylloclades (e.g., species of Colletia
in Argentina). Rod shrubs belonging to different families are very frequently found in North Patagonia.
The evergreen oaks of the Mediterranean region, especially Quercus
ilex, are the ecologically most studied sclerophyllous species (cf. Walter,
1968, p. 44ff; Breckle, 1966).
In contrast to malacophyllous plants, the sclerophyllous xerophytes are
able to retain a favorable internal water balance by restricting transpiration
during drought periods ("xerophytes," after Schimper, 1898, p. 4 ) . The ψ 8
values hardly decrease at all during the summer drought period. These
plants enter a kind of resting period during the summer only when the habitat is exceptionally dry. This means that they reduce their gas exchange (and
therefore also their photosynthesis). A limited amount of moisture seems
to be available over the whole year for the root system which penetrates
deep into the soil. Plants which grow on locations so dry that their stomata
remain closed during the summer drought appear to lack vigor.
In most species, the small sclerophyllous and evergreen leaves remain
on the plant for 2 or more years. Hence, the transpiring surface of the
plant is not reduced during the drought season. The species are hydrostable and have a position midway between the stenohydric and euryhydric groups. Little information is available on φ 8 min values, since
drought-injured plants are rarely found in their natural habitats.
In Arizona, Cercidium microphyllum (its name palo verde refers to the
stem, which remains green even in very old trees), a 5-6-meters (maximum, 8-10-meters) tall tree, which may be considered as an aphyllous
plant, and the evergreen Quercus species of the encinal vegetation in Arizona belong to this subgroup.
The stem and twigs are the main assimilating organs of Cercidium microphyllum, since the paired pinnately compound leaf with three-five pairs
of leaflets (smaller than 1 mm) are ephemeral. The leaves grow only after
longer winter and summer rains and seldom survive more than 6-10 weeks.
At first the leaflets are dropped, later also the rachis. With all leaves on
the plant, their total surface area is still smaller than that of the stems
and twigs. The ψ 8 values of the leaves are quite constant between —26.6
and —31.5 atm, for dry locations —34.7 to —36.8 atm. Leaves from
young plants have surface areas about four times as great as the leaves
from older plants.
Larrea divaricata ( = L. tridentata, Creosote bush) is intermediary between the malacophyllous and sclerophyllous subgroups. This plant is char-
267
coriaceous and not strengthened with sclerenchyma. Aphyllous species are
Spartium junceum and many Genista species of the Mediterranean region.
Other aphyllous rod shrubs often have leaves reduced to very small scales,
or stems transformed into thorny phylloclades (e.g., species of Colletia
in Argentina). Rod shrubs belonging to different families are very frequently found in North Patagonia.
The evergreen oaks of the Mediterranean region, especially Quercus
ilex, are the ecologically most studied sclerophyllous species (cf. Walter,
1968, p. 44ff; Breckle, 1966).
In contrast to malacophyllous plants, the sclerophyllous xerophytes are
able to retain a favorable internal water balance by restricting transpiration
during drought periods ("xerophytes," after Schimper, 1898, p. 4 ) . The ψ 8
values hardly decrease at all during the summer drought period. These
plants enter a kind of resting period during the summer only when the habitat is exceptionally dry. This means that they reduce their gas exchange (and
therefore also their photosynthesis). A limited amount of moisture seems
to be available over the whole year for the root system which penetrates
deep into the soil. Plants which grow on locations so dry that their stomata
remain closed during the summer drought appear to lack vigor.
In most species, the small sclerophyllous and evergreen leaves remain
on the plant for 2 or more years. Hence, the transpiring surface of the
plant is not reduced during the drought season. The species are hydrostable and have a position midway between the stenohydric and euryhydric groups. Little information is available on φ 8 min values, since
drought-injured plants are rarely found in their natural habitats.
In Arizona, Cercidium microphyllum (its name palo verde refers to the
stem, which remains green even in very old trees), a 5-6-meters (maximum, 8-10-meters) tall tree, which may be considered as an aphyllous
plant, and the evergreen Quercus species of the encinal vegetation in Arizona belong to this subgroup.
The stem and twigs are the main assimilating organs of Cercidium microphyllum, since the paired pinnately compound leaf with three-five pairs
of leaflets (smaller than 1 mm) are ephemeral. The leaves grow only after
longer winter and summer rains and seldom survive more than 6-10 weeks.
At first the leaflets are dropped, later also the rachis. With all leaves on
the plant, their total surface area is still smaller than that of the stems
and twigs. The ψ 8 values of the leaves are quite constant between —26.6
and —31.5 atm, for dry locations —34.7 to —36.8 atm. Leaves from
young plants have surface areas about four times as great as the leaves
from older plants.
Larrea divaricata ( = L. tridentata, Creosote bush) is intermediary between the malacophyllous and sclerophyllous subgroups. This plant is char-