V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
257
—23 atm. During drought ψ 8 decreases to —28 atm and the new leaves are
smaller, mesomorphic, and more pubescent than those formed after rains.
At ψ 8 = —32 atm, only small, white, fleshy, and very pubescent xeromorphic leaves develop. The hygromorphic leaves desiccate and abscise at
ψ 8 = —36 atm. Soon the mesomorphic leaves also abscise. When ψ 8
reaches —40 atm, the xeromorphic leaves abscise and only the terminal
buds with the leaf initials (protected by a dense hair cover) which can
withstand osmotic potentials as low as —49 atm remain on the twigs.
When Encelia grows in stony soil between rocks on a north slope with
sufficient water supply during the drought season ψ 8 values do not decrease
as mentioned before. Only hygromorphic leaves develop and flowers are
few.
B. OTHER HETEROPHYLLIC PLANTS
Ecologically caused heterophylly as in Encelia is found quite often.
Poterium spinosum, Artemisia monosperma, and Helianthemum ellipticum
growing in Palestine develop summer leaves with a transpiring mass about
three-six times smaller than in winter leaves (Orshan, 1954, pp. 442 and
443). Both leaf types have similar transpiration rates per gram of leaf
fresh weight in Poterium spinosum, Ononis natrix, and Artemisia monosperma; hence, there is much less water loss during the summer. This
smaller size of the summer leaves is probably caused by a lowering of ψ 8 ;
however, these values were not measured.
C. CACTACEAE
Another example of the relation between ψ 8 and plant anatomical structure is found in Ferocactus (Echinocactus) wislizenii in Arizona (Walter,
1931, p. 141ff). Growth of this plant is reduced most on its southwest side,
which is exposed to the hottest afternoon sun. The unequal growth causes
the tip of the plant to bend toward the southwest (Fig. 25). The cross section of the stem is asymmetric since the ribs are closer together at the southwest side and further apart on the northeast side (Fig. 26). The effect
becomes more pronounced as the plant ages. The vessels and sclerenchyma
of the centrally located xylem are developed more on the southwest side
than on the northeast side. Therefore, the plant has only a single northeast
to southwest plane of symmetry. Samples of similar tissues taken from different places of the cross section indicate that the structural differences are
reflected in changes of the osmotic potential, as can be seen from the
isosmotic curves (Fig. 26). Lowest ψ 8 values (highest potential osmotic
pressures) are found in the southwest side of the stem and highest φ 8 in the
northeast.
257
—23 atm. During drought ψ 8 decreases to —28 atm and the new leaves are
smaller, mesomorphic, and more pubescent than those formed after rains.
At ψ 8 = —32 atm, only small, white, fleshy, and very pubescent xeromorphic leaves develop. The hygromorphic leaves desiccate and abscise at
ψ 8 = —36 atm. Soon the mesomorphic leaves also abscise. When ψ 8
reaches —40 atm, the xeromorphic leaves abscise and only the terminal
buds with the leaf initials (protected by a dense hair cover) which can
withstand osmotic potentials as low as —49 atm remain on the twigs.
When Encelia grows in stony soil between rocks on a north slope with
sufficient water supply during the drought season ψ 8 values do not decrease
as mentioned before. Only hygromorphic leaves develop and flowers are
few.
B. OTHER HETEROPHYLLIC PLANTS
Ecologically caused heterophylly as in Encelia is found quite often.
Poterium spinosum, Artemisia monosperma, and Helianthemum ellipticum
growing in Palestine develop summer leaves with a transpiring mass about
three-six times smaller than in winter leaves (Orshan, 1954, pp. 442 and
443). Both leaf types have similar transpiration rates per gram of leaf
fresh weight in Poterium spinosum, Ononis natrix, and Artemisia monosperma; hence, there is much less water loss during the summer. This
smaller size of the summer leaves is probably caused by a lowering of ψ 8 ;
however, these values were not measured.
C. CACTACEAE
Another example of the relation between ψ 8 and plant anatomical structure is found in Ferocactus (Echinocactus) wislizenii in Arizona (Walter,
1931, p. 141ff). Growth of this plant is reduced most on its southwest side,
which is exposed to the hottest afternoon sun. The unequal growth causes
the tip of the plant to bend toward the southwest (Fig. 25). The cross section of the stem is asymmetric since the ribs are closer together at the southwest side and further apart on the northeast side (Fig. 26). The effect
becomes more pronounced as the plant ages. The vessels and sclerenchyma
of the centrally located xylem are developed more on the southwest side
than on the northeast side. Therefore, the plant has only a single northeast
to southwest plane of symmetry. Samples of similar tissues taken from different places of the cross section indicate that the structural differences are
reflected in changes of the osmotic potential, as can be seen from the
isosmotic curves (Fig. 26). Lowest ψ 8 values (highest potential osmotic
pressures) are found in the southwest side of the stem and highest φ 8 in the
northeast.