V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
263
Europe or the eastern part of North America: The osmotic potentials of
those plants in Arizona are, for instance, for Parietaria debilis —9.5 atm;
Delphinium scaposum, near —18.9 atm; and Galium asperrimum, —9.6
atm. These plants grow in shade (Harris and Lawrence, 1916, pp. 15,
23, and 25), although the ψ 8 values are not much higher even when such
species grow in more sunny places: Lepidium medium, —13.5 to —14.6
atm; Sophia pinnata, —17.7 atm. (Harris and Lawrence, 1916, pp. 39 and
40).
Small moist niches can be found in all arid regions. These niches at
first glance may appear to be quite dry, e.g., north slopes in cleft rocks,
on the lower end of talus, or areas in dry river beds with a relatively high
water table of fresh water. In those habitats plants may have access to
an ample water supply throughout the whole year. Species growing in such
places often have their main distribution area in more humid tropic regions
with summer rains, e.g., various species of Acacia, and other trees and
shrubs, Citrullus, etc. It would certainly be misleading to call these plants
xerophytes.
The other plant species of arid regions, which do not belong to the annuals or geophytes, have above-ground organs which stay alive all year,
even during the dry season. These plants must survive with whatever rainfall reaches their habitat. One group of these plants, the leaf and stem succulents and the less known root succulents, accumulate large amounts of
water and can survive droughts without significant water uptake from the
soil. The other group includes, apart from the halophytes, plants growing
on saline soil, the true xerophytes. The latter can be subdivided with regard
to their features of hydrature into several subgroups.
XV. Hydrature in True Xerophytes
True xerophytes are those nonsucculent and nonhalophytic plants of
arid regions which maintain living, above-ground organs throughout the
year, including drought periods, and obtain their water supply from local
precipitation or atmospheric moisture only.
Four subdivisions of the true xerophytes can be made :
1. Poikilohydrous xerophtes
2. Malacophyllous xerophytes
3. Sclerophyllous or aphyllous xerophytes
4. Stenohydrous xerophytes
Plant types with an intermediary position between these subdivisions
can also be found.
263
Europe or the eastern part of North America: The osmotic potentials of
those plants in Arizona are, for instance, for Parietaria debilis —9.5 atm;
Delphinium scaposum, near —18.9 atm; and Galium asperrimum, —9.6
atm. These plants grow in shade (Harris and Lawrence, 1916, pp. 15,
23, and 25), although the ψ 8 values are not much higher even when such
species grow in more sunny places: Lepidium medium, —13.5 to —14.6
atm; Sophia pinnata, —17.7 atm. (Harris and Lawrence, 1916, pp. 39 and
40).
Small moist niches can be found in all arid regions. These niches at
first glance may appear to be quite dry, e.g., north slopes in cleft rocks,
on the lower end of talus, or areas in dry river beds with a relatively high
water table of fresh water. In those habitats plants may have access to
an ample water supply throughout the whole year. Species growing in such
places often have their main distribution area in more humid tropic regions
with summer rains, e.g., various species of Acacia, and other trees and
shrubs, Citrullus, etc. It would certainly be misleading to call these plants
xerophytes.
The other plant species of arid regions, which do not belong to the annuals or geophytes, have above-ground organs which stay alive all year,
even during the dry season. These plants must survive with whatever rainfall reaches their habitat. One group of these plants, the leaf and stem succulents and the less known root succulents, accumulate large amounts of
water and can survive droughts without significant water uptake from the
soil. The other group includes, apart from the halophytes, plants growing
on saline soil, the true xerophytes. The latter can be subdivided with regard
to their features of hydrature into several subgroups.
XV. Hydrature in True Xerophytes
True xerophytes are those nonsucculent and nonhalophytic plants of
arid regions which maintain living, above-ground organs throughout the
year, including drought periods, and obtain their water supply from local
precipitation or atmospheric moisture only.
Four subdivisions of the true xerophytes can be made :
1. Poikilohydrous xerophtes
2. Malacophyllous xerophytes
3. Sclerophyllous or aphyllous xerophytes
4. Stenohydrous xerophytes
Plant types with an intermediary position between these subdivisions
can also be found.