V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
229
V w is the mole volume of water (18 cm
3
mole
-1
) ; p is the vapor pressure
over the solution; p 0 is the vapor pressure over pure water; hy is the
hydrature of solution; and a is the relative water activity.
The hydrature of protoplasm is not identical with its hydration. The
former indicates the relative thermodynamic activity of water in protoplasm. Hydration measures the imbibition (swelling) as water content per
gram dry weight. However, there is a linear relationship between both
functions in the physiological range (96-100% hydrature, cf. imbibition
curve of protoplasm, Walter, 1923, p. 176; Walter and Stadelmann, 1968,
p. 697, Fig. 4).
The frequently expressed opinion that hydration of protoplasm depends
upon the water potential (suction potential, DPD) and that the protoplasm
shows maximum swelling in a water-saturated cell can be negated by thermodynamic considerations. The relative activity of water (hydrature) of
the cell sap and, therefore, also of protoplasm is always smaller than that
of pure water; therefore maximum swelling of protoplasm is not reached
(Walter and Stadelmann, 1968, p. 696).
When the osmotic potential of the cell sap φ 8 is known, Eq. ( 1 ) can be
used to calculate the hydrature of living protoplasm (see Walter, 1931,
p. 159, Tables I and II), which otherwise cannot be measured directly.
(It should be remembered that ψ 8 is temperature dependent while hy is
not.) The osmotic potential is of special importance for ecological aspects
of the water relations of desert plants.
The many thousands of determinations of potential osmotic pressure
carried out on hundreds of plant species in all climatic regions and under
very different ecological conditions have been used to develop osmotic
spectra (Fig. 10 shows one of these spectra for the Arizona desert). An
osmotic spectrum characterizes the plant life form of a vegetation (e.g.,
grass, leaf succulents, and deciduous trees) by the maxima and minima
values for π* of each constituent plant species (see Walter, 1960, p. 239).
[The potential osmotic pressure π* is the negative value of the osmotic
potential ψ 8 and was used to indicate the hydrature before the thermodynamic terminology was established (see Walter, I960)].
VII. Plant Hydrature and Environmental Water Conditions
Changes in hydrature in the plant with the variations of the available
water from the environment indicate that two different types of plants exist.
a. Poikilohydric Plants. The protoplasmic hydrature of poikilohydric
plants changes with and is directly related to hydrature of the environment
(air humidity). Among the land plants the algae, fungi, lichens, and bryo-
229
V w is the mole volume of water (18 cm
3
mole
-1
) ; p is the vapor pressure
over the solution; p 0 is the vapor pressure over pure water; hy is the
hydrature of solution; and a is the relative water activity.
The hydrature of protoplasm is not identical with its hydration. The
former indicates the relative thermodynamic activity of water in protoplasm. Hydration measures the imbibition (swelling) as water content per
gram dry weight. However, there is a linear relationship between both
functions in the physiological range (96-100% hydrature, cf. imbibition
curve of protoplasm, Walter, 1923, p. 176; Walter and Stadelmann, 1968,
p. 697, Fig. 4).
The frequently expressed opinion that hydration of protoplasm depends
upon the water potential (suction potential, DPD) and that the protoplasm
shows maximum swelling in a water-saturated cell can be negated by thermodynamic considerations. The relative activity of water (hydrature) of
the cell sap and, therefore, also of protoplasm is always smaller than that
of pure water; therefore maximum swelling of protoplasm is not reached
(Walter and Stadelmann, 1968, p. 696).
When the osmotic potential of the cell sap φ 8 is known, Eq. ( 1 ) can be
used to calculate the hydrature of living protoplasm (see Walter, 1931,
p. 159, Tables I and II), which otherwise cannot be measured directly.
(It should be remembered that ψ 8 is temperature dependent while hy is
not.) The osmotic potential is of special importance for ecological aspects
of the water relations of desert plants.
The many thousands of determinations of potential osmotic pressure
carried out on hundreds of plant species in all climatic regions and under
very different ecological conditions have been used to develop osmotic
spectra (Fig. 10 shows one of these spectra for the Arizona desert). An
osmotic spectrum characterizes the plant life form of a vegetation (e.g.,
grass, leaf succulents, and deciduous trees) by the maxima and minima
values for π* of each constituent plant species (see Walter, 1960, p. 239).
[The potential osmotic pressure π* is the negative value of the osmotic
potential ψ 8 and was used to indicate the hydrature before the thermodynamic terminology was established (see Walter, I960)].
VII. Plant Hydrature and Environmental Water Conditions
Changes in hydrature in the plant with the variations of the available
water from the environment indicate that two different types of plants exist.
a. Poikilohydric Plants. The protoplasmic hydrature of poikilohydric
plants changes with and is directly related to hydrature of the environment
(air humidity). Among the land plants the algae, fungi, lichens, and bryo-