V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
225
Ordos Desert (Inner Mongolia) (J), Central Australia (K), Southern
Peruvian region (L), Southern Sahara; (4) arid regions seldom having
rain, which may fall at any time during the year: Lake Eyre basin (South
Australia, Fig. 9 ) ; (5) fog deserts almost without measurable rainfall:
Southern Peruvian-Northern Chilean region (M), Namib Desert [Southwest Africa (N)]; and (6) deserts almost without rain and vegetation:
Central Sahara (O), Southern Egyptian Desert (P).
The floristic kingdom to which the vegetation of an arid region belongs
also has some influence on the type of vegetation. Often particular families
with characteristic life forms strongly predominate. In those American
deserts which belong to the Neotropics, the Cactaceae, in South America,
a few Bromeliaceae are prevalent in addition. In South Africa (Capensis),
Mesembrianthemaceae and also some succulent Euphorbiaceae are characteristic. In the arid regions of the Holarctic, numerous species of halophytic
Chenopodiaceae are found. Australia has no true succulents and in the
driest parts the halophytic saltbush (Atriplex vesicaria) and the bluebush
(Kochia sedoides) are the most important species.
The climatic differences and edaphic factors together with the floristic
composition of each arid region give every desert its uniqueness and make
it difficult to compare one desert with another. Much misunderstanding
in the ecological evaluation of the desert plants has been caused during
the past few decades by overgeneralization. A single investigator usually
knows only one type of desert region or type of desert well and believes
his findings to be applicable to all desert plants. Because of the individual
peculiarities of various deserts, each was dealt with separately by Walter
(1964, 1968); this is not possible in this chapter. Therefore, the similarities will be emphasized more and the differences will be mentioned less.
VI. Hydration of Protoplasm and Osmotic Potential
The thermodynamic activity of water inside the living protoplasm plays
a special role in the life of plants.
The relative thermodynamic activity (a) of water can be defined as
p
vapor pressure over a solution (or an imbibant)
...
a = — =
:
(1)
Po
saturation water pressure over pure water
where p and p 0 are both to be measured under the same temperature and
pressure conditions (Kreeb and Borchard, 1967, p. 200). Walter (1931,
p. 6) introduced the term hydrature (hy) for the product (p/p 0 ) X 100.
225
Ordos Desert (Inner Mongolia) (J), Central Australia (K), Southern
Peruvian region (L), Southern Sahara; (4) arid regions seldom having
rain, which may fall at any time during the year: Lake Eyre basin (South
Australia, Fig. 9 ) ; (5) fog deserts almost without measurable rainfall:
Southern Peruvian-Northern Chilean region (M), Namib Desert [Southwest Africa (N)]; and (6) deserts almost without rain and vegetation:
Central Sahara (O), Southern Egyptian Desert (P).
The floristic kingdom to which the vegetation of an arid region belongs
also has some influence on the type of vegetation. Often particular families
with characteristic life forms strongly predominate. In those American
deserts which belong to the Neotropics, the Cactaceae, in South America,
a few Bromeliaceae are prevalent in addition. In South Africa (Capensis),
Mesembrianthemaceae and also some succulent Euphorbiaceae are characteristic. In the arid regions of the Holarctic, numerous species of halophytic
Chenopodiaceae are found. Australia has no true succulents and in the
driest parts the halophytic saltbush (Atriplex vesicaria) and the bluebush
(Kochia sedoides) are the most important species.
The climatic differences and edaphic factors together with the floristic
composition of each arid region give every desert its uniqueness and make
it difficult to compare one desert with another. Much misunderstanding
in the ecological evaluation of the desert plants has been caused during
the past few decades by overgeneralization. A single investigator usually
knows only one type of desert region or type of desert well and believes
his findings to be applicable to all desert plants. Because of the individual
peculiarities of various deserts, each was dealt with separately by Walter
(1964, 1968); this is not possible in this chapter. Therefore, the similarities will be emphasized more and the differences will be mentioned less.
VI. Hydration of Protoplasm and Osmotic Potential
The thermodynamic activity of water inside the living protoplasm plays
a special role in the life of plants.
The relative thermodynamic activity (a) of water can be defined as
p
vapor pressure over a solution (or an imbibant)
...
a = — =
:
(1)
Po
saturation water pressure over pure water
where p and p 0 are both to be measured under the same temperature and
pressure conditions (Kreeb and Borchard, 1967, p. 200). Walter (1931,
p. 6) introduced the term hydrature (hy) for the product (p/p 0 ) X 100.