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H. WALTER AND E. STADELMANN
ous vegetative growth, but little floral development. Marginal water supply
reduces vegetative growth, but enhances reproductive development. This
relationship may indicate that decreased protoplasmic hydrature is important for flower bud formation. Hydrature changes might possibly act indirectly on flower bud formation by changing the intensity of hormone
synthesis, but no investigations on the relation of hydration of protoplasm
to reproductive development are known to the authors.
XIV. The Ecological Plant Types in Arid Regions
Protoplasmic hydrature becomes less important for plant growth and
development as the climate becomes increasingly humid. This explains why
the concept of hydrature was derived from observations made in an arid
region (Arizona; Walter, 1931, p. 6). It may also be difficult to recognize
the importance of hydrature without field experience in arid regions, since
most laboratory experiments are performed with plants grown with an
abundant water supply.
Classification of plants into ecological types based upon their hydrature
is important mainly for plants in arid regions. Previously, when knowledge
about these plants was restricted, all plants growing in arid regions were
called xerophytes.* At that time the differences among arid regions were
often overlooked (see p. 223ff). Arid regions can be defined as those areas
with low annual precipitation, while potential evaporation is relatively high.
Many of the arid regions so defined have regular rainfall during certain
seasons of the year so that there is a shorter or longer period each year
when water availability is relatively high. It would be inappropriate to consider as xerophytes those plants which restrict their development to a period
of sufficient water supply. Such drought-avoiding plants are the summerand winter-ephemeral species; they are either annuals or geophytes. The
dormant organs of the latter survive the drought period beneath the soil
surface.
Erodium cicutarium, an adventive weed plant in Central Europe, belongs to the winter ephemeral plants of arid regions. In Arizona this species
has an osmotic potential between —8.8 and —14 atm and should never
be called a xerophyte. The range of hydrature of the winter annuals corresponds throughout to the hydrature of the herbaceous plants of Central
* The term "xerophyte" is generally thought to have been introduced by Schouw
(1822). However this author has not yet used this word, which seems to be first
mentioned by Drude (1890, p. 81). "Xerophyte" originally designated plants which
occurred in dry habitats. Later, and incorrectly, the meaning of this term was extended to include all plants growing in arid regions.
H. WALTER AND E. STADELMANN
ous vegetative growth, but little floral development. Marginal water supply
reduces vegetative growth, but enhances reproductive development. This
relationship may indicate that decreased protoplasmic hydrature is important for flower bud formation. Hydrature changes might possibly act indirectly on flower bud formation by changing the intensity of hormone
synthesis, but no investigations on the relation of hydration of protoplasm
to reproductive development are known to the authors.
XIV. The Ecological Plant Types in Arid Regions
Protoplasmic hydrature becomes less important for plant growth and
development as the climate becomes increasingly humid. This explains why
the concept of hydrature was derived from observations made in an arid
region (Arizona; Walter, 1931, p. 6). It may also be difficult to recognize
the importance of hydrature without field experience in arid regions, since
most laboratory experiments are performed with plants grown with an
abundant water supply.
Classification of plants into ecological types based upon their hydrature
is important mainly for plants in arid regions. Previously, when knowledge
about these plants was restricted, all plants growing in arid regions were
called xerophytes.* At that time the differences among arid regions were
often overlooked (see p. 223ff). Arid regions can be defined as those areas
with low annual precipitation, while potential evaporation is relatively high.
Many of the arid regions so defined have regular rainfall during certain
seasons of the year so that there is a shorter or longer period each year
when water availability is relatively high. It would be inappropriate to consider as xerophytes those plants which restrict their development to a period
of sufficient water supply. Such drought-avoiding plants are the summerand winter-ephemeral species; they are either annuals or geophytes. The
dormant organs of the latter survive the drought period beneath the soil
surface.
Erodium cicutarium, an adventive weed plant in Central Europe, belongs to the winter ephemeral plants of arid regions. In Arizona this species
has an osmotic potential between —8.8 and —14 atm and should never
be called a xerophyte. The range of hydrature of the winter annuals corresponds throughout to the hydrature of the herbaceous plants of Central
* The term "xerophyte" is generally thought to have been introduced by Schouw
(1822). However this author has not yet used this word, which seems to be first
mentioned by Drude (1890, p. 81). "Xerophyte" originally designated plants which
occurred in dry habitats. Later, and incorrectly, the meaning of this term was extended to include all plants growing in arid regions.