V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
255
Yield is always influenced by balances among physiological processes
such as photosynthesis and respiration. Equally important is the conversion
of photosynthetic products into plant tissues. Organic matter may be used
in a productive way for increase in leaf size or number, or, nonproductively, for formation of mechanical tissues in the stem only (Walter, 1960,
p. 399). Different patterns in the utilization of assimilate cause, for example, differences in dry matter production between a Helianthus cmnuus
plant and a 1-year-old Fagus seedling under the same environmental conditions: whereas intensity of photosynthesis of the unit of leaf surface area in
Helianthus is only four to five times higher than in leaves of Fagus, the
annual production of dry matter is about forty times higher in Helianthus
than in Fagus. The latter plant develops already in the first year a woody
stem and only two or three leaves.
In the final analysis, all the physiological factors in the plant depend
upon the living protoplasm and its activity, which in turn depends upon
protoplasmic hydrature. Secondary control mechanisms may also cooperate, for instance, in photosynthesis, the regulation of stomatal opening
which influences rates of photosynthesis. An analysis of all the factors
which affect yield and utilization of assimilates is not yet available (Walter
ndKrçeb, 1970, p. 191).
The discussion above dealt with quantitative changes caused by drought.
However, these changes are always accompanied by qualitative alterations
of structure and the development of plant organs. These differences in
structure are especially evident in desert plants, and for their development
water stress generally is the controlling factor.
XIII. The Relation between the Osmotic Potential (ψ 8 ) and the
Anatomical and Morphological Characteristics of Plants
A. Encelia farinosa
Interactions often exist between osmotic potential and the anatomical
and morphological features of plants. The case of the shade leaves and
sun leaves was mentioned previously. An even more striking example involves the composite Encelia farinosa, a semishrub characteristic of the
Sonora Desert of Arizona (Walter, 1931, p. 134ff). The plant (Fig. 23)
has soft and hairy leaves which vary considerably in form (Fig. 24). The
close correlation between leaf form and ψ 8 indicates a dependence of leaf
form on osmotic potential.
After a rainy period the leaves of Encelia are large, greenish, and
hygromorphic with few hairs; their osmotic potential is about —22 to
255
Yield is always influenced by balances among physiological processes
such as photosynthesis and respiration. Equally important is the conversion
of photosynthetic products into plant tissues. Organic matter may be used
in a productive way for increase in leaf size or number, or, nonproductively, for formation of mechanical tissues in the stem only (Walter, 1960,
p. 399). Different patterns in the utilization of assimilate cause, for example, differences in dry matter production between a Helianthus cmnuus
plant and a 1-year-old Fagus seedling under the same environmental conditions: whereas intensity of photosynthesis of the unit of leaf surface area in
Helianthus is only four to five times higher than in leaves of Fagus, the
annual production of dry matter is about forty times higher in Helianthus
than in Fagus. The latter plant develops already in the first year a woody
stem and only two or three leaves.
In the final analysis, all the physiological factors in the plant depend
upon the living protoplasm and its activity, which in turn depends upon
protoplasmic hydrature. Secondary control mechanisms may also cooperate, for instance, in photosynthesis, the regulation of stomatal opening
which influences rates of photosynthesis. An analysis of all the factors
which affect yield and utilization of assimilates is not yet available (Walter
ndKrçeb, 1970, p. 191).
The discussion above dealt with quantitative changes caused by drought.
However, these changes are always accompanied by qualitative alterations
of structure and the development of plant organs. These differences in
structure are especially evident in desert plants, and for their development
water stress generally is the controlling factor.
XIII. The Relation between the Osmotic Potential (ψ 8 ) and the
Anatomical and Morphological Characteristics of Plants
A. Encelia farinosa
Interactions often exist between osmotic potential and the anatomical
and morphological features of plants. The case of the shade leaves and
sun leaves was mentioned previously. An even more striking example involves the composite Encelia farinosa, a semishrub characteristic of the
Sonora Desert of Arizona (Walter, 1931, p. 134ff). The plant (Fig. 23)
has soft and hairy leaves which vary considerably in form (Fig. 24). The
close correlation between leaf form and ψ 8 indicates a dependence of leaf
form on osmotic potential.
After a rainy period the leaves of Encelia are large, greenish, and
hygromorphic with few hairs; their osmotic potential is about —22 to