V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
249
than in shade leaves due to a higher transpiration of sun leaves. This
example illustrates the very close relationship between osmotic potential
of the leaves and the hydrature of the protoplasm in the meristematic cells
and thus between ψ 8 and xeromorphism of the leaf.
XII. Osmotic Potential (ψ 8 ) 9 Growth, and
Production of Organic Matter
The water balance of a plant is determined by the water content and
water retention of soil and by potential evaporation. The measurement of
environmental conditions, however, indicates little about their actual effects
on plants. For homoiohydric plants, the effect of external conditions on
hydrature is the most important factor for growth. This can be demonstrated for plants of the same species which are growing under different
environmental conditions when the osmotic potential of their vacuome is
determined.
Such a test was made for Solanum elaeagnifolium growing on river terraces of the Santa Cruz River in Arizona. Following a rain in that area,
water is retained in small, inconspicuous depressions of soil. While the
level ground quickly dries out again, the areas in the depressions are more
intensively moistened the deeper they are located. Thorough moistening
of soil at the bottom of the depression is shown by the presence of Prosopis
shrubs. After a rain, summer ephemeral plants grow. Solanum elaeagnifolium is found in the center as well as at the edges of a depression. The
size of these plants decreases from approximately 60 cm to only 1 cm in
height as their position is closer to the edge of the depression. The osmotic
potential of tall plants is about —15 atm; of the dwarf plants about —30
atm. Plant height and osmotic potential are parallel to each other (see
Fig. 18).
Dwarf plants of ephemeral species are seen in a desert after an occasionally heavy rainfall. Their shoot growth is always considerably less than
is their root growth. This shift in growth rate may be a valuable adaptation
to the arid environment.
An experiment with seedlings of Pisum sativum clearly shows the different effects of a decrease in hydrature for shoots and roots (Walter, 1962,
p. 344ff). At first, Pisum seeds were germinated in wet saw dust. In a
series of sealed vessels sugar solutions of stepwise increased concentration
were prepared. In each of these vessels the germinated seeds were placed
over the sugar solution so that only the roots were immersed in the solution. In this way the germinating seeds were exposed to air humidity corresponding to relative vapor pressure over the solution, and the same hy-
249
than in shade leaves due to a higher transpiration of sun leaves. This
example illustrates the very close relationship between osmotic potential
of the leaves and the hydrature of the protoplasm in the meristematic cells
and thus between ψ 8 and xeromorphism of the leaf.
XII. Osmotic Potential (ψ 8 ) 9 Growth, and
Production of Organic Matter
The water balance of a plant is determined by the water content and
water retention of soil and by potential evaporation. The measurement of
environmental conditions, however, indicates little about their actual effects
on plants. For homoiohydric plants, the effect of external conditions on
hydrature is the most important factor for growth. This can be demonstrated for plants of the same species which are growing under different
environmental conditions when the osmotic potential of their vacuome is
determined.
Such a test was made for Solanum elaeagnifolium growing on river terraces of the Santa Cruz River in Arizona. Following a rain in that area,
water is retained in small, inconspicuous depressions of soil. While the
level ground quickly dries out again, the areas in the depressions are more
intensively moistened the deeper they are located. Thorough moistening
of soil at the bottom of the depression is shown by the presence of Prosopis
shrubs. After a rain, summer ephemeral plants grow. Solanum elaeagnifolium is found in the center as well as at the edges of a depression. The
size of these plants decreases from approximately 60 cm to only 1 cm in
height as their position is closer to the edge of the depression. The osmotic
potential of tall plants is about —15 atm; of the dwarf plants about —30
atm. Plant height and osmotic potential are parallel to each other (see
Fig. 18).
Dwarf plants of ephemeral species are seen in a desert after an occasionally heavy rainfall. Their shoot growth is always considerably less than
is their root growth. This shift in growth rate may be a valuable adaptation
to the arid environment.
An experiment with seedlings of Pisum sativum clearly shows the different effects of a decrease in hydrature for shoots and roots (Walter, 1962,
p. 344ff). At first, Pisum seeds were germinated in wet saw dust. In a
series of sealed vessels sugar solutions of stepwise increased concentration
were prepared. In each of these vessels the germinated seeds were placed
over the sugar solution so that only the roots were immersed in the solution. In this way the germinating seeds were exposed to air humidity corresponding to relative vapor pressure over the solution, and the same hy-