270
H. WALTER AND E. STADELMANN
The stenohydric xerophytes for which ψ θ opt is almost identical with
xjjsmin exhibit a high degree of stability for the ψ 8 value and are in this
respect similar to the succulents. However, the stenohydric plants do not
develop a water storage organ and the yellowing of their assimilatory
organs often begins already after a relatively short dry period, whereas
succulents yellow rarely, in extreme cases only, after months or even 1
year of drought. Plants with latex ducts (Euphorbia, Apocynaceae), thick
tap root (Umbelliferae), bulbs, or tubers are stenohydric.
XVI. The Hydrature of Succulents
Plants with water storage tissues differ so significantly from the types described above that they should not be considered to be true xerophytes,
although they could be included in Section XV as a fifth subgroup, "succulent xerophytes."
Water storage, the most characteristic feature of succulents, makes them
independent for periods of varying length of water uptake from the soil.
Of other plant groups, only the poikilohydric plants are able to survive
temporarily without requiring external water supply. However, the poikilohydric plants lapse during the drought into a dormant state, whereas the
succulents do not.
The water-storage tissues of succulents are filled when rain falls and
the uppermost layers of the soil become moistened. The succulents do not
need deep roots. The tender, absorbing, lateral roots die when the upper
soil layers become dry during a drought, but new lateral roots are formed
very fast when the soil is remoistened by rainfall. New lateral root formation was tested in the laboratory after a 6-month drought period. Such formation began in Opuntia puberula after 8 hours, for the other cacti tested
50% of the plants had clearly visible roots after 24 hours, and 80% after
72 hours. Pseudolobivia kermesina, however, needed 8 days to form new
roots (Kausch, 1965, p. 231). The ability of succulents to live without
water uptake from the soil for an extended period makes it possible for
them to grow in humid areas on shallow soil, where species with higher
water requirements cannot survive, for instance, on almost bare rocks, or
as epiphytes on trees. In other locations in humid areas the succulents cannot survive competition with nonsucculents, since their production of dry
matter, and therefore their growth, is slow. The stored water is used economically, the intensity of transpiration is low for all succulents (especially
low transpiration values result when calculated on a fresh weight basis)
and therefore gas exchange with the atmosphere is also limited, with consequently low photosynthetic activity. The assimilating surface of the plant
is small, further lowering dry matter increment. Little organic material,
H. WALTER AND E. STADELMANN
The stenohydric xerophytes for which ψ θ opt is almost identical with
xjjsmin exhibit a high degree of stability for the ψ 8 value and are in this
respect similar to the succulents. However, the stenohydric plants do not
develop a water storage organ and the yellowing of their assimilatory
organs often begins already after a relatively short dry period, whereas
succulents yellow rarely, in extreme cases only, after months or even 1
year of drought. Plants with latex ducts (Euphorbia, Apocynaceae), thick
tap root (Umbelliferae), bulbs, or tubers are stenohydric.
XVI. The Hydrature of Succulents
Plants with water storage tissues differ so significantly from the types described above that they should not be considered to be true xerophytes,
although they could be included in Section XV as a fifth subgroup, "succulent xerophytes."
Water storage, the most characteristic feature of succulents, makes them
independent for periods of varying length of water uptake from the soil.
Of other plant groups, only the poikilohydric plants are able to survive
temporarily without requiring external water supply. However, the poikilohydric plants lapse during the drought into a dormant state, whereas the
succulents do not.
The water-storage tissues of succulents are filled when rain falls and
the uppermost layers of the soil become moistened. The succulents do not
need deep roots. The tender, absorbing, lateral roots die when the upper
soil layers become dry during a drought, but new lateral roots are formed
very fast when the soil is remoistened by rainfall. New lateral root formation was tested in the laboratory after a 6-month drought period. Such formation began in Opuntia puberula after 8 hours, for the other cacti tested
50% of the plants had clearly visible roots after 24 hours, and 80% after
72 hours. Pseudolobivia kermesina, however, needed 8 days to form new
roots (Kausch, 1965, p. 231). The ability of succulents to live without
water uptake from the soil for an extended period makes it possible for
them to grow in humid areas on shallow soil, where species with higher
water requirements cannot survive, for instance, on almost bare rocks, or
as epiphytes on trees. In other locations in humid areas the succulents cannot survive competition with nonsucculents, since their production of dry
matter, and therefore their growth, is slow. The stored water is used economically, the intensity of transpiration is low for all succulents (especially
low transpiration values result when calculated on a fresh weight basis)
and therefore gas exchange with the atmosphere is also limited, with consequently low photosynthetic activity. The assimilating surface of the plant
is small, further lowering dry matter increment. Little organic material,