232
H. WALTER AND E. STADELMANN
phytes belong to this group as well as some pteridophytes of arid regions
which withstand complete desiccation without damage. Only very few
angiosperms (e.g., Myrothamnus) are in this group.
b. Homoiohydric Plants. The protoplasmic hydrature of homoiohydric
plants is (within certain limits) independent of hydrature (air humidity)
of the atmosphere because the vacuome (a term which refers to all of
the vacuoles in a plant collectively) is an internal aqueous medium for
the protoplasm. In this way the hydrature of protoplasm becomes essentially independent of the external conditions. Most land plants are homoioplants survive complete desiccation. The leaves are rolled during drought and less
conspicuous. After rain the leaves spread out. During this period of active life,
the potential osmotic pressure is relatively low.)
(Villa) Perennial shrubs, slightly xeromorphic: 1 Clematis ligusticifolia;
2
Prosopis velutina (26 determinations); 3 Olneya tesota; 4 Acacia paucispina; 5
Acacia greggii; 6 Cassia covesii; 1 Cercidium (Parkinsonia)
microphylla (23 determinations); 8 C. torreyana; 9 C. aculeata. (These species favor more humid habitats
between rocks or in river valleys. Under extreme drought the leguminosae drop
their leaflets.) (Vlllb) Heavily xeromorphic perennial desert shrubs: 1 Simmondsia
calif ornica; 2 Mortonia scabrella; 3 Lycium californicum
(with succulent leaves)
and Frankenia palmeri; 4 Larrea divaricata ( = L. tridentata = Covillea glutinosa;
68 values); 5 Celtis pallida; 6 Ephedra trifurca. (The optimal potential osmotic
pressure of these plants is relatively high. Increase of π* occurs only after prolonged
drought, since these plants have the ability to maintain their osmotic potential
under unfavorable conditions for a considerable length of time, probably by reduction
of transpiration.)
(IX) Perennial malacophyllous xerophytes: 1 Lippia wrightii; 2 Gaertneria deltoidea; 3 Encelia farinosa (over 100 determinations). (The potential osmotic pressure
increases considerably under water shortage. When the maximum value is reached,
the leaves dry out and are dropped. The shrubs survive the drought period in
the leafless stage.)
(X) Sclerophyllous woody species from the lower forest level: 1 Quercus oblongifolia (30 determinations); 2 Q. emoryi (23 determinations); 3 Q. arizonica;
4 Q. reticulata; 5 Q. hypoleuca; 6 Arbutus arizonica; 1 Arctostaphylos
pungens;
8 Garry a wrightii; 9 Cercocarpus ledifolius and Vauquelinia calif ornica; 10 Dodonaea
angustifolia and Lepargyrea ( = Shephardia) rotundifolia; 11 Rhamnus sp., Berberis
haematocarpa, Mahonia wilcoxii, Rhus ovata, R. coriophylla and Garrya veatchii;
12 Juniperus pachyphloeus and /. utahensis; 13 J. scopulorum, and Cupressus arizonica; 14 Pinus chihuahuensis
and P. monophylla.
(These truly sclerophyllous
species do not occur in the desert itself, but in the next higher level of vegetation,
the evergreen oak forests. The measurements of potential osmotic pressure are incomplete since they were made only during the winter season. The sclerophyllous
plants correspond to the Mediterranean species in Europe.)
(XI) Herbaceous plants from the lower forest level (humid locations): 1 Aquilegia chrysantha; 2 Nasturtium officinale; 3 Rumex sp., 4 Adiantum capillus veneris;
5 Epicampes rigens (xeromorphic grass); 6 Dudley a sp. (Sedum-like succulent).
(These samples show low values for the potential osmotic pressure of herbaceous
species in favorable habitats during the winter season.)
H. WALTER AND E. STADELMANN
phytes belong to this group as well as some pteridophytes of arid regions
which withstand complete desiccation without damage. Only very few
angiosperms (e.g., Myrothamnus) are in this group.
b. Homoiohydric Plants. The protoplasmic hydrature of homoiohydric
plants is (within certain limits) independent of hydrature (air humidity)
of the atmosphere because the vacuome (a term which refers to all of
the vacuoles in a plant collectively) is an internal aqueous medium for
the protoplasm. In this way the hydrature of protoplasm becomes essentially independent of the external conditions. Most land plants are homoioplants survive complete desiccation. The leaves are rolled during drought and less
conspicuous. After rain the leaves spread out. During this period of active life,
the potential osmotic pressure is relatively low.)
(Villa) Perennial shrubs, slightly xeromorphic: 1 Clematis ligusticifolia;
2
Prosopis velutina (26 determinations); 3 Olneya tesota; 4 Acacia paucispina; 5
Acacia greggii; 6 Cassia covesii; 1 Cercidium (Parkinsonia)
microphylla (23 determinations); 8 C. torreyana; 9 C. aculeata. (These species favor more humid habitats
between rocks or in river valleys. Under extreme drought the leguminosae drop
their leaflets.) (Vlllb) Heavily xeromorphic perennial desert shrubs: 1 Simmondsia
calif ornica; 2 Mortonia scabrella; 3 Lycium californicum
(with succulent leaves)
and Frankenia palmeri; 4 Larrea divaricata ( = L. tridentata = Covillea glutinosa;
68 values); 5 Celtis pallida; 6 Ephedra trifurca. (The optimal potential osmotic
pressure of these plants is relatively high. Increase of π* occurs only after prolonged
drought, since these plants have the ability to maintain their osmotic potential
under unfavorable conditions for a considerable length of time, probably by reduction
of transpiration.)
(IX) Perennial malacophyllous xerophytes: 1 Lippia wrightii; 2 Gaertneria deltoidea; 3 Encelia farinosa (over 100 determinations). (The potential osmotic pressure
increases considerably under water shortage. When the maximum value is reached,
the leaves dry out and are dropped. The shrubs survive the drought period in
the leafless stage.)
(X) Sclerophyllous woody species from the lower forest level: 1 Quercus oblongifolia (30 determinations); 2 Q. emoryi (23 determinations); 3 Q. arizonica;
4 Q. reticulata; 5 Q. hypoleuca; 6 Arbutus arizonica; 1 Arctostaphylos
pungens;
8 Garry a wrightii; 9 Cercocarpus ledifolius and Vauquelinia calif ornica; 10 Dodonaea
angustifolia and Lepargyrea ( = Shephardia) rotundifolia; 11 Rhamnus sp., Berberis
haematocarpa, Mahonia wilcoxii, Rhus ovata, R. coriophylla and Garrya veatchii;
12 Juniperus pachyphloeus and /. utahensis; 13 J. scopulorum, and Cupressus arizonica; 14 Pinus chihuahuensis
and P. monophylla.
(These truly sclerophyllous
species do not occur in the desert itself, but in the next higher level of vegetation,
the evergreen oak forests. The measurements of potential osmotic pressure are incomplete since they were made only during the winter season. The sclerophyllous
plants correspond to the Mediterranean species in Europe.)
(XI) Herbaceous plants from the lower forest level (humid locations): 1 Aquilegia chrysantha; 2 Nasturtium officinale; 3 Rumex sp., 4 Adiantum capillus veneris;
5 Epicampes rigens (xeromorphic grass); 6 Dudley a sp. (Sedum-like succulent).
(These samples show low values for the potential osmotic pressure of herbaceous
species in favorable habitats during the winter season.)