V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
231
6 Mammilaria grahami; 1 Opuntia phaeacantha-toumeyi
(45 determinations); 8 O.
phaeacantha-blakeana;
9 O. castillae, O. santa-rita and O. discata; 10 O. laevis
and O. chlorotica; 11 O. versicolor; 12 O. fulgida, O. lepticaulis, O. spinosior,
O. basilaris, O. polyacantha and O. arbuscula. (The potential osmotic pressures
are low: the more water stored by the plant species, the lower the potential osmotic
pressures.)
(II) Leaf succulents: 1 Agave americana; 2 A. palmeri; 3 A. schottii; 4 Yucca
schottii; 5 Y. elata and Y. baccata; 6 Y. sp. (Mexico); 7 Dasylirion
wheeled;
8 Nolina microcarpa. (The potential osmotic pressure increases in these leaf succulents, as in other plants, with decreasing degree of succulence.)
(III) Winter ephemeral plants: 1 Parietaria obtusa; 2 Streptanthus
arizonicus;
3 Senecio douglasii, Plantago ignota, Er odium texanum and Phacelia
tanacetifolia;
4 Erodium cicutarium; 5 Sonchus asper, Hordeum murinum, Malva silvestris, Marrubium vulgäre (all ruderal); 6 Brodiaea sp., Delphinium scaposum, Verbena ciliata;
1 Sicyos sp., Rumex hymenosepala, Penstemon wrightii; 8 Anemone
sphenophylla,
Sphaeralcea pedata. [The growth period is completely restricted to the winter rainy
season. These species survive the drought season and the summer as seeds (few
species with underground organs) and germinate (or bud) after the first winter
rain. The winter climate is similar to the summer climate in Central Europe,
except that night temperatures are lower in this desert. Therefore, the potential
osmotic pressures of these species are of the same magnitude as those of the
summer ephemeral plants of Central Europe.]
(IV) Summer ephemeral plants: 1 Amaranthus palmeri; 2 Euphorbia
heterophylla;
3 Kallstroemia
grandiflora; 4 Allionia incarnata; 5 Trianthema
portulacastrum;
6 Tribulus terrestris; 7 Martynia louisiana, Cucurbita digitata, Maurandia antirrhinoides, and Menodora scabra; 8 Cladothrix lanuginosa; 9 Ambrosia
psilostachya
(in shade); 10 Euphorbia glyptosperma; 11 Solanum elaeagnifolium (15 determinations). (Most of these plants are annual therophytes, which germinate after the
first summer rain and dry out during the fall drought period.)
(V) Rain phanerophytes: 1 Fouquieria s pi ende ns (28 determinations, maximum
21.4 atm, yellowing leaves 11.7 a t m ) ; 2 Idria columnaris (lower values from the
succulent stem); 3 Jatropha cardiophylla (yellowing); 4 Jatropha cinerea (leafless
stem), Elaphrium
macdougali
(leafless stem), Jatropha spathulifolia
(young
leaves), Elaphrium microphyllum
(old leaves). (The leaves of these plants are
photosynthetic active as long as there is enough water supply from the soil to
cover transpiration loss. When water deficits occur, the leaves do not dry but
yellow and abscise. New leaves grow after each substantial rainfall. The potential
osmotic pressure remains constant or may even decrease.)
(VI) Deciduous trees and shrubs: 1 Populus macdougali; 2 Fraxinus tourneyi;
3 Vitis arizonica (budding) and Celtis reticulata (leaves just reached full size).
(These species depend upon ground water and therefore have no water shortage
during the drought period. Leaves are dropped at the beginning of the low temperature season. The potential osmotic pressure is similar to that of the corresponding
species in Central Europe.)
(VII) Poikilohydric ferns: 1 Cheilanthes wrightii (with very tender leaves, hygromorphic); 2 C. fendleri (abaxial leaf surface with dense hair cover); 3 C.
lindheimeri
(both leaf surfaces with dense hair cover); 4 Gymnopteris
hispida
and Pellaea mucronata
(sclerophyllous); 5 Selaginella arizonica (green variety);
6 S. arizonica (gray variety); 7 Notholaena hookeri; 8 TV. aschenborniana (with
scales as on Ceterach); 9 N. sinuata var. integerrima (leaves xeromorphous). (These
231
6 Mammilaria grahami; 1 Opuntia phaeacantha-toumeyi
(45 determinations); 8 O.
phaeacantha-blakeana;
9 O. castillae, O. santa-rita and O. discata; 10 O. laevis
and O. chlorotica; 11 O. versicolor; 12 O. fulgida, O. lepticaulis, O. spinosior,
O. basilaris, O. polyacantha and O. arbuscula. (The potential osmotic pressures
are low: the more water stored by the plant species, the lower the potential osmotic
pressures.)
(II) Leaf succulents: 1 Agave americana; 2 A. palmeri; 3 A. schottii; 4 Yucca
schottii; 5 Y. elata and Y. baccata; 6 Y. sp. (Mexico); 7 Dasylirion
wheeled;
8 Nolina microcarpa. (The potential osmotic pressure increases in these leaf succulents, as in other plants, with decreasing degree of succulence.)
(III) Winter ephemeral plants: 1 Parietaria obtusa; 2 Streptanthus
arizonicus;
3 Senecio douglasii, Plantago ignota, Er odium texanum and Phacelia
tanacetifolia;
4 Erodium cicutarium; 5 Sonchus asper, Hordeum murinum, Malva silvestris, Marrubium vulgäre (all ruderal); 6 Brodiaea sp., Delphinium scaposum, Verbena ciliata;
1 Sicyos sp., Rumex hymenosepala, Penstemon wrightii; 8 Anemone
sphenophylla,
Sphaeralcea pedata. [The growth period is completely restricted to the winter rainy
season. These species survive the drought season and the summer as seeds (few
species with underground organs) and germinate (or bud) after the first winter
rain. The winter climate is similar to the summer climate in Central Europe,
except that night temperatures are lower in this desert. Therefore, the potential
osmotic pressures of these species are of the same magnitude as those of the
summer ephemeral plants of Central Europe.]
(IV) Summer ephemeral plants: 1 Amaranthus palmeri; 2 Euphorbia
heterophylla;
3 Kallstroemia
grandiflora; 4 Allionia incarnata; 5 Trianthema
portulacastrum;
6 Tribulus terrestris; 7 Martynia louisiana, Cucurbita digitata, Maurandia antirrhinoides, and Menodora scabra; 8 Cladothrix lanuginosa; 9 Ambrosia
psilostachya
(in shade); 10 Euphorbia glyptosperma; 11 Solanum elaeagnifolium (15 determinations). (Most of these plants are annual therophytes, which germinate after the
first summer rain and dry out during the fall drought period.)
(V) Rain phanerophytes: 1 Fouquieria s pi ende ns (28 determinations, maximum
21.4 atm, yellowing leaves 11.7 a t m ) ; 2 Idria columnaris (lower values from the
succulent stem); 3 Jatropha cardiophylla (yellowing); 4 Jatropha cinerea (leafless
stem), Elaphrium
macdougali
(leafless stem), Jatropha spathulifolia
(young
leaves), Elaphrium microphyllum
(old leaves). (The leaves of these plants are
photosynthetic active as long as there is enough water supply from the soil to
cover transpiration loss. When water deficits occur, the leaves do not dry but
yellow and abscise. New leaves grow after each substantial rainfall. The potential
osmotic pressure remains constant or may even decrease.)
(VI) Deciduous trees and shrubs: 1 Populus macdougali; 2 Fraxinus tourneyi;
3 Vitis arizonica (budding) and Celtis reticulata (leaves just reached full size).
(These species depend upon ground water and therefore have no water shortage
during the drought period. Leaves are dropped at the beginning of the low temperature season. The potential osmotic pressure is similar to that of the corresponding
species in Central Europe.)
(VII) Poikilohydric ferns: 1 Cheilanthes wrightii (with very tender leaves, hygromorphic); 2 C. fendleri (abaxial leaf surface with dense hair cover); 3 C.
lindheimeri
(both leaf surfaces with dense hair cover); 4 Gymnopteris
hispida
and Pellaea mucronata
(sclerophyllous); 5 Selaginella arizonica (green variety);
6 S. arizonica (gray variety); 7 Notholaena hookeri; 8 TV. aschenborniana (with
scales as on Ceterach); 9 N. sinuata var. integerrima (leaves xeromorphous). (These