V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
239
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• Ouercus ilex, shade leaves
Δ Ouercus coccifera
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Pistaciu lemiscus
Pistacia terebinthus
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Viburnum tinus
Cistus albidus
Thymus vulgaris
A
April
May June July Aug. Sept. Oct.
Month
Nov. Dec. Jan. Feb. Mar.
Fig. 13. Annual variations of the osmotic potential φ, of sclerophyllous and
malacophyllous species. Abscissa: April to March of next year (redrawn after
Braun-Blanquet and Walter, 1931, pp. 716, 717, and 720). !, young leaves.
damaged by water deficits. In contrast, euryhydric species can withstand
large water deficits and a great decrease of ψ 8 (i.e., decrease in hydrature
of the protoplasm). The difference φ 8 op t — ψ 8 min is relatively great.
Another grouping distinguishes between hydrolabile and hydrostabile
species. Hydrolabile species respond to droughts very rapidly with a
decrease of φ 8 . Hydrostabile species exhibit a highly stable osmotic
potential which results in a relatively constant hydrature of protoplasm.
Faster closure of stomata to reduce water loss under drought conditions or