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SUMMARY

The possible role in drought resistance played by leaf water potential, osmotic potential, weight-averaged turgor pressure and bulk modulus of elasticity is studied in mature and young leaves of Senecio Candidus and S. ambiguus. Small water losses cause relatively large reductions in leaf water potential in, S. candidus without greatly altering its osmotic potential. This occurs because S. candidus develops thicker leaf cell walls with high values of leaf bulk elastic modulus. Recovery from water stress is very rapid in this species. Leaf growth is influenced by the structural pattern of the cell walls in that S. candidus leaves complete their expansion within a shorter period of time than those of S. ambiguus. The possible adaptive significance of changing the leaf bulk modulus of elasticity in mature and young leaves of the two species is discussed.