图22.2 Stress resistance can involve tolerating the stressful condition or avoiding it. Some resistance mechanisms are constitutive and are active before exposure to stress. In other cases, plants exposed to stress alter their physiology in response, thereby acclimating themselves to an unfavorable environment, Examples of constitutive mechanism of drought resistance include the succulent, photosynthetic stem of the saguaro catus(Cereus giganteus), a drought-tolerant species; and the wet-season life cycle of the Mohave desert star(Monoptilon bellioides). Examples of acclimation mechanisms include osmotic adjustment in plants such as spinach and freezing tolerance in cold-hardy trees such as black spruce (Picea Mill).图22.2 Stress resistance can involve tolerating the stressful condition or avoiding it. Some resistance mechanisms are constitutive and are active before exposure to stress. In other cases, plants exposed to stress alter their physiology in response, thereby acclimating themselves to an unfavorable environment, Examples of constitutive mechanism of drought resistance include the succulent, photosynthetic stem of the saguaro catus(Cereus giganteus), a drought-tolerant species; and the wet-season life cycle of the Mohave desert star(Monoptilon bellioides). Examples of acclimation mechanisms include osmotic adjustment in plants such as spinach and freezing tolerance in cold-hardy trees such as black spruce (Picea Mill)