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Poikilohydry is the capacity to tolerate dehydration to low cell or tissue water content and to recover from it without physiological damage. This condition occurs in such organisms as the lichens and bryophytes that lack mechanisms, such as a waterproofing cuticle and stomata that can help to prevent desiccation. Poikilohydry is also noted in many forms of algae, which may be able to survive desiccation between successive high tides, or during occasional stranding due to the drying of a lake or pond. Similarly, poikilohydry enables land plants to survive when water supplies are seasonal or intermittent, as in the liverwort genus Targionia, which lives in Mediterranean habitats with hot dry summers.

The antonym of poikilohydry is homoiohydry, a suite of morphological adaptations and strategies that enable plants to regulate or achieve homeostasis of cell and tissue water content. The vascular plants have largely lost the capacity to tolerate dehydration. Some exceptions are the seeds of many vascular plants, the resurrection plants (such as Selaginella lepidophylla) and aerophytes (including some species of Tillandsia).

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