Poikilohydry is the lack of ability (structural or functional mechanism) to maintain and/or regulate water content to achieve homeostasis of cells and tissue connected with quick equilibration of cell/tissue water content to that of the environment. Frequently, it is coupled with 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 occurs in land plants which survive environmetal conditions 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. Aside from most seeds and spores, only about 300 species of vascular plants are desiccation-tolerant, including resurrection plants (such as Selaginella lepidophylla) and aerophytes (including some species of Tillandsia).