Poikilohydry is a condition in organisms that lack a mechanism to prevent desiccation, such as the lichens and bryophytes. The structures supporting poikilohydry are typically small enough to reside in the diffusion boundary layer, where the cuticle or stomata do not have a sufficient guarding effect on the amount of water lost from the organism. Poikilohydry is also noted in many forms of aquatic 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.
The antonym is Homoiohydry.