Poikilohydry and homoihydry: antithesis or spectrum of possibilities?

Plants have followed two principal (and contrasting) strategies of adaptation to the irregular supply of water on land, which are closely bound up with scale. Vascular plants evolved internal transport from the soil to the leafy canopy (but their 'homoihydry' is far from absolute, and some are desiccation tolerant (DT)). Bryophytes depended on desiccation tolerance, suspending metabolism when water was not available; their cells are generally either fully turgid or desiccated. Desiccation tolerance requires preservation intact through drying-re-wetting cycles of essential cell components and their functional relationships, and controlled cessation and restarting of metabolism. In many bryophytes and some vascular plants tolerance is essentially constitutive. In other vascular plants (particularly poikilochlorophyllous species) and some bryophytes tolerance is induced by water stress. Desiccation tolerance is adaptively optimal on hard substrates impenetrable to roots, and on poor dry soils in seasonally dry climates. DT vascular plants are commonest in warm semiarid climates; DT mosses and lichens occur from tropical to polar regions. DT plants vary widely in their inertia to changing water content. Some mosses and lichens dry out and recover within an hour or less; vascular species typically respond on a time scale of one to a few days. Contents Summary 327 I. Introduction 328 II. The soil-plant-atmosphere continuum 329 III. Desiccation-tolerant plants: taxonomic distribution and functional characteristics 331 IV. Anatomical and physiological requirements and implications of desiccation tolerance 333 V. Time-scale considerations and ecological adaptation 340 VI. Conclusion 344 Acknowledgements 344 References 344.