The minimum size of seeds and spores in relation to the ontogeny of homoiohydric plants

Abstract

Homoiohydry may be defined as the capacity of nondormant terrestrial plants to remain hydrated, albeit without the capacity for net CO2 fixation, when the soil can supply water to the plant less rapidly than the atmosphere can potentially remove water in transpiration. Homoiohydry is an attribute of the sporophytes of most extant vascular plants and involves significant differentiation with roots (or functionally analogous structures), xylem, cuticle, stomata and intercellular gas spaces (Raven 1977; Edwards, Abbott & Raven 1996; Bell & Mooers 1997). Homoiohydry was a major physiological innovation in the evolution of embryophytes and has enabled plants 1-100 m high to grow in terrestrial habitats with fluctuating soil water availability and evaporative demand of the atmosphere (Raven 1977, 1984a, 1993, 1997a, 1998). The possession of homoiohydric characteristics permits non-dormant sporophytic structures to be desiccation-intolerant, i.e. unable to survive equilibration with an atmospheric relative humidity of less than c. 50% (i.e. a water potential of c. -100 MPa). Whatever the mechanistic basis for this height restriction of desiccation-tolerant growing vegetative structures of land plants to less than 1 m, it appears that the occurrence of terrestrial plants in the 1-100 m height range requires desiccation-intolerance in the growing sporophyte combined with homoiohydry. It must be remembered that desiccation-tolerance (as defined above) is common in dormant structures (e.g. seeds) of those plants which are desiccation-intolerant when growing and apparently relates to the widespread occurrence in vascular plants of a number of closely related genes encoding 'late-embryogenesisabundant' proteins, one group of which is termed the dehydrins (Ingrams & Bartels 1996). Major reproductive innovations in the evolution of embryophytes were the polyphyletic evolution of heterospory and, in a few of these parallel heterosporous lines of evolution, the seed habit, and culminated in the plant supplying all of the water needed to its reproductive structures, so that above-ground sexual reproductive processes relied only on soil water supply to the