Reflections on the absence of stomata on the gametophyte generation of extant land plants: a focus on poikilohydry

Abstract The recent realization that bryophyte sporophytes are homoiohydric enabled the present analysis of morphological features specifically associated with poikilohydry. The only morphological feature absolutely diagnostic of poikilohydry is schizolytic intercellular spaces. In vascular plants these are gas-filled from the outset, but in bryophytes are liquid-filled initially. They remain in this condition in liverwort gametophytes but become gas-filled following stomatal opening in hornworts and mosses. These have fixed apertures and are completely unresponsive to environmental cues. Stomata in Devonian fossils may have had similarly fixed apertures. Unistratose lamellae, characteristic of moss and liverwort leaves and pteridophyte gametophytes, are optimal structures for reversible cell shrinkage and recovery accompanying de- and rehydration. In 1 M sucrose, gametophytic tissues and filmy fern leaves undergo shrinkage, whereas sporophyte cells plasmolyse. Under extreme desiccation, hydroids are the only bryophyte cells to undergo cavitation. Like bryophytes, desiccation-tolerant streptophyte algae undergo reversible cell shrinkages. Mucilage secretion is unimportant in bryophyte desiccation biology and developmental differences rule out homology between the mucilage clefts and stomata in hornworts and Blasiales. Elaborate placental walls in basal liverwort lineages and a stomatal toolkit in the capsule walls of Haplomitrium suggest that liverworts’ ancestors may have had more complex sporophytes than those in extant taxa.