Stomata in early land plants: an anatomical and ecophysiological approach

Abstract

direct comparisons of functions and functioning with extant forms and the conclusion that they are among Descriptions of Silurian and Lower Devonian stomata the most conservative of embryophyte vegetative characbased on cuticles, coalified compressions and permin- ters. Their study in the fossil record has received a major eralizations reveal similarities with those in mosses boost from the current preoccupation with recording and certain ferns, and facilitate understanding about their frequencies (McElwain and Chaloner, 1995; the mechanism of guard cell movements. A detailed Beerling and Woodward, 1997). However, in considering survey of stomatal complexes, substomatal chambers the broader physiological and ecological issues of and cortical tissues in Rhynie Chert plants suggest stomata, they should not be considered in isolation from adaptations to reduce water loss in peristomatal the surrounding and, particularly, the underlying tissues. regions and these, together with a specialized paren- Earlier observers, for example, Whitehouse (1952) and chymatous tissue with an extensive intercellular space Corner (1964) had, in discussing the major anatomical system at the base of the substomatal chamber, point innovations associated with fitness to the terrestrial to high water use efficiency. Lower stomatal frequen- environment, focused on the indispensible role of an cies are discussed in relation to water stress and intercellular space system in association with the postulated high atmospheric carbon dioxide concen- stomatal apparatus. This aspect has received comparattrations. Stomatal numbers and distribution in axial ively little attention in studies based on cuticles or Silurian and basal Devonian fossils form the basis for compression fossils in which ‘soft’ tissues are now speculation on the selective pressures (e.g. the gen- represented by a film of coal. Here, in addition to an eration of a transpiration stream, H 2 O and nutrient update on these conventional approaches, new data are acquisition, temperature control) that led to the evolupresented based on the three-dimensionally preserved tion of stomata, although the fossil record provides no tissues of the Pragian silicified Rhynie Chert plants, direct evidence for evolutionary pathways. which provide a unique insight into the ecophysiology of early land plants.