The Transition To Land

The transition from an aquatic habitat to life on land was a major evolutionary event in the history of photosynthetic organisms, which involved a number of important physiological changes and structural modifications to the plant body (T. Taylor, 1982a, 1988a; Graham et al., 2000). When plants first moved onto the land, the Earth's surface was probably already inhabited by various cyanobacteria, algae, fungi, and perhaps lichens (Taylor et al., 1997). As in arid regions today, microbial mats and biological crusts consisting of communities of microorganisms were probably important in soil formation, and there is evidence of fossil soils (paleosols) as early as the Proterozoic (Hasiotis, 2002; Prave, 2002). Molecular data obtained from the scaly green flagellate Mesostigma viride (Streptophyta) suggest that several major physiological changes, for example, in the regulation of photosynthesis and photorespiration, took place early during the evolution of the streptophytes, that is before the transition to land (Simon et al., 2006).

The transition to a desiccating terrestrial habitat probably occurred sometime in the Ordovician or earliest Silurian. By the Late Silurian-Early Devonian, there is evidence of a radiation of land plants and a number of unique adaptations are found in several groups of organisms (Bateman et al., 1998). In some ways, the move of plants onto the land is similar to the evolution of the soft-bodied, late Neoproterozoic marine faunas—the Ediacaran faunas. In both cases, there were apparently a number of open niches and the plants that filled these niches exhibited a number of unusual morphologies, many of which did not survive beyond the Devonian. Although perhaps overly simplistic, it is now apparent that the ability to exist on land is the result of numerous complex interactions that involved the interplay between structural and physiological adaptations in the plants themselves, symbiotic interactions at several levels, and physical and chemical changes in the environment. In the following sections, we will consider the major adaptations that are necessary for life on the land, including anchorage and water uptake, support for the upright plant body, water movement through the plant, desiccation prevention, some mechanism for gas exchange, reproduction in a terrestrial environment, and life history strategies. All of these features are interrelated and some structures perform multiple roles.

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