Conducting Units: Tracheids and Vessels
The development of upright land plants depended on the development of a water-conducting system. Many of the earliest land plants, e.g., species of Rhynia and Cooksonia, had little woody stem, depending mostly on turgor pressure of soft tissues for support (Niklas 1993a,b). As plants evolved to taller sizes, water conduction and mechanical support were more closely linked; in fact, this still is the case in many present-day plants that have no vessels, like the conifers. Both water conduction and rigidity depend largely upon cell-wall lignification, and it is thought that it was the evolution of the biochemical synthesis of lignin that made upright land plants possible (Barghoorn 1964).
KeywordsCellulose Lignin Polysaccharide Hexagonal Cretaceous
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