Efficiency and evolution of water transport systems in higher plants: a modelling approach. II. Stelar evolution

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Abstract
Different stelar arrangements have developed through evolution of land plants. The first stele to appear was a central strand (protostele) consisting of tracheids or hydroid-like cells. In more derived steles (e.g. actinostele, siphonostele), a location of the conducting elements at relatively more peripheral regions of the axis can be observed. It has been shown that the trend in stelar evolution in early land plants from protostele to actinostele or siphonostele has little to do with an increase of the flexural stiffness in the axis. Hence, it is to be expected, that the (early) stelar evolution reflects an optimization process of the water conducting capabilities of the stem. To test this hypothesis, the effectiveness of protostele and siphonostele in water conduction was analysed numerically. The results demonstrate that the hydrodynamic behaviour of a plant axis depends not only on the relative amount of its conducting tissues, but also on the arrangement of the xylem within an axis. A protostele and a siphonostele with identical distance between outer xylem boundary and site of transpiration may, therefore, be identical with regard to water transport efficiency.