EFFICIENCY AND EVOLUTION OF WATER TRANSPORT-SYSTEMS IN HIGHER-PLANTS - A MODELING APPROACH .1. THE EARLIEST LAND PLANTS

The evolution of the stele was studied under the functional aspect of water transport problems by using a numerical approach. The underlying mathematical model describes the behaviour of a fluid-filled porous m edium and is based on the coupling of Hooke's law and Darcy's law incl uding a dynamic permeability approach which leads to a self-organizati on of the considered structure according to the resulting fluid-pressu re field. Calculations dealing with two problems were performed. The e ssential demand of a water conducting system for a plant was demonstra ted quantitatively. As soon as the plant shows an upright habit, the n eed for efficient water transport occurring through a highly porous ap oplastic pathway becomes evident. In a second approach, the evolution of the protostele was simulated using the concept of dynamic permeabil ity. The simulations of structures with self-regulating hydraulic cond uctivity yielded two strategies according to the pressure-permeability relationship. Increasing hydraulic conductivity with increasing negat ive fluid pressure results in peripheral layers of the conducting tiss ues, whereas the inverse pressure-permeability relationship yields a c entral position of the conducting tissues. The latter arrangement corr esponds to the protostelar construction of early vascular plants.