NUMERICAL-STUDIES OF WATER CONDUCTION IN LAND PLANTS - EVOLUTION OF EARLY STELE TYPES

During land plant evolution, a change in stelar architecture, i.e., in the geometric arrangement of the water-conducting tissue inside the p lant axis, can be observed. In the most primitive stele type, the prot ostele, the vascular tissue is organized as a simple central strand. F urther evolutionary changes led to more peripherally arranged vascular tissues. In the siphonostele, for example, the vascular tissue forms a hollow cylinder filled with pith. A functional explanation of this e arly stelar evolution is provided in the present paper. Using a numeri cal simulation approach, we analyze the water transport properties of various protostelic and siphonostelic model axes. The results indicate that several geometric parameters are relevant for understanding the water transport properties of various stele types and for explaining t he early stelar evolution: the parenchymatic path lengths (i.e., the d istance between the xylem surface and the transpiring plant surface), the ratio of xylem surface over transpiring surface, and the ratio of cross-sectional area of xylem to cross-sectional area of the parenchym a outside of the xylem. As a whole, the evolution of early stele types may be viewed as a size-related multi-criteria optimization process i n which the xylem volume as well as the fluid pressure gradients in th e parenchyma and in the xylem are minimized. For slender plant axes, a protostele appears to be the optimal stelar architecture. In wider pl ant axes, however, other stelar architectures (such as a siphonostele) prove to be more efficient than a protostele.