A general model for the structure and allometry of plant vascular systems

Vascular plants vary in size by about twelve orders of magnitude, and a sin gle individual sequoia spans nearly this entire range as it grows from a se edling to a mature tree. Size influences nearly all of the structural, func tional and ecological characteristics of organisms(1,2). Here we present an integrated model for the hydrodynamics, biomechanics and branching geometr y of plants, based on the application of a general theory of resource distr ibution through hierarchical branching networks(3) to the case of vascular plants. The model successfully predicts a fractal-like architecture and man y known scaling laws, both between and within individual plants, including allometric exponents which are simple multiple of 1/4. We show-that conduct ing tubes must taper and, consequently, that the resistance and fluid flow per tube are independent of the total path length and plant size. This reso lves the problem of resistance increasing with length, thereby allowing pla nts to evolve vertical architectures and explaining why the maximum height of trees is about 100 m. It also explains why the energy use of plants in e cosystems is size independent.