Control of Development in the Bifurcating Branch System of Tabebuia rosea: A Computer Simulation

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Abstract
A computer model, simulating geometry and development of the branch system of T. rosea DC., was constructed. Early in development there is a transition from symmetric bifurcation and exponential increase in branch number to asymmetric branching, manifest in the morphological differentiation into leaders and lateral shoots and in the progressive reduction of increase in branch numbers. This reduction is an inherent geometric property of botanical trees and similar, branched systems. It results from the discrepancy between the increase in crown surface with the 2nd power of branch order and the exponential increase in branches supplying the surface. In symmetric branching, mother and daughter branches are arranged in the same plane; in asymmetric branching, branch planes of leaders and laterals are inclined against each other. Development was assumed to be controlled by flux distribution within the trees. Flux supplied to a terminal branch determines its vigor and branching potential. Asymmetric branching results from asymmetric flux distribution. Feedback interaction between mean and lateral branches causes "apical control", i.e., enhanced growth of leaders as compared with lateral branches. Growth of the branch system as a whole is limited by the sigmoid increase with time in flux to the tree. Reiteration of the set of geometric and developmental rules simulates time-dependent changes during normal development and regeneration of the branch system, which are strikingly similar to those observed in growing Tabebuia trees, suggesting that simulated controls are similar to those operating in real trees.