Overview

There are a large number of organisms that do not grow by a pattern similar to the simple cycle of cell growth followed by a division yielding two new cells. In filamentous organisms such as molds and in root cultures of higher plants, growth occurs by elongation and branching. New cells and/or biomass is formed primarily in the tip or apical region, and formation of lateral branches occurs in a region behind the tip of the parent branch or, in fungal hyphae, by bifurcation of the parent tip. In roots of higher plants, cells that leave the tip cease to divide and undergo a process known as terminal differentiation, in the process forming the structures that make up the root. Thus, cell age in a root equals zero in the tip and increases with distance from the tip. It is thus reasonable to characterize the state of a root by its age distribution and to model the dynamics of the distribution by using some model of the branching kinetics. Several PBMs developed along these lines have been proposed in the literature [36, 51, 56].

The situation for filamentous fungi is somewhat different. The hyphae of higher fungi consist of individual cells, while in lower fungi the cell walls or septa break down and the hyphae consist of a multinucleate branching tube without clearly defined individual cells. Growth occurs primarily from the tip and elongation, and tip bifurcation is usually assumed to be controlled by a cell organ known as...