Section 8 Table of Contents Section 10

9. Branching patterns of herbaceous plants

In spite of its simplicity, the simple model of a compound leaf shown in Plate 13 (see caption) properly reflects one of the general principles of the development of higher plants. It is called the principle of subapical growth, and means that only apices can create new branches. Internodes may elongate, but cannot initiate branches. These rules are illustrated schematically in Plate 14 (see caption).

A variety of structures can be produced through subapical growth, depending on which apices remain active and initiate new branches, and which ones do not. In line with the research interests of Lindenmayer and other biologists using L-systems, this section focuses on the modeling and simulation of the inflorescences of herbaceous plants. For a more complete description see Chapter 3 of [Pru1990]

The simplest case occurs when only the apex of the main axis of the growing plant produces new branches, as shown in Plate 15 (see caption). These lateral branches carry buds and then flowers, but do not branch any further. The resulting structure is called a monopodial branching structure, and the corresponding inflorescence is called a raceme. Plate 16 (see caption) illustrates the development of a common weed Capsella bursa-pastoris, or shepherd's purse, following this principle. The creeping bellflower Campanula rapunculoides, presented in Animation 9 (see caption), develops in the same manner. Additional views of this development are included in Animation 10 (see caption), which combines models of several plants in a single scene, and Animation 11 (see caption), which presents a view of the flowers.

Plate 17 (see caption) shows a form of branching which, in a sense, is opposite to the previous one. In this case, called sympodial branching, the apex produces a flower bud terminating the development of the current axis, but in addition produces one or more active lateral apices. This process repeats recursively, with the main thrust of the development always going into the lateral branches. The resulting inflorescence type is called a cyme. The development of Lychnis coronaria, or rose campion, shown in Animation 12 (see caption) and Animation 13 (see caption), provides a good example of the described process. Animation 14 (see caption) presents a view of the development of a single flower.

Yet another branching type, called polypodial branching, is presented schematically in Plate 18 (see caption). In this case, both the apex of the main axis and the lateral apices produce new branches until, at some point in time, the apices undergo a developmental switch and are transformed into flower buds. The resulting inflorescence, called a panicle, may adopt various forms depending on the geometric parameters. For example, Plate 19 (see caption) shows a so-called decussate branching pattern, in which pairs of lateral branches are arranged in mutually perpendicular planes. By terminating each of these branches with a flower we can model lilac inflorescences, as presented in Plate 20 (see caption).

Section 8 Table of Contents Section 10