The bud is a propagation system, since each new branch begins in a bud. We differentiate between terminal main buds and side buds in the axis of carrying leaves (bud axis). The inside of the bud is here protected by fast initial growth of the leaves. In leaf buds, for example, it develops into a branch with leaves, and in the case of flower buds to a bloom or bloom conditions.
Figure 2.2
Various leaf formations: (a) Distichy; (b) Dispersion; (c) Decussation
The circular formation of the leaves at a branch axis is called a node (nodus), the intermediate range is called an internode. A node may initiate one or more leaves, whereby the deviation angles (also called divergence angles) between neighboring leaves always remain constant according to the so-called equal distance rule. In Fig. 2.2 different forms of leaf arrangement can be seen:
■ Distichy
In the distichous (1/2 alternate) mode, a leaf primordium is initiated per node with a divergence angle of 180°. The leaves stand in two orthostichous (vertical) rows.
■ Dispersion
In the dispersion case, the divergence angle is smaller and results in a spiral leaf formation. The angles usually are within the range of 135 – 144°, and thus approximate the Golden Angle (see Sect. 3.4). The advantage of this arrangement is the minimizing of the mutual shadowing through a sequential leaf formation.
■ Decussation
In this formation, several leaf primordia are formed at the node. The leaves of sequential nodes are set in an alternating pattern in the gap of the preceding set of leaves, which also results in a low amount of self-shadowing. If two leaves per node are present, the nodes alternate at 90°, and a cross-like leaf arrangement emerges.
Starting with the leaf buds and their different arrangements along the branch axis, plants grow in a recursive pattern by generating branch axes. A synthesizing algorithm must be able to copy these basic forms of branching, as distichy, dispersion and decussation are usually selected over separately selectable parameters that trigger corresponding branching procedures.
Chapter 2 The leaf arrangement can be seen as the fundamental structural characteristic
Plants of a branching plant. The branching formation and different spatial arrange
ments described in the sections that follow, on the other hand, represent geometrical characteristics. With a purely structural viewpoint, the appearance and the function of a branch are of no significance, only the type of branching is important. Thus distichy and dispersion are in this sense binary trees, while decussation represents a tertiary tree that is branched from one knot in three directions.
