According to its genetic options and the available space, a plant tries to optimize growth. Independent of the branching form, the primary axes grow vertically, the secondary axes rather horizontally. Thereby, the genetically specified differences in the growth form result from the different growth conditions in the lower and upper parts of the plant. We can differentiate between trees and shrubs:
■ Trees
The shape of trees shows a clear difference between the growth of the trunk and the crown. While the usually sympodial arranged side branches die off, the crown develops out of a complete branching system (see Fig. 2.4a). In the crown, the terminal buds and the closest side buds are most strongly promoted and therefore grow faster (ahrotony). Some tree species develop long and short shoots. While the long shoots advance the structure of the crown, subordinate branches are often not fully pronounced and are thrown off later.
At this point, it should be noted that the different shapes of trees are heavily dependent on the location of the tree. If a tree is part of a forest, meaning in the direct vicinity of other trees, the light conditions cause the lower side branches to quickly die off and the tree will strive toward the light, thereby developing a more pronounced crown shape. A free-standing tree will, even with lower side branches, try to fill up as much space as possible to gather a maximum of light.
■ Shrubs
Shrub-like forms of plants usually display a sympodial branching form. Here, the growth of the buds and/or side shoots located at the tips of the buds are promoted more strongly than those located above (basitony), which produces a low, stout shape. We differentiate between epitonic forms, in which in particular upper-lateral buds are promoted and in this way form a concave structure (Fig. 2.4c), and hypotonic forms, in which the lower – lateral shoots are promoted to form a convex, more closed shape (see Fig. 2.4b).
Aside from these classical growth forms there are still other kinds, such as for instance, climbing plants, which do not display their own mechanical stability but coil around other objects for support. in the context of an evolutionary
Chapter 2 adjustment to living conditions, the branch axis can be transformed in such a
Plants way that they are able to store water such as a cactus, defend themselves using
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thorns against game, or provide additional stability and support to the plant.
Figure 2.4 Spatial subdivision: (a) tree;
(b) convex shrub;
(c) concave shrub
(a) (b) (c)
In contrast to the forms of branching global characteristics just described in the global geometric characteristic ^ last paragraph, the spatial division is a global geometric characteristic. In order
to generate a shape or form, an algorithm must contain global information. For example, somewhere along the tree trunk there must be some kind of function for the thickness and the type of branching depending on the vertical position. This can explicitly happen through specifying a mathematical function or within a growth simulation, in which the lower branches die off due to global influences, such as the diminishing strength of light on the ground. In both cases, an interplay between local branching procedures and global parameters is necessary for the realization of the algorithms.
