Tropisms
The appearance of a plant is, except for the branch formation, substantially determined by the formation of the shoot axes according to growth preferences, so-called tropism. The direction of the shoots is controlled by gravity and the incidence of light. These growth mechanisms are decisive factors for the optimization of the living space of a plant.
As already mentioned, the main shoot axes mostly grow vertically in order to allow the axes to follow an optimal use of space. The adaptation here is caused by the gravity field of the earth - that is why this orientation is described as gravitropism. In contrast, the main roots grow according to positive gravit - ropism into the ground, while the leaves grow at a right angle to the gravity vector, which is called plagiogravitropism.
For a number of plants the hereby emerging directions and angles towards gravity are not static, but rather change in daily intervals, meaning that activities such as directioning and unfolding of the leaves take place during the night. The movement is controlled on a biological basis by a physiological clock, that is itself regulated by temperature changes and light intensity. As soon as the
control is discontinued, the length of this period diverges significantly between 22 and 28 hours, depending on the species and individual factors.
In contrast to this, phototropism controls the growth of a shoot relative to the incident light direction. A transversal phototropism directs the shoots at right angle to the light. Many herbs are positive phototropic, meaning that they bend their shoot axes towards the light source as they grow. Another phenomenon is shadow evasion, where partially illuminated leaves move towards the illuminated side. An example here is the sunflower which follows the sun with its broad surface. There also are opposite variants, where the plants evade sunlight or incident light.
Tropisms are thus considerably responsible for forming the shapes of plants. However, if the growth of a shoot is to be controlled by a real source of light, the light condition within the whole respective area must be determined in order to guarantee a correct growth process. In this sense, the growth process also depends on all other shoots, since the shading and reflection of those further shoots influences the growth. In Sect. 4.12 a description of controlling the growth of climbing plants using such light simulations can be found. The method is used in order to simulate the propagation of such plants on walls.
In many cases, photo - and gravitropism can also be computationally approximated using local mechanisms. For example, the direction of newly generated internodes of a plant which displays negative gravitropism can in each case be turned vertically and/or towards the given light source. The shape of a branch which has been manipulated in this way is unique for such plants. An example is the weeping willow, whose branches grow towards the ground. The computational efficiency of such a local approximation counts in many cases much more than the small visual errors that might occur as a result.
