StarLogo Project:

Grass

WHAT IS IT?

This project is an extremely simplified simulation of grass growing. Using a few simple rules a complex and almost realistic behavior emerges.

To simulate the energy of the sun there is a certain amount of sun energy in every patch of sky. This energy is mapped to color (light = high energy, dark = low energy). Every iteration there is a burst of energy added to a random patch of sky to simulate the constant addition of energy by the sun. Each strand of grass uses up sun energy to survive. It takes that energy from the patch of sky at its tip. If the energy in that patch is below a certain level, then the strand of grass dies. If the energy in that patch is a above a certain level the stand will begin hatching seeds. This process will continue until the energy in the patch again falls below the energy ceiling. Each time a seed is hatched energy is used up. Energy is also used up by grass strands in every iteration simply for survival. If a seed lands on an empty space of dirt (i.e. no other strands are there), it will spring up into a strand. The height of this newly hatched strand depends on certain "genetic" information carried by the seed. This information is the amount of sun energy present at the strand from which the seed was hatched at the time of the seed hatching. Thus the height of the hatching strand depends on the amount of energy present around its parent at the time of hatching. For example if there is very little energy around a strand hatching a seed, its seed will yeild a very short strand. If on the other hand there is a lot of energy around the parent strand, then the offspring strand will be very tall. There is a limit to how tall a strand of grass can grow.

The simulation starts out with three strands growing to a specified height. As soon as these strands are full grown they each drop a yellow seed. From here the simulation takes off on its own using the simple rules outlined above.

HOW TO USE IT

Click on the SETUP button to set up the screen and initialize the variables. Then click on SHINE to add sun energy to the sky and to diffuse it. Finally click on GO to begin growing the initial three strands. From there the simulation will begin.

The BURST slider controls the amount of energy added to the sky at every random burst. The simulation runs best with BURST around 100. The MAP slider controls the color mapping of the sun energy in the sky. The greater MAP is, the greater the amount of energy white represents. In other words, the greater MAP is, the wider is the range of energy represented by the different shades of blue.

THINGS TO NOTICE

Initially the population of grass explodes and nearly every space is taken. Then the population begins to die off as less and less energy is available. Finally one or two of the strands may benifit from a burst near their parent strands at the time of hatching. These bursts make the offspring strands extremely tall and allow them to exploit unused energy. Such strands may send down so many seeds that the population may once again explode. At this point the cycle continues.

Very different behaviors take place depending on what value BURST is set to. If it is too low the situation is very static and all the strands may eventually die off. If BURST is too high the population will constantly be full and excessively tall strands will be widespread. If BURST is just right, the cycle described above will take place (just right is somewhere between 80 and 150).

EXPLORATIONS

* What kind of "genetic" information could you add to the seeds to make the simulation more realistic or just more interesting?

* How could you simulate the sun's energy and its distribution more realistically?

* How could you change the simulation so that the grass strands require water aswell as the sun's energy?