Copy/Paste the following code to your GroIMP project:
//**********************************************************
/*
You will learn:
– how to modify properties of objects by actualization rules.
– Objects and their connections control topology,
properties of objects (attributes) control geometry and color.
*/// a simple internode with a fixed, very small initial size
module Internode(int age, int f) extends F(0.02, 0.0005)
{{if (f == 0) setShader(GREEN);
else setShader(BLUE);}};// a simple meristem
module Meristem(int age, int f) extends Sphere(0.002)
{{setShader(RED);}};

// The initial structure consists of the meristem
protected void init()
[ Axiom ==> Meristem(30, 0); ]

public void run()
[

// Two alternative rules for the meristem:
// first rule for a new meristem: Creation of the internode
m:Meristem, (m[age] == 30) ==>
Internode(0, m[f]) RH(140) RU(random(-10, 10)) Meristem(0, 1-m[f]);

// The meristem ages by 1 day
m:Meristem, (m[age] < 30) ::> m[age]++;

// rule for the increase of the internodium
i:Internode ::>
{
if(i[age] < 30)
i[length] += 0.005; // length growth: the first 30 days
i[diameter] += 0.0001; // Thickness growth always
i[age]++; // incrementation of the age
}
]

//**********************************************************