Copy/Paste the following code to your GroIMP project:
//**********************************************************
/* You learn:
– how you can specify that an object from the left-hand side of
a rule continues to exist after rule application by naming it.
– use the method enlargeSpheres to enlarge the sphere and watch
the behavior of the spheres when you apply the run() rule.
*/

// sphere 1:
module RightSphere(float len, super.radius) extends Sphere(radius);

// sphere 2:
module LeftSphere(float len, super.radius) extends Sphere(radius);

// both objects get the radius 0.1 when they are created

protected void init()
[
Axiom ==> [ RU(30) F(2) RightSphere(1, 0.1) ]
[ RU(-30) F(2) LeftSphere(1, 0.1) ];
]

// Use this method to enlarge the spheres:
public void enlargeSpheres()
[
a:RightSphere ::> a[radius] += 0.2;
a:LeftSphere ::> a[radius] += 0.2;
]

// run() lets the stem grow
public void run()
[

// rule 1: the object RightSphere is replaced by a new object
// of the same type
RightSphere(x, r) ==> F(x) RightSphere(0.8*x, 0.1);

// rule 2: the object LeftSphere is named and will be inserted
// by the right-hand side again into the graph. It maintains
// its attributes.
a:LeftSphere(x, r) ==> F(x) a {a[len] *= 0.8;};
]

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