Copy/Paste the following code to your GroIMP project:
//**********************************************************
/*
You will learn:
– how a gradient-controlled transport process (e.g., for assimilates)
can be modeled (linked with diameter to make it visible) */

module Internode(super.diameter) extends F(100, diameter);

protected void init()
[
Axiom ==> P(14) Internode(1) P(2) Internode(1)
P(4) Internode(1) P(15) Internode(60);
]

public void transport()
[
i_top:Internode < < i_bottom:Internode ::>
{
float r = 0.1 * (i_bottom[diameter] – i_top[diameter]);
i_bottom[diameter] :-= r;
i_top[diameter] :+= r;
}
]

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