You can visualize the structure of wood as being like a bundle of microscopic
straws that are glued together. The straws themselves are made mostly of a
material called cellulose, (cotton is almost pure cellulose) which is pretty
stable. In engineering terms it is a 'thermo-setting' resin, which means that
it has a well-defined structure, and doesn't react much to heating until you
get up to the charring point. The glue (called 'lignin'), on the other hand,
is a thermo-plastic resin. It is more like a thick liquid, and becomes softer
when you heat it. Most of the things we refer to in everyday life as
'plastics' are like this.
Materials like cellulose, that have a defined structure, behave in a
predictable way when they are put under load. If the load is light, they bend
'elasticly'(sp?), which means that they store the energy that is used to
deform them, and spring back to shape when the load is removed. At some load,
however, they lose the ability to spring back, and are permanently deformed,
if not broken. This is called the 'yield stress'.
Thermoplastics don't have a defined structure, and so they will deform
permanently under any load; their yield stress is effectively zero in the
long term. This is the kind of 'cold creep' I was talking about. All woods
will do this to some extent, but weeping willow is the worst. It has to do
with things like the internal structure of the wood, the amount and
composition of the lignin, the phase of the moon, the price of bananas in
Burundi; all the usual stuff.
Bethca didn't know you were going to get an engineering lecture! :-)