The pumpkin shoot "hook" that points downwards

Here is what I meant about the "hook" of the growing pumpkin shoot. A photo from August 24 2006.

This pumpkin shoot is so vigorous that it feels like a snake, it's "meaty" and thick, flexible but so "pumped" with turgidity it might "burst" if I hit it against something..! A living cord or rope that stretches 6 inches or more a day !

I have taken this photo to show the shape of the growing tip; on healthy plants it always points downward. When ever I have found a pumpkin shoot-tip that is not doing this, the vine has stopped growing. It must be a plant-hormone signal that keeps this "hook" shape and growth continuing..?