Climbing vines need to find a suitable support on which to grow. Plant parts in need of suppoprt begin what appears to be a hunting motion whereby shoot tips rotate in a nutational movement. This is a mechanism that helps plants find needed support in their attempt to reach optimim light. When the shoot rubs against a support, the rubbing induces another response called a thigmotropic response (induced by touch), in which usually the shoot usually begins to curl around the support. Of course, vines typically show the most extreme nutational movements.

So, if you're convinced you're not seeing phototropism in action, I would bet that you are seeing initial stages of nutation.

Al

Well, it must be the nutational movement. It was straight up until the other plant was placed behind it. I was impressed by the ivy covering the 6" gap. (The ivy is only 10" tall).