With plants, the "default" is to grow a shoot in a straight line. When an adjustment needs to be made, the shoot's two sides grow at different rates, resulting

With plants, the "default" is to grow a shoot in a straight line. When an adjustment needs to be made, the shoot's two sides grow at different rates, resulting in a bend. If there is no stimulus that makes the plant want to "turn", it will just combine growing in the same direction (probably with some random small variations thrown in). By analogy to cars, if you disable the steering wheel, the car can still move, but it won't have a particular destination.