This term is often used about runner beans and other seed-bearing crops, such as apples. When fruit appears on a plant, it is referred to as setting. For fruit to set, it needs to be pollinated (unless it is a self-pollinating variety), usually by insects, although corn, for example is wind-pollinated. Some fruit requires three different contributors to the mix, for example, the so-called Triploid varieties of apple, although most only require a single partner (which may need to be of a different variety in the case of tree fruit). It is believed that runner beans also require a cool, moist root run to aid pollination.