Why is ginkgo leaf venation described as an open pattern?

Ginkgo leaf venation is described as an open pattern because the veins are characterized by a dichotomous branching system that is generally devoid of vein fusions. In this arrangement, the vascular bundles extend through the petiole and branch repeatedly without coalescing or forming networks. While this system is typically referred to as an open type, it has been observed that vein unions can occur with some regularity in Ginkgo leaves.1