Quercus lobata | Palo Alto
Valley Oak, Q. lobata, is closely related to an oak that’s quite widespread in the US, the Bur Oak, Q. macrocarpa. The key identifying trait of Bur Oak is what’s referred to as an enlarged central sinus - a prominent gap in the center of the leaf. Usually the leaves are quite large, also. In this photo we see a leaf with an enlarged sinus, but this time it's a leaf from a Valley Oak, a leaf much smaller than a typical Bur Oak leaf. Most Valley Oak leaves do not have such an enlarged sinus, but they can be found regularly by an attentive searcher. This central sinus is a visual indicator of the genetic relatedness of Bur Oak and Valley Oak.