An oak gall
This sphere on a dead oak leaf is a gall. An insect grub was living there, and produced a waste product that the oak leaf interpreted as a growth hormone; the leaf produced a sphere that fed and protected the grub. The evolutionary response of the oak tree is that they shed the infected leaves. Leaf of post oak (Quercus stellata) in Oklahoma. Photograph by Stanley A. Rice. Appears in Encyclopedia of Biodiversity (Facts on File).