The term "geniculus" was introduced by R. Potonie (1934) for "a small peculiarity as an inwardly or outwardly turned equatorial knee in pollen grains." An attempt is made here to determine the exact nature of this "small peculiarity" in pollen of Quercus prinoides by using a new technique which permits reconstruction of the pollen grain from serial photographs taken at every half micron through it. The geniculus is found to be an equatorial thinning and bulging of the exine, which is thought to serve as an aid in germination. It is present in several species of the genus Quercus and appears to be characteristic of the genus. The geniculus is also found to occur in species of several genera that are assigned to families other than the Fagaceae.