The walls of normal root hairs of White Mustard (Sinapis alba) have been examined by the methods of polarization microscopy, X-ray diffraction analysis, and electron microscopy, and hairs have also been examined after growth in media containing 14C labelled glucose. Autoradiographs show that rapid wall synthesis occurs over a length of about 120μ back from the tip. The microfibrillar material deposited is Cellulose I, and over the hemispherical tip of the hair the microfibrils are arranged at random. The lamella carrying these microfibrils, which passes back as the outermost lamella of the hair lower down, is classified as an α layer. Within this, reaching almost to the tip, the inner wall lamella has microfibrils running approximately longitudinally. Both these lamellae are incrusted, the outer lamella much more intensely. On plasmoptyais the material ejected from the tip of the hair can be seen after the appropriate treatment to contain microfibrils. It is suggested that the extreme tip of the hair is the site of cellulose synthesis and rapid growth in wall area; and that growth here may be by intussusception. In the region lower down, where the inner lamella is also present, wall deposition is apparently by apposition. This structure appears to lend no support to a simple multi-net growth hypothesis.