1. The structure of the embryo and development of the seedling of Allium cepa are described. 2. The primary root is diarch. In its growing point are two histogens, the inner giving rise to the stele and the outer to the remainder of the root. 3. The adventitious root is usually pentarch, originates in the pericyclic region of the stem at the level of the apical meristem, and develops in a manner similar to that of the primary root. Secondary roots may be developed from the adventitious roots. 4. The first vascular tissue to mature is in the cotyledon. Xylem matures centripetally throughout this structure. 5. Transition from an exarch to an endarch condition of the xylem in the lower part of the stem is described. 6. The stem consists of a parenchymatous cortex, a stele which contains branched and anastomosed amphivasal vascular bundles in a groundwork of parenchyma, and a central pith surrounded by a cylindrical network of bundles to which most of the leaf traces are connected. 7. The development of the l...