Small interfascicular and fascicular tissue blocks were excised from the hypocotyl of mature castor bean embryos. When cultured on a liquid medium containing mineral salts, sucrose, and kinetin, many of the interfascicular tissue blocks produced a cambium. Fascicular tissue blocks exhibited remarkably little development of vascular bundle tissue, particularly in the morphologically apical side of the block. Cambial development was frequently observed in this kind of tissue block, however, and was apparently not influenced by the lack of vascular bundle differentiation. Omission of kinetin from the medium prevented cambial development in both interfascicular and fascicular tissue blocks but did not lead to any further reduction of vascular bundle differentiation in the latter. It is therefore concluded that vascular bundle differentiation and cambial development represent two essentially unrelated developmental processes, both initiated in the mature embryo but each depending for its realization on different specific internal and external conditions. The precambial layer, i.e. the layer(s) considered to represent the initiated cambium, can be identified in the interfascicular zone as the innermost of two cell layers characterized by the relatively small size of the cells and nuclei. Attempts to obtain enzyme-histochemical evidence corroborating the special character of the precambial layer have been unsuccesful.