Two species of Leguminosae and five species of Rosaceae have been studied in an attempt to determine the relationship between the development of primary vascular bundles and their morphology, and the variation in morphology among vascular bundles of different types in the stem. A residual meristem occurs in all species. The direction of differentiation of provascular tissue and primary phloem is acropetal and continuous, but differentiation of primary xylem is usually basipetal in the stem. Rare acropetal differentiation of primary xylem was observed in two species only, and in these it is restricted to axial bundles. In order to identify the different types of vascular bundles and to understand their relationship to each other and to the organs they supply, the architecture of the primary vascular system was determined. The several types of vascular bundles are highly variable in size and tracheary cell content. Of all the vascular bundles, the leaf traces are the largest and contain the greatest number of files of tracheary elements. Whereas leaf traces gradually increase in size from the levels of their divergence from axial bundles to the levels at which they enter leaf bases, axial bundles maintain a size in approximate proportion to the diameter of the vascular cylinder. Branch traces decrease in size acropetally. In axial bundles there are usually only one or two files of tracheary elements in contrast to as many as nine files in the median leaf traces of some species. In apical regions where the axial bundles are the sole means of continuous longitudinal transport in the stem, effective water and mineral conduction occur through a relatively small number of tracheary elements. In leaf traces, the much larger number of tracheary elements may have, among other functions, adaptive value in compartmentalization of transport.