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 pro vascular 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.

The structural patterns of the primary vascular systems in some species of Leguminosae and Rosaceae have been determined by tracing the longitudinal course of the vascular bundles in terminal stem segments. These systems are interpreted as consisting of sympodia, Each sympodium is composed of an axial bundle which is continuous through the length of the segment and from which arise trace bundles that supply leaves and axillary buds. A compact arrangement of vascular bundles seems to correlate with the woody. habit. Regardless of the degree of compactness of the primary vascular system, the structural identity of the individual sympodia is maintained. The total number of vascular bundles at a particular level is related to the number of axial bundles in the system, the number of traces per leaf and per axillary bud, and the number of internodes traversed by the traces prior to entering a lateral appendage. Shrubs and trees have more vascular bundles than herbs. Data from this study and the literature indicate that the vascular system is predominantly of the open type in dicotyledonous plants which have helically arranged leaves and, further, that in such plants with a 3-trace, trilacunar nodal structure, the number of sympodia coincides with the number of orthostichies (which is also the denominator of the phyllotactic fraction). In open systems leaf gaps cannot be morphologically delimited. Because of the resemblance of the open type of angiosperm vascular system to that of certain gymnosperms, previously interpreted to have evolved from a protostele, we suggest that the eustele of angiosperms is homologous with the stele of gymnosperms. We believe, also, that angiosperms, like gymnosperms, are probably not characterized by leaf gaps of filicinean type. We provide, furthermore, a rationale for the view that the axial bundle of a sympodium is a cauline structure.

The structural patterns of the primary vascular systems in some species of Leguminosae and Rosaceae have been determined by tracing the longitudinal course of the vascular bundles in terminal stem segments. These systems are interpreted as consisting of sympodia, Each sympodium is composed of an axial bundle which is continuous through the length of the segment and from which arise trace bundles that supply leaves and axillary buds. A compact arrangement of vascular bundles seems to correlate with the woody. habit. Regardless of the degree of compactness of the primary vascular system, the structural identity of the individual sympodia is maintained. The total number of vascular bundles at a particular level is related to the number of axial bundles in the system, the number of traces per leaf and per axillary bud, and the number of internodes traversed by the traces prior to entering a lateral appendage. Shrubs and trees have more vascular bundles than herbs. Data from this study and the literature indicate that the vascular system is predominantly of the open type in dicotyledonous plants which have helically arranged leaves and, further, that in such plants with a 3-trace, trilacunar nodal structure, the number of sympodia coincides with the number of orthostichies (which is also the denominator of the phyllotactic fraction). In open systems leaf gaps cannot be morphologically delimited. Because of the resemblance of the open type of angiosperm vascular system to that of certain gymnosperms, previously interpreted to have evolved from a protostele, we suggest that the eustele of angiosperms is homologous with the stele of gymnosperms. We believe, also, that angiosperms, like gymnosperms, are probably not characterized by leaf gaps of filicinean type. We provide, furthermore, a rationale for the view that the axial bundle of a sympodium is a cauline structure.

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.

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