Abstract: The origin, early development and ultrastrueture of the minor vein procambium and associated bundle sheath cells were studied in the leaf blades of maize ( Zea mays L. ). Apparently three contiguous cells in the middle layer of ground meristem apparently might be involved in the initiation of a minor vein and bundle sheath. The minor veins were formed from two unequal periclinal divisions of the inner cell, which gave rise to three smaller cells, one centrally located procambial initial cell, two bundle sheath cell precursors adaxially and abaxially located in respect. One of the two lateral cells immediately adjacent to either side of the inner cell directly differentiated into a precursor also divided radially, producing two abaxial bundle sheath cells. At the early stage of minor vein development, the procambial cells were essentially similar in ultrastructure to their precursor procambial initial cell and had all the protoplasmic constituents typically encountered in the meristematic cells. They contained a large nucleus with aggregates of heterochromatin and a dense cytoplasm rich in ribosomes. Proplastids possessed a simple fine structure with a few or without any internal membranes. Short segments of rough endoplasmic reticulum were sparsely distributed in the cytoplasm or associated with the plasmodesmata and with the outer membrane of the nuclear envelope. In many cases, the cytoplasm contained a few small vacuoles, many of which were in close association with the dilated endoplasmic reticulum. Numerous plasmodesmata were also observed in the walls between procambial cells and between procambium or procambial initial and surrounding cells. At the later stage of procambium development, some procambial cells showed an increased thickness of the cell wall, which was accompanied by a substantial decrease in the number of plasmodesmata between these cells, while other procambial cells transformed into the highly vacuolated parenchymatous state. The bundle sheath cells began to differentiate very shortly after vein initiation. The differentiation of the meristematic sheath cells into the highly vacuolated, parenchymatous photosynthetic cells was a gradual process. That was accompanied by the transition of proplastid to chloroplast, cell volume enlargement, and formation of large central vacuoles. The latter was also associated with fusion of small vacuoles as well as invagination of cytoplasmic material, including small vesicles, mimicking phagocytosis. The results indicated that adaxial and abaxial bundle sheath cells of the minor veins were more closely related ontogenetically to the initial procambial cell than were the lateral bundle sheath cells which were closely related ontogenetically to the mesophyll cells in the middle layer of the leaf.

Key words: Zea mays, Minor vein, Bundle sheath, Ontogeny, Ultrastructure