Ultrastructural Study on Minor Veins in Watermelon Leaf Blades Ultrastructural Study on Companion Cells in Leaf Minor Veins of Morus alba Ultrastructural Study on the Ontogeny of Minor Veins and Associated Sheath Tissues in the Maize Leaf Blades Differentiation and Ultrastructure of the Protophloem Sieve Elements of Minor Veins in the Maize Leaves: Changes in the Protoplast Cytochemical Localization of ATPase and Acid Phosphatase in Minor Veins of the Leaf of Vicia faba During Different Developmental Stages