Ultrastructural Study on Minor Veins in Watermelon Leaf Blades

Minor veins in watermelon leaf blades were studied by transmission electron microscopy.The minor vein was a vascular bundle surrounded by large bundle sheath cells.Xylem parenchyma cells and vessel elements were arranged in a linear form, whereas phloem parenchyma cells, companion cells, and sieve elements aggregated to form the head.Two types of companion cells can be distinguished based on their ultrastructural features.One type (2a type) was smaller, contains larger vacuoles, and possessed few plasmodesmata, which did not branch.The other type was located on both sides of the phloem and contacted the vascular bundle sheath with a large area.This type was typically characterized by the presence of abundant vesicles in cytoplasm and numerous branched plasmodesmata across its walls.These features define the cells as intermediary cells.Therefore, it was proposed that watermelon belongs to plants that possess two types of companion cells in the same minor veins.