Ultrastructural Changes in the Scale Leaf of Onion Epidermal Cells During Peduncle Growth

Changes of ultrastructure of the epidermal cells of onions were observed with electron microscope during peduncle growth. The young cells of inner scale leaf were found to be normal: the vacuole occupies most of the central space within the cell, and the cytoplasm is spread along the periphery of the cell, against the cell wall. The cytoplasm is rich in organelles: plastids,mitochondria,ribosomes, etc. The diameter of a plasmodesma is about 50 nm. During cell senescence the cytoplasm becomes loose and a lot of flucculent material is found in the vacuole. The cell organelles degenerate gradually. There are some enlarged plasmodesmata,with a diameter of about 80 nm. This indicates that modified plasmodesmata are playing an important role in intercellular transport of macromolecules. In a senescing cell the nucleus and plastids are disorganized but most of plasmodesmata regain their normal structure.