Ultrastructure of the Large Vascular Bundles in the Leaves of Rice Plant

Ultrastructure of the large vascular bundles in expanded and folded leaves of rice plants were observed by an electron microscope, and the results were compared with those of the small vascular bundles previously reported. 1. The large bundles have more developed mestome sheath than the small bundles. The suberized lamellae occur in the outer and inner tangential walls of the mestome sheath cells, but are absent in the middle portion of the radial walls. The lamella occurs between primary and secondary walls of the mestome sheath cells. 2. The protophloem sieve elements and companion cells located abaxially within the large bundles, mature in the elongating zone of the folded leat and begin to degenerate before emergence of the leaf. So far as the same portion of the leaf is concerned, the maturation and degeneration of the protophloem in large bundles are earlier than those in the small bundles. 3. In the border region between phloem and xylem of the large bundles, the parenchyma cells of metaphloem and metaxylem, and the cells of mestome sheath are interconnected by numerous plasmodesmata. According to this observations, the following symplastic pathways for photosynthate are suggested. [figure] 4. The large bundles have many metaxylem parenchyma cells, in which abundant mitochondria and endoplasmic reticulum are contained. The function of the metaxylem parenchyma cells in absorption and transfer of solutes is discussed.