Abstract:Protophloem sieve element differentiation in the minor veins of the maize ( Zea mays L. ) leaves was first evidenced as an increase of the wall thickness, which began in the comers of the cell and then extended to other parts of the wall, and the appearance of long rough endoplasmic reticulum cistemae distributed throughout the cytoplasm, and then the presence of characteristic crystalloid inclusions within the plastids. As differentiation progressed, long cisternae of rough endoplasmic reticulum appeared to transform into shorter forms and eventually aggregated into small stacks, losing their ribosomes during the process. The nuclei degenerated, although frequently persisted until very late in differentiation the stages of maturation, as darkly stained amorphous aggregates surrounded by double nuclear envelope or only inner membrane of nuclear envelope. Subsequently, the nuclear envelope collapsed and became discontinuous. At the beginning of nuclear degeneration the perinuclear spaces were partly dilated and sometimes the outer nuclear envelope in the dilated portions then ruptured, and was accompanied by the disappearance of the cytoplasmic portion near it. During the peried of nuclear degeneration, in addition to the endoplasmic reticulum, plastids and mitochondria underwent structural modification, while components such as ribosomes, cytoplasmic ground substances, vacuoles and dictyosomes disintegrated and disappeared. At maturity, the surviving protoplasmic components, including plasmalemma, mitochondria, small stacked smooth endoplasmic reticulum and P-type plastids with crystalloids, became parietal in position. As differentiation of adjacent metaphloem sieve elements proceeded, the protoplasmic components of the mature protophloem sieve elements progresively degenerated and finally obliterated.