Developmental anatomy of major lateral leaf veins of healthy and of pear-decline diseased pear trees

Following the initiation of a leaf primordium in healthy Pyrus sp. ‘Variolosa,’ the marginal meristems initiate the wings of the lamina. A superficial sheet of anticlinally dividing cells gives rise to the protoderm. Submarginally, a uniseriate strand of initial cells gives rise by anticlinal divisions to the adaxial and abaxial subprotodermal layers, and by periclinal divisions to the middle layer. The abaxial subprotoderm, by periclinal divisions, also contributes cells to the middle layer. The procambia that give rise to lateral veins differentiate from undifferentiated cells in the middle layer. Sieve tubes that form in the primary phloem from the procambium are arranged somewhat irregularly, while the cambium produces radially arranged rows of secondary phloem. In trees affected by pear decline necrosis does not occur in the primary phloem of new shoots that form following chilling; possibly they are pathogen free. Sieve-tube necrosis in secondary phloem is accompanied by excessive and prolonged abnormal phloem formation, and groups of sieve tubes in this abnormal phloem also become necrotic. The necrotic process consists of degeneration of organelles, membranes, and ground cytoplasm.