Abstract
The protoplast of maturing axial tracheids in the secondary xylem of shortleaf pine (Pinus echinata Mill.) was studied by transmission and scanning electron microscopy. The mature protoplast is differentiated into two interconnected components: (1) the commonly observed peripheral layer lining the secondary cell wall, and (2) an elaborate reticulum of cytoplasmic filaments and placoids within the central vacuole. The reticulum provides an extensive surface area of vacuolar membranes for rapid exchange of nutrients and metabolites with the vacuolar sap, which is envisaged to function as a vital medium during the period of secondary cell wall synthesis. The breakdown of the protoplast which terminates tracheid maturation is associated with poorly defined alterations of the vacuolar membranes. This is indicated by increased formation of cytoplasmic spherules and membraneous vesicles which may be portions of separated vacuolar membrane during early stages of degradation. Autolysis is supposed to occur when the cytoplasm is exposed to the vacuolar sap after rupture and separation of the vacuolar membranes. The Gomori acid phosphatase technique as combined with electron microscopy produced no evidence of autolysosomal segresomes in strands of intravacuolar reticulum of the cytoplasm.
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