Abstract
The cytoplasm of the interior of maturing pine tracheids is revealed by scanning electron microscopy as a lamellate and filamentous intravacuolar reticulum immersed in vacuolar sap. Formation of cytoplasmic spherules and the breakdown of the cytoplasmic reticulum initiate protoplast disintegration that terminates in tracheid maturation. The process is believed to involve vacuolar membrane transformations.