The relationship between cell organelles and cell wall thickenings in primary tracheary elements of the cucumber. II. Quantitative aspects

The relationship between the endoplasmic reticulum and the Golgi vesicles and the formation of cell wall thickenings in primary tracheary elements of the hypocotyl of the cucumber was investigated. In order to study the degree to which the endoplasmic reticulum and the Golgi vesicles might contribute to thickening formation, the relationship between the size of the thickening and the density of these cell organelles in the cytoplasm near the thickening was studied quantitatively. It appeared that the density, and hence the contribution, of the endoplasmic reticulum and the Golgi vesicles reaches an optimum in the mid-stage of the process of thickening formation. The endoplasmic reticulum and, to some extent, the Golgi vesicles occur in the cytoplasm between the cell wall thickenings in higher densities than in the cytoplasm facing the thickenings. This supports the hypothesis that cell wall substances brought by the endoplasmic reticulum and the Golgi vesicles pass through the plasmalemma between the thickenings and are then transported through the primary cell wall towards the sites of the thickenings. Lomasomes seem to be involved in the initiation of the thickenings. It seems likely that cell wall substances pass through the plasmalemma by way of the lomasomes. The substances are then transported sideways through the primary cell wall and the thickenings arise at each side of the vanishing lomasomes.