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

During the formation of cell wall thickenings in primary tracheary elements of the hypocotyl of cucumber, the morphology or topography of cell organelles was studied in relation to the size of the developing thickenings. In the initial stage, endoplasmic reticulum is smooth and vesicular, whereas afterwards it is rough and cisternal. The diameter and the electron density of the contents of Golgi vesicles in the successive stages differ. In the initial stage, cortical microtubules are present along the whole plasmalemma; in the following stages they occur only around the thickenings; and, in the final stage, they are again present along the whole plasmalemma. Lomasomes and cytoplasmic fibrils are present only in the beginning of thickening formation. The changing morphology of the endoplasmic reticulum in the different stages is related to different functions in thickening formation. The smooth vesicular form buds off vesicles which are probably added to the lomasomes. The rough cisternal form synthesizes proteins (enzymes). In addition, it seems that it transports substances and conducts vesicles. The changing morphology of Golgi vesicles is probably related to varying amounts and nature of substances in the vesicles in various stages. The orientation of microtubules and the distance between them indicate that microtubules form a barrier against Golgi vesicles. Lomasomes probably deposit substances during the initial stage in the developing thickenings. There are no indications that cytoplasmic fibrils are involved in thickening formation.