Tissue slices from living root caps as a model system in which to study cytodifferentiation of polar cells.

Tissue slices of living root caps of cress (Lepidium sativum L.), two to three cell layers in thickness, were prepared by a microsurgical procedure. The viability, cellular structures and cytoplasmic movement of the cells were examined in the light microscope. Nuclei, amyloplasts, vacuoles and endoplasmic reticulum were identified and their positions confirmed after fixation and observation of the same cells in the electron microscope. The distribution of microtubules was shown by immunocytochemistry. During germination, microtubules appear first at the distal edges of the statocytes, while in mature statocytes a distal domain of criss-crossed microtubules could be distinguished from a proximal domain with transversally oriented microtubules. Microfilaments in young statocytes form a nuclear enclosure; in mature statocytes bundles of microfilaments fan out into the cell cortex. The transition from statocytes to secretion cells is accompanied by a more pronounced cortical network of microfilaments, while the nucleus-associated microfilaments remain visible. It is suggested that these microfilaments play a role in the positioning of the nucleus and the translocation of endoplasmic reticulum.