Cytoplasmic MTs are recognized as important elements in plant development. They are categorized in four principal arrays: the PPB, the MA, the phragmoplast, and the cortical array, and they appear to participate in fundamental aspects of cell division, growth, and differentiation. Thus, such diverse activities as prediction of the alignment of the cell plate, the movement of chromosomes and transport of cell plate vesicles, and the orientation of the cellulose wall microfibrils are all mediated by MTs to varying degrees. Our knowledge about the distribution and function of MTs has been gleaned largely from studies at the electron microscope level, and more recently at the light microscope level using immunofluorescence methods. Because most of these observations have been based on cells that have been fixed, it has not been possible to examine directly the dynamic properties and behavior of MTs. Transitional states have been inferred from looking at numerous isolated examples, which have then been placed into a sequence. Other aspects, such as the sensitivity of MTs to drugs, ions, or cold temperature, have also been approximated through studies in which cells have been fixed at periodic intervals following application of an agent. Some important aspects of MT behavior, including, notably, their turnover, have been beyond the reach of these indirect methods. To enlarge our understanding of the dynamic properties of MTs, we have introduced the technique of fluorescent analog cytochemistry as applied to plant cells (Zhang et al., 1990b). In this procedure fluorescently labeled exogenous tubulin, which is injected into the cell, incorporates into the endogenous pool and acts as a reporter. MTs, which are observed with the CLSM, can be examined under any experimental condition to which the cells are exposed. The results have produced new information about MT transitions and dynamic behavior in live cells, which we address in this Update. Other aspects of plant MTs, including the identification of MT organizing centers, tubulin isoforms, and MAPs have been considered in other recent topical reviews (Lambert, 1993; Cyr, 1994; Goddard et al., 1994; Shibaoka, 1994; Cyr and Palevitz, 1995; Joshi and Palevitz, 1996), so they will not be covered here. INCORPORATION OF HETEROLOGOUS ANIMAL TUBULIN INTO PLANT MTS; OBSERVATIONS BY CLSM