There are two quite different modes of polar cell expansion in plant c
ells, namely, diffuse growth and tip growth. The direction of diffuse
growth is determined by the orientation of cellulose microfibrils in t
he cell wall, which in turn are aligned by microtubules in the cell co
rtex. The orientation of the cortical microtubule array changes in res
ponse to developmental and environmental signals, and recent evidence
indicates that microtubule disassembly/reassembly and microtubule tran
slocation participate in reorientation of the array. Tip growth, in co
ntrast, is governed mainly by F-actin, which has several putative form
s and functions in elongating cells. Longitudinal cables are involved
in vesicle transport to the expanding apical dome and, in some tip gro
wers, a subapical ring of F-actin may participate in wall-membrane adh
esions. The structure and function of F-actin within the apical dome m
ay be variable, ranging from a dense meshwork to sparse single filamen
ts. The presence of multiple F-actin structures in elongating tips sug
gests extensive regulation of this cytoskeletal array.