Aspects of the structure and ultrastructure of the fusiform
cambial cells of the taproot of Aesculus hippocastanum
L. (horse chestnut) are described in relation to the seasonal cycle of
cambial activity and dormancy. Particular
attention is directed at cell walls and the microtubule and
microfilament components of the cytoskeleton, using a
range of cytochemical and immunolocalization techniques at the
optical and electron-microscopical levels. During
the dormant phase, cambial cell walls are thick and multi-layered,
the cells possess a helical array of cortical
microtubules, and microfilament bundles are oriented axially. In
the early stages of reactivation, vesicle-like
profiles are associated with the cell walls, whereas arrangement of
the cytoskeletal elements remains unchanged.
In the succeeding active phase, the cell walls are thin, and
cortical microtubules form a random array, although
microfilament bundles maintain a near-axial orientation. The
observations are discussed in relation to the seasonal
cycle of wall structure and cortical microtubule rearrangement
within the vascular cambium of hardwood trees.
It is suggested that the cell-wall thickening at the onset of
cambial dormancy, which is associated with the presence
of a helical cortical microtubule array, should be considered to
be secondary wall thickening, and that selective
lysis of this secondary wall layer during cambial reactivation
restores the thinner, primary wall found around active cambial cells.