Pseudotransverse divisions and intrusive elongation of fusiform initials in the storeyed cambium of Tilia

Abstract

The inclination, shape, and disappearance of individual fusiform initials were studied in the storeyed cambium of Tilia. Changes in inclination were accomplished through oriented cell-end shifting and pseudotransverse divisions combined with intrusive elongation. Rapid reorientation was associated with an increase in both radial and pseudotransverse anticlinal divisions, resulting in an overproduction of daughter cells; surplus cells were eliminated, creating space for the intrusive elongation of adjoining initials. The storeyed pattern was both disrupted and restored as a consequence of pseudotransverse divisions and intrusive elongation. Abnormally elongated cell ends shortened to the level of the storey border. Movement between storeys occurred as a result of intrusive elongation of one cell end in association with either pseudotransverse division or a gradual shortening of the opposite end. The combined findings of fluctuations in cell inclination, movement between storeys, and continual disturbance and restoration of the storeyed pattern indicate that developmental changes in the storeyed cambium are much more dynamic than previously suspected.