Shifts in plant cell axiality: histogenetic influences on cellulose orientation in the succulent, Graptopetalum.

The elongation of typical plant meristems can be explained, biophysically, by the cellulosic hoop reinforcement in the longitudinal walls of most of the cells. A cortical microtubule array is believed to govern the reinforcement alignment. The orientation of the reinforcement is studied here with polarized light. Transverse orientation appears to be invariant, regardless of division direction, in interior cells of at least some established meristems. Exceptional longitudinal reinforcement (i.e., in the direction of growth) is occasionally seen, however, in the outer epidermal wall of established Graptopetalum roots, leaves, and stems. It is found in pairs of elongate cells that arise from a longitudinal division. This behavior is prominent when the epidermis is shifting reinforcement to initiate new organs. Usually the division direction and the new cell long axis coincide; reinforcement parallels both. When these two factors are in marked opposition, as in certain longitudinal divisions of very broad cells, the reinforcement can follow the new cell's long axis even though this orients alignment perpendicular to the new wall. The normal prevailing effect of the division direction may relate to the formation of the cortical microtubule array parallel to the orientation of the preprophase band. The normally synergistic effect of the new cell's long axis may stem from the fact that when the band-like array occupies the smaller pair of anticlinal faces of the cuboidal cell (and hence parallels the cell's long dimension in surface view), it will have the densest and presumably most stable structure. Reinforcement orientation is seen as a function of at least two factors which bear on the stability of the newly forming array.