Control of shoot apical development via cell division.

Cell division in plants not only partitions the protoplast but also provides the architectural framework for plant form. The shape of the shoot apical meristem is produced and maintained by gradients in the rate and plane of cell division, from the summit to the base of the apical dome, which also determine the region in which primordia can be formed. In Pisum leaf initiation is mainly the result of changes in the frequency of periclinal divisions at the leaf site whereas in Silene an increase in the rate of cell division seems more important since periclinal divisions are always present. Periclinal divisions may be permissive of primordium initiation rather than causal and may define the maximum area over which primordia can form. The occurrence in Pisum and Silene of periclinal divisions which do not seem to be related to concurrent outward growth suggests that the plane of division and the direction of growth may be controlled separately and in different ways. The control of outward growth during primordium initiation may lie in the epidermis, which necessarily grows faster at the leaf site. The initial orientation of epidermal growth at the primordial site, inferred as being normal to the plane of cell division, is predominantly longitudinal in Pisum but transverse in Silene. Longitudinal growth becomes dominant later in leaf development in Silene, as in Pisum. Several lines of evidence suggest a crucial role for the epidermis in the initial stages of primordium formation although the initial orientation of division planes in it may be concerned more with the shape of the young leaf than with initiation itself. In flower initiation primordial size becomes reduced, and in Silene there are alternations of higher and lower rates of cell division in successively initiated primordial types. A fuller understanding of the role of cell division in apical growth depends on better knowledge of the functional relationships between the plane of cell division, the orientation of microtubules and wall microfibrils, and the effect that division in one cell has on its neighbours.