Growth in plants
At the lowest level, the growth of any organism proceeds by either division or expansion of cells, together with occasional formation of cavities between cells. In plants, cells typically expand—normally through intake of water—only for a limited period, after which the cellulose in their walls crystallizes to make them quite rigid. In most plants—at least after the embryonic stage—cells typically divide only in localized regions known as meristems, and each division yields one cell that can divide again, and one that cannot. Often the very tip of a stem consists of a single cell in the shape of an inverted tetrahedron, and in lower plants such as mosses this is essentially the only cell that divides. In flowering plants, cell division normally occurs around the edge of a region of size 0.2-1 mm containing many tens of cells. (Hearts of palm in palm trees can however be much larger.) The details of how cell division works in plants remain largely unknown. There is some evidence that orientation of new cells is in part controlled by microscopic fibers. Various small molecules that can diffuse between cells (such as so-called auxins) are known to affect growth and production of new stems (see below).