In considering the apical or primary meristems of the plant body, one of the most perplexing problems is the permanently meristematic condition of these regions, which are somehow spared from the processes of maturation occurring in their derivatives. One might be tempted to relate this property to their terminal position, their three-dimensional mass, or their organization, which is distinct from that of the mature structures they produce. However, the lateral meristems, which share the capacity for continued growth but are strikingly different in every other respect, prevent an easy acquiescence to this temptation. The vascular cambium and the cork cambium, or phellogen, are lateral in position, have the form of cylindrical sheets encircling the plant axis, and are organized in close conformity with the tissues to which they give rise. They initiate only specific tissues rather than whole organs as in the case of the terminal meristems. Furthermore, it must be borne in mind that whereas every vascular plant body must have terminal meristems in order to exist at all, the lateral meristems have a supplemental role and are by no means universal (Barghoorn, 1964).

THE INITIATION OF CAMBIAL ACTIVITY

Nothing emphasizes the differences between primary and secondary meristems more effectively than a consideration of the origin of the vascular cambium. Whereas the shoot and root apical meristems are initiated among the cells of the embryo early in the development of the plant, the cambium has its origin from a partially differentiated vascular tissue, the procambium.