In another chapter of this volume 1 some of the more general aspects of morphogenesis at the shoot apical meristem have been considered. Here the aim is to examine what is meant by apical organization 2 , i.e. the constructional and functional unity which the apex exhibits throughout the development of the plant, and to see how the apex, during its metabolism and growth, gives rise to organs of characteristic size, position and symmetry and to the tissue pattern within.A. General considerations.
I. Diverse histology of shoot apices.In species from different taxonomic groups the shoot apex (comprising the apical meristem and the subjacent region of expansion) may be very different in respect of its shape, size, pattern of organ formation, histological constitution and pattern of tissue differentiation. In some species, too, the apex may show more or less marked ontogenetic changes in shape, size, phyllotaxis, etc. For more than a hundred years, plant anatomists have been interested in these phenomena. At least seven "types" of apical construction have been recognised, these ranging from the "apical cell type" exemplified by the leptosporangiate ferns, Equisetum, species of Selaginella, etc., to those in which evident apical initial cells are completely absent, but in which embryonic cells of approximately the same size are arranged more or less precisely in some characteristic number of "histogenic" layers. The diversity of shoot apices in plants, but also some fundamental similarities in their structure, are illustrated in Figs. 1 and 2.1 "The morphogenetic role of apical meristems: fundamental aspects", pp. 443-451. 2 Organization. In biology, this term connotes an organized body or structure. During its ontogenetic development from a zygote, with specific hereditary properties, a plant typically undergoes characteristic and harmonious changes in size and shape and i.n the diversification of its organs and tissues; functional activities and mutual relationships of parts are also involved at all stages. Collectively, these several phenomena constitute what is implied by organization. Different species are distinguishable because of their different specific organizations. In referring to a plant as an organization, we imply that its characteristic form, structure and functional activities have come into being by a succession of orderly processes, that some basis for the progressively manifested organization is present at the outset i.e. in the zygote, young embryo, generative cell, gemma, or apical meristem, and that the ontogenetic process is essentially one in which the potential organization of the species is given expression. By its very nature the concept is thus comprehensive and pervasive: it implies constructional and functional unity. This is an essential consideration in assessing the regulated growth and morphogenetic activity of the shoot apex.