The shoot tip

The gametophytes of mosses and liverworts and the sporophytes of many higher plants have a shoot, or early stem, with a single cell at its tip, or apex, from which all the tissues of the stem arise. This apical cell is usually four-sided (tetrahedral), with three faces directed downward, and the fourth capping the apex. Daughter cells are continually cut off sequentially from the three inner faces, the apical cell preserving its tetrahedral shape. In cell lineages derived from the daughter cells, the division planes may remain oriented in a more or less regular manner, so that, for some distance below the apex, the three sectors can be recognized in the stem. This basic pattern occurs in the arrangement of the “leaves” of some mosses, which lie in three ranks. In many plants, however, division planes in the lower part of the apex show no particular correlation with the planes of cleavage of the apical cell, and the lateral appendages do not reflect any three-part arrangement.

Gymnosperm and angiosperm apices do not possess apical cells. The generative role is discharged by an ill-defined zone of tissue called the promeristem. Regularities may appear in the distribution of division planes only in the extreme tip region. Over the outer part of the apex, the cells often appear to lie in one to three layers, which constitute the tunica. Enclosed by the tunica lies a core of cells that exhibits no distinct layering; this zone is the corpus. The layers of the tunica normally contribute to the surface layers of the plant, and the corpus provides the deeper lying tissues.

The tunica–corpus analysis emphasizes the orientation of division planes, but apices can be examined from other points of view—the sizes of cells, the degree of vacuolation, and the concentration of various cell constituents, especially ribonucleic acid (RNA), vary through the apex and this sometimes results in more or less distinctive zones. Both gymnosperm and angiosperm apices have been classified on the bases of such zonal patterns, but the validity of this approach, as well as its usefulness for understanding the function of the apex as a morphogenetic centre, has been questioned.

Since 1950, a theory of angiosperm apical zonation developed by French and Belgian botanists has been gaining support. This theory proposes that the central region of the apical dome constitutes a mass of cells with relatively low division rates, the méristème d’attente, or “waiting meristem.” Surrounding this region is an annular zone of cells with higher division rates, the anneau initial, or “initiating ring.” Features other than division rates characterize these zones: RNA and protein content are lower in the méristème d’attente than in the anneau initial, and the nucleoli are smaller. In longitudinal section, the differences contribute to the patterns distinguishable in apices, some of which have been used as bases for structural classification. The main contention of the Franco-Belgian school, however, is that the zonation represents a functional difference. The méristème d’attente is regarded as a region mainly concerned with controlling the geometry of the apex. The cells have a restricted metabolism concerned primarily in maintaining a low rate of increase in cell number, and they themselves, as well as their immediate derivatives, take no part in organogenesis or associated differentiation. The anneau initial, by contrast, is that part of the apex that produces the beginnings, or primordia, of lateral organs. Not only is the division rate higher, but the tissue as a whole is involved in metabolic syntheses that precede morphogenesis.

One difficulty in investigating the stem apex arises from the uncertainty about which aspects are important for the overall function: division planes, division frequency, metabolic patterns, or some combination of these. Still another complication results because the apex is in a state of constant change during the growth of the plant. A long-term developmental trend begins after the definition of the growing point in early embryogenesis and continues thereafter through juvenility and the period of vegetative growth into the reproductive phase. Superimposed on this trend is a cyclical change reflecting the periodic generation of the primordia of leaves and lateral shoots in the region immediately under the apex.