Plant Biology: Characteristics of Shoot Systems

The aboveground, conspicuous part of flowering plants constitutes the shoot system, which is composed of erect stems on which are attached leaves, flowers, and buds. Leaves are attached to the stem at regions called nodes. The section of stem between nodes is an internode, and the upper angle between the stem and the leaf at the node is called the leaf axil. Axillary (lateral) buds located in the leaf axils give rise to vegetative branch stems or to flowers. Terminal buds are present at the tips of the main stem and branches and contain the apical meristem tissues. The shoot originates in the embryo at the end opposite the root and develops a complex shoot apex, different from that of the root (see Table 1 ).

TABLE 1

The Shoot Apex of Roots and Stems

	Roots	Shoots
Apical meristem	covered by a cup-shaped root cap	exposed, no cellular cap
Apical appendages	absent	produce primordia (leaves and buds)
Orientation of cell division	occurs in all planes	oriented; two locations: inner ( corpus) and an outer ( tunica)
Zones	separate areas of division, elongation, and differentiation	no such recognizable zones present

The growing point of the shoot—the apical meristem—is surrounded by developing leaves ( leaf primordia) that have in their axils bud primordia. The buds are of two kinds: Some are vegetative and will develop into leafy branches; others contain rudimentary reproductive tissues and will produce flowers.

Auxins (hormones) produced in the tip ordinarily inhibit the growth of the lateral bud primordia, and they remain dormant for some time; if the apical bud exerts apical dominance over the lateral buds, the plants produced will be conically shaped with a single leader and shorter lateral branches. If apical dominance is weak, axillary buds develop into branches soon after the terminal shoot elongates, resulting in a plant with many, branched stems, none the clear leader. Auxins produced in the leaf primordia control the elongation and differentiation of the primary meristems.