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Shoot
Primary Growth
(Campbell 6th Ed. 729-734; 7th Ed. 720-725)
The shoot is composed of two
organs: the stem and leaves. The points on the stem where leaves
are attached are called nodes
and the stem segments between nodes are the internodes.
The primary growth
of the shoot is concentrated at the shoot tip.
Examine a prepared slide of the shoot tip of Coleus
The dome-shaped structure at the tip of the stem is the shoot apical meristem. The cells of this meristem are small, dense staining and contain prominent nuclei. All tissues of the stem and leaves arise from this terminal meristem. Leaf primordia originate on the lateral flanks of the apical meristem (surface view. In sectional view the very young leaves appear to be long and narrow but they soon develop, on a miniature scale the characteristic shape of the mature leaves. They curve up and over the meristem with the older and larger primordia protecting the shoot apical meristem. The shoot apical meristem and the young primordia is commonly termed the "bud". Growth of the stem axis is by cell enlargement in the internodal region. There is a progressive increase in the distance between the young nodes and the apex of the shoot is pushed upward. In the axils of the older leaf primordia, dome-shaped masses of meristematic cells, like the apical meristem itself, develop. These are the branch primordia (lateral buds). In time these primordia become active apical meristems and form the lateral branches of the stem system. Commonly, the growing terminal bud of a shoot inhibits development of lateral buds, a phenomenon known as "apical dominance". As the distance between the terminal bud and the lateral bud increases, the inhibitory influence of the terminal bud is lessened and the lateral branch primordia become active.
Stem Structure
(Campbell 6th Ed. 733, Fig. 35.18; 7th Ed. 724, Fig. 35.16)
The cell derivatives of the apical meristem differentiate into the epidermis, pith, cortex and vascular bundles of the stem as well as the epidermis, vascular bundles and mesophyll of the leaf. There is, of course, a continuity between root, stem and leaf tissue.
By the time internodal elongation has stopped, the primary tissues of the stem have fully differentiated.
Note the following features:
The single layered (uniseriate) epidermis is covered by a thin cuticle. Beneath the epidermis are two to three rows of small thick walled collenchyma cells followed by three to eight rows of large, thin walled parenchyma cells . These two tissues constitute the cortex of the stem. Unlike the root, the vascular tissue of the dicot stem is composed of discrete vascular bundles arranged in a ring surrounding a central pith. Each vascular bundle is composed of xylem and phloem tissue with the phloem typically external to the xylem. The xylem includes thick walled fiber cells, vessel cell and parenchyma. The vessel cells are relatively thick walled and possess a large empty lumen. The phloem is composed of an external cap of fibers and just internal to these fibers, sieve tubes and companion cells. The sieve tubes and companion cells are very difficult to discern in most sections. The thin walled more or less rectangular shaped cells between the xylem and phloem constitute the vascular cambium. Future mitotic divisions of this lateral meristem will produce secondary xylem and phloem and contribute to the secondary growth phase of the stem.
Note the structural features and arrangement of the stem tissues.
