Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Organs of Plants
3.5. Stem Morphology and Anatomy. The Concept of the Shoot. Shoot Morphology. The Bud, Shoot Development, Types of Buds. Shoot Branching. Tillering. Phyllotaxy

The stem is a vital axial vegetative organ of a plant. Morphologically and functionally, it connects the underground Organs of absorption (roots) and the aerial organs of Nutrition (leaves). It serves as a conduit for the upward flow of Water and mineral salts from the roots to the leaves, and the downward flow of assimilates from the leaves to sites of consumption and storage. The stem exhibits apical growth, negative geotropism, radial Symmetry, and a complex internal Structure adapted to performing diverse physiological Functions. It is capable of producing and supporting leaves, branches, flowers, and fruits, carrying out Photosynthesis, storing reserve nutrients, developing buds, regenerating, and acting as an organ of reproduction.

A stem together with the leaves and buds it bears is called a SHOOT. In ontogeny, a shoot develops from a bud. The point of stem attachment for a leaf is called a node, and the segment between adjacent nodes is an internode. Based on internodal development, Three types of shoots are distinguished: shortened, normal, and elongated. Elongated shoots feature greatly stretched internodes with sparsely arranged leaves (such as the tree of heaven), while the majority of plants possess normal shoots with moderately proportioned internodes. Shortened shoots are characterized by extreme compression of the internodes, sometimes creating the illusion of a stemless plant. Such plants form rosettes, for instance, primroses and dandelions.

The angle between the stem and the petiole of an emerging leaf is called the axil. The stem terminates in an apex, which functions as a shortened rudimentary shoot. A bud consists of a growing tip (apex), embryonic leaf primordia, nodes, and heavily compressed internodes. All these structures are externally protected by scales, which are modified leaves.

Types of buds: naked and protected. Naked buds are typical of annuals, whereas protected ones predominate in perennial herbs, trees, and shrubs, and are often covered with tough, leathery scales.

Depending on their position, buds are classified as terminal (apical) and axillary (lateral). Terminal buds are located at the apex of the shoot and drive its longitudinal growth, while lateral or axillary buds are situated further down the stem axis, and their development promotes branching.

Buds can be vegetative, generative, or mixed. Vegetative buds give rise to shoots and leaves, generative ones develop into a flower or inflorescence, and mixed buds produce both leaves and flowers.

Buds may occur singly or in groups. When buds are arranged vertically one above another on a shoot, they are called serial; side-by-side or horizontal arrangements are termed collateral; and those encircling the stem are whorled.

Buds that form on internodes, roots, or leaves are known as adventitious buds, while those that remain dormant for extended periods and develop only under specific conditions (such as pruning or freezing) are called latent or resting buds.

Stem development is accompanied by an increase in trunk and foliage mass, the synthesis of abundant organic matter, and progressive branching. Several types of branching are distinguished: monopodial, sympodial, dichotomous, and false dichotomous. In monopodial branching, the main stem continues to grow via the terminal bud for many years, while lateral shoots originate from the main and secondary axes without exceeding the main stem in size (e.g., pine, spruce). Sympodial branching is a type where the terminal bud ceases growth after a time, and shoot elongation is continued by a lateral bud, which eventually also stops growing, prompting a new axillary bud to take over, and so on (e.g., linden, plum). Dichotomous branching occurs through the bifurcation of the apical growth point into two new ones that retain this property indefinitely (e.g., clubmosses, spike mosses). In false dichotomous branching, the terminal bud dies, and two oppositely positioned buds below it sprout to form two leading axes, which in turn experience apical dieback and give rise to a new pair of opposite lower buds, and so forth (e.g., common lilac). A specialized type of branching is tillering, which results in The formation of numerous aerial shoots. These arise from tillering nodes established at the Base of the parent plant either in or near the soil surface. In tillering plants, the buds at the base of the shoot are densely clustered, as are the renewal buds. Together with the shortened internodes and the developing shoot buds, this complex is referred to as the tillering node or tillering zone. Tillering is the characteristic branching pattern of grasses and sedges.

According to their tillering habits, grasses are categorized into: densely tufted, where the buds at the nodes are tightly clustered (e.g., matgrass); loosely tufted, where the buds on the tillering nodes are spaced apart (e.g., bluegrass); or rhizomatous, where the buds are situated along a rhizome (e.g., couch grass).

In most plants, stems are clothed in leaves. Each plant species is characterized by a specific pattern of leaf arrangement, known as phyllotaxy. The MAIN TYPES OF phyllotaxy are spiral (alternate), opposite, and whorled. In spiral (alternate) phyllotaxy, a single leaf arises from each node of the shoot. This is the most widespread type of leaf arrangement (found in most members of the rose, bean, and nightshade families, for instance). When two leaves are attached at a node, positioned directly across from each other, the arrangement is opposite, which occurs in lilacs, maples, sages, and pinks. Even less common is whorled phyllotaxy, where three or more leaves are attached at a single node, as seen in herb paris, oleander, or bedstraws.