BOTANY. PLANT MORPHOLOGY - O. A. Shevchuk - 2014

SHOOT MORPHOLOGY. THE SHOOT AND ITS PARTS

Definition, Functions, Structure. The SHOOT (Latin: blastos) is the axial organ of higher plants capable of apical growth, consisting of an axis (the stem), leaves, and buds originating from it. In phylogenesis, shoots emerged as an ADAPTATION TO A terrestrial lifestyle resulting from the Transformation of a system of leafless cylindrical Organs—telomes—in rhyniophytes. In ontogenesis, the shoot develops from an embryonic bud or from an axillary or adventitious bud.

The shoot provides aerial Nutrition, enables Vegetative Reproduction, and is capable of metamorphism.

The constituent elements of a shoot are the stem with nodes and internodes, buds, and leaves. A bud is an embryonic shoot. The stem and leaves are structural elements (second-order organs) of the shoot that form from a common mass of meristem (the shoot apical meristem) and share a unified Vascular System. The stem Supports the spatial arrangement of leaves, flowers, and fruits, which ensures the optimal performance of their functions. It facilitates the Transport of substances in both upward and downward streams, thus acting as an intermediary between the roots and leaves. The leaf is a flat lateral organ extending from the stem with limited growth, performing the functions of Photosynthesis, gas exchange, and Transpiration. Buds ensure the continuous growth and branching of the shoot, leading to The formation of a shoot system. The shoot differs from the ROOT by the presence of leaves.

The Characteristic Features of a shoot include:

✵ unlimited growth and the formation of new organs due to meristem activity;

✵ metamerism, or longitudinal Symmetry—the segmentation of the organ into similar parts (metameres): a node, a leaf, an axillary bud, and the underlying internode;

✵ radial symmetry (multiple planes can be drawn along the radius).

The region of the stem where one or several leaves develop is called a node. Open (incomplete) and closed (complete) nodes are distinguished. An open or incomplete node is one where the leaf base encompasses only a portion of the stem circumference. For instance, such nodes are characteristic of maples, oaks, poplars, etc. A closed or complete node is one where the leaf base entirely encircles the stem, as seen in Caryophyllaceae, Apiaceae, Poaceae, etc.

Lateral roots may form at the nodes. Each stem node has the potential ability to produce a lateral shoot from a bud and lateral roots. The section of the stem between two adjacent nodes is an internode. The upper angle between a leaf and the stem is the leaf axil. Axillary buds develop within the axils. After leaf fall, a leaf scar with vascular bundle traces remains on the stem. In the case of leaf reduction, leafless shoots develop.

Classification of shoots. According to their origin, the following structural and morphological types of shoots are distinguished: main and lateral.

The main shoot, or first-order shoot, is the shoot that forms from the apical meristem of the embryonic stem axis. The main shoot is the primary one by origin. It consists of the hypocotyl, cotyledons extending from the cotyledonary node, and a bud. The bud forms the constituent PARTS OF THE main vegetative shoot: the epicotyl, the stem with leaves, and buds. The hypocotyl, or subcotyledonary axis, is the region of the plant seedling stem between the root collar and the cotyledons. Shoots that initiate on the hypocotyl region of the stem are called hypocotyl shoots (e.g., stump sprouts in poplars, hornbeams, etc.). The epicotyl, or supercotyledonary axis, is the part of the plant seedling stem between the cotyledons and the first leaves, representing the first internode. Shoots initiating on the epicotyl region of the stem are called epicotyl shoots (for example, some basal shoots and stolons in marsh tea / Rhododendron tomentosum).

A lateral shoot is a shoot that develops from a lateral bud and causes branching, as well as an increase in the size and surface area of the plant's vegetative body.

According to the presence of reproductive organs, the following shoot types are distinguished: vegetative, reproductive, and vegetative-reproductive.

A vegetative or sterile shoot is a shoot bearing leaves and buds. It consists of a radially symmetrical stem axis with nodes and internodes, leaves positioned along its axis, and buds. Photosynthesis, as The primary function of the shoot, is carried out by the leaves. The stem is the part of the shoot that supports other organs and performs conducting, mechanical, and sometimes storage functions. Vegetative shoots primarily provide aerial nutrition and are also capable of various metamorphoses. Vegetative shoots and their metamorphoses sustain the individual life of the plant, ensure vegetative reproduction, defense, etc.

Vegetative shoots are divided into aerial and subterranean. Among aerial vegetative shoots, annual herbaceous assimilating shoots (which develop during a single growing season) and perennial woody shoots, or branches, are distinguished. Aerial modifications of vegetative shoots include thorns, tendrils, runners, phylloclades, and cladodes.

Subterranean vegetative shoots include stolons, rhizomes, bulbs, tubers, etc. They ensure the accumulation of nutrients and vegetative reproduction in plants.

A generative or spore-bearing shoot is a shoot on which reproductive organs ensuring propagation are formed. Such shoots produce spore-bearing spikes or strobili containing spores, which originates their name. Spore-bearing shoots are characteristic of Water/56.html">LOWER SPORE PLANTS such as horsetails, ferns, clubmosses, and gymnosperms.

A reproductive or floriferous (fruit-bearing) shoot is a plant shoot on which flowers or infructescences develop, from which fruits or multiple fruits with seeds subsequently ripen. Such shoots are produced by angiosperms.

Some plants exhibit a mixed type of shoots, known as vegetative-reproductive.

According to internode length, shoots are classified into: elongated, shortened, normal, rosette, semirosette, and rosulateless.

In a typical shoot, internodes elongate during its development. Therefore, a shoot with internodes whose length exceeds the stem diameter in the same region is called elongated. Elongated shoots are also referred to as dolichoblasts or auxiblasts. The internodes of such shoots are excessively elongated, leafless, and flowerless. They are characteristic of lianas and tree of heaven (Ailanthus). Elongated internodes can form epicormic shoots or water sprouts. These develop from adventitious buds in the crowns of fruit trees following the pruning of the tree apex, lateral branches, etc. In woody plant species, such shoots are called growth shoots.

If internodes do not develop and nodes are densely packed against each other, such a shoot is called shortened, or a brachiblast. The stem portion of a brachiblast practically consists of nodes bearing closely crowded leaves. Their annual increment is only a few millimeters. The Development of shortened shoots increases crown foliage density and the number of flowers and infructescences. It should be noted that such shoots are well-defined in aspen, birch, poplar, pear, and apple trees. In plants such as pine, barberry, and larch, leaves develop exclusively on shortened shoots. In pear and apple trees, flowers and fruits predominantly form on such shoots as well; they are referred to as fruit spurs.

A normal shoot is a shoot with normal or medium-length internodes. Such shoots are typical of many wild and agricultural plants (e.g., plum, linden, hazel, pea, etc.).

In herbaceous plants, shortened shoots form basal rosettes, or more rarely, apical ones. Accordingly, the following shoot types are distinguished: arosulate, rosulate, and semirosulate.

An arosulate shoot is a shoot in which all internodes are elongated, bearing both median and upper leaves (for example, in sunflowers, dahlias, phlox, etc.).

A rosulate shoot is a shoot in which the basal part of the internode is shortened (forming a rosette), while a single elongated internode—a scape bearing a flower or inflorescence—develops above it (for example, in daffodils, tulips, hyacinths, dandelions, etc.).

A semirosulate shoot is a shoot featuring a basal rosette, above which several metamers develop with elongated internodes, as well as median and upper leaves (for example, in delphiniums, lupines, etc.).

Depending on the direction of growth and spatial orientation, shoots are classified into the following types: orthotropic (vertical), plagiotropic (horizontal), and heterotropic (mixed).

An orthotropic shoot is a shoot oriented vertically relative to the substrate surface, such as the shoots of the common oak or Scots pine. Among orthotropic shoots, erect forms predominate, as seen in sunflowers, ficuses, rye, and others.

In plants incapable of maintaining a vertical position on their own (lianas), shoots can be twining—winding around a support (such as hops, field bindweed, beans, etc.)—or climbing and gripping, utilizing various adaptations to stay upright. Examples include tendrils in grapes and pumpkins, hooks in bedstraw, adventitious rootlets in ivy, and holdfasts in wild grapes.

A plagiotropic shoot is a shoot oriented more or less horizontally relative to the substrate surface, such as the creeping runners of strawberries or the prostrate stems of watermelons, melons, and cucumbers.

Plagiotropic shoots may be prostrate or trailing—stems that do not root (e.g., knotweed, rupturewort, chickweed, tradescantia)—or creeping, which do root (e.g., periwinkle, wild strawberry, garden strawberry, purslane, etc.). Creeping shoots with relatively short internodes are referred to as runners (or whips), while those with elongated internodes are called stolons.

A heterotropic or anisotropic shoot is a shoot characterized by mixed growth direction; most commonly, it grows as a plagiotropic underground structure before turning into an orthotropic aerial shoot, as observed in thyme and heather.

Anisotropic shoots also include ascending or rising forms. Occasionally, weeping forms occur due to branches growing downward (as seen in birch, rowan, and willow).

Based on their position relative to the soil surface, shoots are categorized as aerial or underground.

Depending on their developmental duration, shoots are classified as annual or perennial.

An annual shoot is one whose development is completed within a single year, such as the shoots of herbaceous plants or the annual growth increments of woody plants.

A perennial shoot is one that develops over multiple years as part of the skeletal axes of woody plants.

Based on their developmental rhythm, the following shoot types are distinguished: skeletal, replacement, renewal, and enrichment shoots.

A skeletal shoot is a woody, perennial shoot covered with secondary protective Tissues, characterized by intensive growth and large dimensions, such as the trunk of a tree or shrub.

A replacement shoot is a daughter shoot that outgrows the parent shoot and, together with it, becomes part of the Skeletal System. Such shoots develop in trees with sympodial branching. In herbaceous plants, this shoot is annual and is referred to as a renewal shoot.

A renewal shoot is a shoot of perennial herbaceous plants that develops from a renewal bud following a period of dormancy.

An enrichment shoot is an annual lateral shoot that grows actively without entering dormancy, increasing the plant's overall surface area through branching.

Based on apex specialization, shoots are divided into monocarpic and polycarpic types.

A monocarpic shoot has a short life cycle during which it forms a flower or inflorescence, after which it dies off entirely (in annuals) or partially (in perennials). In perennial plants, growth is resumed through renewal buds. Monocarpic shoots are further classified into monocyclic (fruiting in the first year), dicyclic (developing over two years: vegetative in the first year, and reproductive or mixed In the second), and polycyclic (developing over more than two years).

A polycarpic shoot is a perennial shoot whose apical growth is not terminated by the formation of reproductive organs, which instead occupy a lateral position. Such shoots bear fruit over many years.

Shoot GROWTH AND DEVELOPMENT. The embryonic state of a shoot is the bud. The core part of the bud is the apical meristem (growing cone). It occupies the terminal position in the plant, consists of primary meristem, and gives rise to all primary permanent tissues and organs. Within the bud, the growing cone is surrounded by leaf primordia at various Selection/3.html">Stages of development, as well as modified leaves known as bud scales, which serve a protective function.

The Cells of the growing cone divide intensively, increasing in number and causing the stem to elongate. Concurrently, Primary and secondary meristematic protuberances emerge, which develop into new leaf primordia and axillary buds.

Leaves in the shoot apex originate as lateral protuberances in the form of ridges. These leaf initials are called primordia. The number of leaves on a shoot, their arrangement, and The rate of their formation vary among plant species and serve as a constant characteristic of a given species.

The time required for the development of each successive leaf is known as the plastochron and is constant for each plant species. For instance, the time interval between the initiation of two successive leaves is 4.3 hours in spruce, compared to 12.5 days in maple.

Inside the bud, leaves are packed tightly against one another because the stem portion is shortened. As the bud unfolds, the stem internodes elongate, spacing the leaves apart.

The shortened nodes reach their normal length, while the leaves attain their proper shape and size. Cells in the terminal bud proliferate more intensively and continuously than the apical Meristems of lateral buds and shoots. As a result, the main stem stands out for its vigor and growth rate among the mass of branches.

Shoot elongation and branching types

Apical growth of a shoot is driven by The activity of the apical bud, ensuring lengthwise elongation through the formation of new metameres. Certain plants (such as corn, rye, etc.) exhibit intercalary growth, which is driven by an intercalary meristem located at the Base of the internodes.

Apical growth can be either monopodial or sympodial.

Shoot branching is the formation of subsidiary axes of lower orders from a parent axis. This crucial biological process increases the surface area and, consequently, the productivity of the plant. Shoot branching occurs through the simultaneous unfolding of numerous lateral buds. During apical branching, a bud may give rise to two or multiple axes at the same time—a phenomenon known as polytomy. The development and initiation of lateral buds can proceed acropetally (from the base to the apex) or basipetally (from the apex to the base).

There are two MAIN TYPES OF branching: apical and lateral.

Apical branching is a type of branching in which the growing point of the parent shoot divides into two equal or unequal elements that occupy a terminal position. In this type of branching, the growth of the parent axis ceases at the moment the lateral axes are initiated.

Dichotomous or forked branching is a type of apical branching where the apex of the main axis divides into two, giving rise to two subsidiary axes, each of which in turn produces two further axes, and so on. This is an archaic, primitive type characteristic of ancient plant groups, and today it occurs in Algae, mosses, clubmosses, many ferns, and some gymnosperms. Forked branching, in turn, can be isotomous dichotomy, where the resulting shoots are of equal size (e.g., fir clubmoss), or anisotomous dichotomy, where one shoot develops more vigorously than the other (e.g., spikemoss).

Lateral branching is a type of branching in which subsidiary axes are formed from lateral buds originating below the apex of the parent axis. There are Two Types of lateral branching: monopodial and sympodial.

Monopodial branching is a type of lateral branching in which the apical bud provides continuous, gradual elongation of the main axis, while lateral axes form below the growing point, are less developed, and do not exceed the main axis in length. Consequently, with this branching pattern, the main axis—the monopodium—exhibits unlimited apical growth. Axes of subordinate orders diverge from the monopodium. Monopodial branching is characteristic of ferns, all gymnosperms, and many angiosperms, such as oak, bird cherry, lily of the valley, and certain species of the genus *Populus*, among others.

Monopodial branching lacks the drawbacks inherent to dichotomous branching: it provides reliable support for the plant and is

more intensive. This higher intensity is explained by the fact that the growing point is represented not by a single initial Cell, but by a multicellular apical meristem. Lateral shoots are produced in large quantities rather than just one or two per growing season.

In woody plants, this branching habit promotes the formation of polysymmetric crowns with straight, evenly thickened trunks, which are highly valued in construction, shipbuilding, and other industries.

Sympodial branching is a type of lateral branching in which the apical bud of the main axis either dies off or lags in growth, and a shoot develops from a lateral bud situated directly beneath or lower down. The main shoot—the sympodium—develops as a result of the activity of various meristems. Its base incorporates dichotomous and monopodial branching. Sympodial branching is characteristic of angiosperms, especially among herbaceous forms. Examples include birch, linden, hazel, Solomon's seal, grasses, sedges, and tomatoes.

Compared to monopodial branching, sympodial branching is more intensive and produces a denser crown. This is because the death of the apical bud stimulates the development of numerous lateral buds. As a result, A large number of flowers, fruits, and seeds are produced, which plays a positive evolutionary role in angiosperms by helping to increase offspring numbers. However, it should be noted that this type of branching does not promote intensive longitudinal growth of the plant.

Three distinct types of sympodial branching are distinguished: acrosympodial, mesosympodial, and basisympodial.

Acrosympodial branching is a type of branching in which the shoot axis of the next order is formed exclusively from the apical bud of the preceding axis.

Mesosympodial branching refers to a pattern in which the growth and branching of the shoot continue from buds located in the middle section of the axis (for example, in honeysuckle and spiraea).

Basisympodial branching occurs when only the base of the shoot axis of each order contributes to the sympodium (for example, the rhizome of Solomon's seal).

A special type of sympodial branching is false-dichotomous branching.

False-dichotomous branching is a type of branching in which the growing point of the main axis is expended on Flower Formation or dies off, and stem branching is carried out by means of opposite lateral buds located below. As a result, lower-order axes successively cease growth, being replaced by opposite higher-order axes. This type of branching is observed in Representatives of the Caryophyllaceae family, mistletoe, lilac, chickweed, and horse chestnut.

Tillering is a specialized type of branching found in both herbaceous and woody plants. During tillering, lateral shoots (tillers) develop exclusively from basal or underground buds of the parent shoot. The internodes at the base of the shoot are shortened, and accordingly, the vast majority of lateral buds are located close to one another. The region of shortened internodes where tiller formation takes place is called the tillering zone or tillering node. The tillering zone is quite well developed in grasses. Adventitious roots develop on the tillers. The primary root in grasses dies off early, and the aggregate of adventitious roots forms a fibrous root system.

During tillering, every shoot in a bush terminates in a flower or an inflorescence.

Intravaginal is a shoot that develops inside the Sheath of the covering leaf. Extravaginal is a shoot that breaks through the sheath and grows away from the parent stem.

Depending on the length of plagiotropic shoots and the shape of the tillering node, grasses are divided into: densely tufted, loosely tufted, and rhizomatous.

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Fig. 2. Types of stems and shoots based on their spatial orientation: 1 — erect, 2 — inclined, 3 — bent, 4 — arcuate, 5 — drooping, 6 — prostrate, 7 — creeping, 8 — ascending, 9 — geniculate-ascending, 10 — fractiflex, 11 — zigzag, 12 — twining (clockwise), 13 — twining (counterclockwise), 14 — clinging, 15 — climbing, 16 — creeping-rooting, 17 — interwoven, 18 — pendulous, 19 — floating, 20 — emergent, 21 — submerged.