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

THE STEM

General characteristics of the stem and its Functions

The stem (caulis) is the axial, vegetative, predominantly above-ground, radially symmetrical supporting part of the SHOOT. It exhibits unlimited apical growth, positive heliotropism, and negative geotropism, bears leaves, buds, and their modifications, and participates in the Transport of substances. A stem consists of nodes and internodes. Depending on the extent of elongation, stems can be shortened or elongated. Stems that consist essentially of nodes alone are termed shortened.

The stem grows in length through The activity of apical (in the growth cone) and intercalary Meristems. It bears leaves, buds, and spore-bearing Organs, as well as flowers in angiosperms.

The primary Functions of the stem are conduction and support. Through conduction, the stem connects the above-ground and underground organs (roots and leaves), ensuring both upward and downward transport of substances. The supporting or mechanical function consists in holding the entire above-ground mass (leaves, buds, and lateral shoots) in space.

Additional functions of the stem include metameric, assimilatory, storage, protective, regenerative, and reproductive functions.

The stem is metameric because it consists of repeating units: nodes bearing leaves, buds, and shoots, and internodes, which are the sections of the stem between two adjacent nodes. Thus, the metameric function of the stem is associated with The formation of homologous organs (branches and leaves), which helps increase the plant's surface area for aerial Nutrition.

In the primary cortex of young stems, chlorenchyma is well-developed and Photosynthesis takes place, meaning that for a certain period they perform an assimilatory function (for example, young stems, Cladonia, and the stems of succulents). The storage function involves the accumulation of reserve nutrients in the parenchyma of the pith in perennial plants (e.g., kohlrabi stems, sugar cane, potato tubers), and Water in succulents (e.g., cacti). Some plants develop various outgrowths (thorns, prickles) that protect them from being eaten by animals, thereby performing a protective function.

The reproductive and regenerative functions involve the stem participating in the vegetative Propagation of the plant, which is facilitated by the presence of regeneration buds (e.g., rhizomes, stolons).

Stem shape. The shape of stems is typically cylindrical, characterized by radial Symmetry and a circular cross-section (e.g., in most species, grasses, rushes, woody plants, etc.). In addition, based on cross-sectional shape, stems can be: triangular (e.g., species of the genus Carex), quadrangular (e.g., in members of the Lamiaceae family), polygonal (e.g., in species of the genus Rumex), flattened (e.g., in Opuntia), winged (e.g., in Lathyrus), ribbed (e.g., in Apiaceae), hollow at the internodes (e.g., in grasses), and so on.

Stems can also be ribbed, furrowed, nodose, articulate (jointed), or moniliform. A stem is called ribbed if narrow ridges run parallel to its longitudinal axis, alternating with wide depressions. A furrowed stem features shallow longitudinal grooves. A moniliform (beaded) stem has strongly thickened sections separated by deep constrictions (e.g., in Schlumbergera), whereas an articulate stem has weakly defined constrictions (e.g., in Salicornia). A stem is considered nodose if its diameter at the nodes is greater than at the internodes (e.g., in grasses). A winged stem is one in which two opposite Ribs are strongly developed.

In many plants, the shape of the stem serves as a systematic (taxonomic) diagnostic feature.

Based on their orientation in space, stems can be upright (erect), ascending, twining, climbing, creeping, prostrate, decumbent, etc. Among the great variety of stems, leafless stems also occur; these are usually green and perform the functions of leaves (e.g., in certain cacti). If a leafless stem terminates in a flower or an inflorescence, it is called a scape (e.g., in dandelions and onions). A scape is a strongly developed apical internode of a shortened shoot.

Depending on the degree of branching, stems are classified as branched or unbranched.

According to their consistency (texture), stems can be herbaceous (e.g., in herbaceous plants), woody (e.g., trees, shrubs, subshrubs), or semi-woody (e.g., certain species of wormwood/Artemisia).

Based on the presence of pubescence, stems can be pubescent or glabrous.

A glabrous or smooth stem is one devoid of any outgrowths and typically featuring a well-defined waxy coating (e.g., Sempervivum, Sedum species, etc.).

In many plants, various appendages and outgrowths—such as prickles, hairs, notches, scales, etc.—develop On the surface of the stem or parts of it.

Trichomes, or hairs, are outgrowths of Cytology/practical/5.html">Plant Epidermal Cells that vary in shape, Structure, and function (e.g., simple and glandular hairs, scales, prickles, etc.).

Emergences are superficial outgrowths on the stem formed by the epidermis and the underlying Tissues (e.g., stinging hairs of nettles, prickles of roses, raspberries, gooseberries, and briars, and hook-like hairs of hops).

Scales and hairs differ in structure. Hairs can be simple, branched, unicellular, or multicellular, and their shapes vary widely. For example, in nettles and potatoes, the hairs are simple, unicellular, and pointed, whereas in mullein (Verbascum), they are branched and consist of several single-celled tiers arranged in rings. Hairs perform a protective function by reflecting sunlight, thereby shielding the plant from excessive overheating and Transpiration. The cells of hairs are living at early stages and subsequently die off, forming a whitish-gray pubescence.

Multicellular epidermal hairs include stellate hairs. In such hairs, numerous axes of equal length radiate from a single point and lie in the same plane.

The cells of glandular hairs produce a secretion that is released externally under certain conditions. The tip of a glandular Hair is expanded into a HEAD. In mint, the head is multicellular, whereas in geranium and stinging nettle, it is unicellular.

Scales are multicellular plates that vary in size and are attached to the organ by a broad base via a stalk (e.g., the scales of sea buckthorn and oleaster).

Trichomes may fall off as the organ ages, or they may persist throughout its entire life span.

Pubescent refers to a stem bearing a collection of trichomes. Pubescence can be uniform or non-uniform. Several types of pubescence are distinguished based on the size, shape, orientation relative to the organ surface, and density of trichomes: velvety, arachnoid, tomentose, silky, woolly, bristly, Ciliate, scaly, and glandular.

Soft, short trichomes form velvety pubescence. Long, tortuous, thin trichomes pressed against the surface form arachnoid pubescence. Branched or simple, densely intertwined trichomes form tomentose pubescence. Soft, long trichomes appressed to the surface form silky pubescence. Woolly pubescence is formed by curved, long trichomes through which the stem surface is clearly visible. Bristly pubescence is formed by sparse, coarse, and long trichomes. When long, straight trichomes are arranged in a single row along the angles of the stem, such pubescence is called ciliate. Scaly pubescence is formed by scales, and glandular pubescence is formed by glandular capitate trichomes.

Dimensions and life span of the stem. Plant stems vary in size and age.

The maximum length (200....300 m) is reached by the stems of rattan palms (lianas). In the vast majority of herbs, stems are low (several tens of centimeters), yet there are herbs whose stem height reaches 2 m or more (e.g., in maize, sunflower, common reed, etc.).

The perennial stem of woody plants, which is covered with a thick layer of secondary protective tissues and forms the main skeletal axis, is called the trunk. The trunk persists throughout the entire life of trees, and for a significant number of years in shrubs.

According to trunk height, trees are classified into: the first magnitude (above 25 m), the second magnitude (16–25 m), and the third magnitude (5–15 m). The tallest trees on Earth are the giant sequoia (80–100 [135] m), North American Douglas-firs (100 m), coast redwood (110–112 m), and Australian eucalypts (around 150 m).

Tree trunks vary in diameter. For instance, in the baobab and giant sequoia, it reaches 10–12 m; in the common oak, 4 m; in the silver poplar, 2 m, etc.

The life span of stems varies. Two MAIN TYPES OF stems are distinguished: herbaceous, which typically exists for a single growing season, exhibits weak lignification and thickening, is strongly parenchymatous, and lacks periderm; woody, which is typically perennial, thickens indefinitely, and is formed by lignified tissues.

The age of stems in ephemeral plants is 30...45 days, in most herbs 120...150 days, and in trees several centuries (e.g., apple up to 200, pine up to 500, etc.). Trunks of certain trees live for more than 1 thousand years (e.g., sequoia, yew, cypress — 3...5 thousand years).

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Fig. 3. Stem shape: 1 - cylindrical, 2 - conical, 3 - fusiform (terete), 4 - compressed (elliptic), 4a - compressed (lenticular), 4b - compressed (notched in the middle), 4c - compressed (acuminate), 5 - globose (spherical), 6 - angular triangular, 6a - quadrangular, 6b - pentangular, 6c - multangular, 7 - bluntly triangular, 8 - bluntly quadrangular, 9 - concavely triangular, 10 - concavely quadrangular, 11 - convexly triangular, 12 - compress-triangular, 13 - compress-quadrangular, 14 - compress-concavely quadrangular, 15 - compress-convexly quadrangular, 16 - ribbed, 17-20 - furrowed, 21 - nodose, 22 - articulate, 23 - moniliform.