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

SHOOT SPECIALIZATIONS AND METAMORPHOSES

Typically, a vegetative SHOOT performs the function of aerial Nutrition. However, sometimes its function changes in The process of ADAPTATION TO ENVIRONMENTAL conditions. This leads to the Modification of the shoot, i.e., metamorphosis. In nature, among herbaceous perennials, shoot metamorphoses are often associated with the storage of nutrients and, in most cases, also serve for Vegetative Reproduction. Shoot metamorphoses (modifications) can be underground and aboveground. Underground shoot modifications. These modifications perform very important Functions: they serve as Organs for nutrient storage, ensure plant vegetative reproduction, and also protect plants under unfavorable environmental conditions (low temperatures, prolonged moisture deficiency). In spring, new aboveground shoots develop from them.

A rhizome is an underground shoot that outwardly resembles a ROOT, but unlike the latter, it has a well-defined metameric Structure (at least in the young part): internodes, nodes, and scale-like leaves with buds located in their axils (couch grass, horsetails, lily of the valley). Lateral Branches of the rhizome (herb Paris) or aboveground shoots (Solomon's seal) are formed from these buds. Adventitious roots develop on the nodes of the rhizome, replacing The Root System. Reserve nutrients are deposited in the stem of the rhizome (and sometimes in the leaves).

Depending on the direction of growth, rhizomes are distinguished as horizontal, oblique, or vertical, and according to their origin, as epigeogenic and hypogeogenic. An epigeogenic rhizome is formed through the gradual transformation of the axes of aboveground shoots into underground ones, due to their being covered and pulled into the soil. The hypogeogenic rhizome comprises only those shoot areas that have never grown aboveground. It performs the functions of storing reserve substances, initiating renewal buds, and vegetative reproduction. The latter function of the rhizome is observed in perennial plants which, as a rule, lack a main root in their adult state.

The Water/140.html">Anatomical Structure of the rhizome is typically stem-like. However, under the Influence of the underground lifestyle, structural elements characteristic of the root sometimes develop in the rhizome, namely: a two-layered endodermis with Cells whose walls are horseshoe-thickened appears between the cortex and the central cylinder.

Taking into account the length of the rhizome internodes and the annual growth rate, long (couch grass, wood small-reed, foxtail), short, and shortened rhizomes (irises, Solomon's seal) are distinguished. Based on this feature, short-rhizome, medium-rhizome, and long-rhizome Morphology/18.html">PLANT LIFE FORMS are identified among rhizomatous species. Plants with horizontal long rhizomes that form many aboveground shoots quickly occupy large areas. If these are weeds (couch grass), combating them is quite difficult. Such plants are used for sand stabilization (sedge). In meadow management, grasses with long horizontal rhizomes are called rhizomatous (bluegrass, bentgrass), and those with short ones are called tufted (timothy-grass, matgrass). Rhizomes are characteristic mainly of perennial herbaceous plants, but sometimes they also develop in shrubs (spindle tree) and subshrubs (lingonberry, bilberry).

A caudex is a perennial organ of shoot origin. Together with the root, it serves as a site for nutrient storage and bears numerous renewal buds, some of which may be dormant. The caudex is underground, less commonly aboveground. It is formed from the short bases of dead semi-rosette flowering shoots or shortened axes of rosette shoots that sink into the soil.

The caudex differs from the rhizome in its mode of dying off. Specifically, the caudex does not die off from the lower end, but grows and thickens at the expense of the cambium, gradually passing into a perennial root that also thickens. The caudex and thickened root gradually die off and break down from the center to the periphery due to the decay of the parenchymatous Tissues of the pith and wood. A cavity is formed in the center of the caudex. Later, the caudex, together with the root, may divide longitudinally into separate parts — particules. Caudex plants are numerous among Representatives of the Fabaceae, Apiaceae, and Asteraceae families.

Tubers are thickened underground modified shoots with one or several closely spaced internodes. They are formed at the ends of elongated underground shoots called stolons. The apical bud of the stolon thickens, its axis grows and transforms into a tuber, while only eyebrows remain of the scale-like leaves. Groups of eye buds sit in the axils of each eyebrow, one of which, under favorable conditions, sprouts into a new shoot. Stolons eventually break down, and tubers serve as organs of vegetative reproduction.

The Internal Structure of a potato tuber indicates its shoot nature. In a young state, the tuber is externally covered by an epidermis, which is eventually replaced by a periderm. The bulk of the tuber is represented by pith parenchyma. A large amount of starch grains accumulates in both the cortical and pith parenchyma. Weakly developed vascular bundles are located at a great distance from each other along the cambial ring. Tubers of shoot origin are characteristic not only of potatoes, but also of some other plants, such as Jerusalem artichoke (earth pear), where they are formed on shortened underground shoots.

A bulb is a modified underground, highly shortened shoot that structurally resembles a bud. Externally, the entire bulb is covered with dry brownish scales. The shortened stem part is called the basal plate (donets). Tightly arranged fleshy modified leaves, in which reserve nutrients accumulate, are attached to it. Numerous adventitious roots develop from the basal plate. Bulbs are characteristic of ephemeroids—plants that have a short growing season and grow predominantly in arid climates. Bulbs with and without rhizomes are distinguished. Bulbs with rhizomes reproduce by means of offshoots developing from the basal plate; these elongate and form a new small bulb that roots at some distance from the parent bulb. This method of reproduction is typical of Bieberstein's tulip. Bulbs of different plants vary in both biological and morphological features. Thus, in some plants (onion, hyacinth, tulip), they are covered with solid membranous scales. Such bulbs are called tunicate. In other plants, such as the lily, they are covered with small scales. Such bulbs are called scaly. In most bulbs, the outer dry scales play a protective role for the inner juicy and fleshy ones. A replacement bud arises in the axil of the last leaf, from which a scape develops the next year, and a flower on it. Daughter bulbs, commonly referred to as "cloves" or "babies", are laid down in the axils of the remaining leaves. Many of them are formed in garlic, forming a compound bulb out of simple daughter bulbs.

A corm is an underground modified organ that externally resembles a bulb and in cross-section—a tuber. Unlike a bulb, in a corm reserve substances accumulate in the stem part—the basal plate (saffron, gladiolus).

Underground stolons and underground tubers are short-lived horizontal weak underground shoots with scale-like colorless leaves and tubers at the apex. Unlike the rhizome, they do not perform the function of nutrient storage, and their apical buds thicken, turning into tubers. Underground stolons are formed in potatoes. In its homeland, this is a perennial plant that regenerates and reproduces by tubers. Here, it is artificially planted every year with tubers to obtain a yield of new tubers. In spring, flower shoots grow from the eyes, and white stolons, slightly thicker than numerous adventitious roots, are formed on these shoots from the axils of underground scale-like leaves, and partly aboveground ones covered with soil. After tuber formation, the stolons break down, and the tubers become organs of vegetative reproduction. Tubers differ from underground-origin rhizomes in their spherical or oval shape, greater thickness, and further reduction of leaves, which become merely eyebrows. However, in nature There are many transitional forms between tubers and thick rhizomes, such as in figwort (Scrophularia nodosa), marsh woundwort (Stachys palustris), corn mint (Mentha arvensis), etc.

Aboveground shoot modifications. These modifications arise As a result of plant adaptation to moisture regimes and are aimed at conserving water in the aboveground parts of plants during the dry period.

Phylloclades are formed in the axils of modified scale-like leaves and have limited growth. These are flat, leaf-like expanded shoots that perform the functions of leaves. Scale-like leaves are formed on phylloclades, and in their axils on pedicels—flowers or inflorescences and fruits, which indicates their shoot nature (Ruscus and Phyllanthus). The anatomy of phylloclades also confirms their stem structure.

Cladodes, like phylloclades, are leaf-like flattened stems that perform the function of leaves, but unlike phylloclades, they are capable of unlimited growth. Cladodes are found in asparagus and Australian muehlenbeckia.

Phyllodes are flattened, leaf-like expanded petioles of a leaf with a xeromorphic structure, which take over the function of Photosynthesis due to the reduction of the leaf blade. Such modifications are characteristic of plants growing in arid climates, for example, in many Australian acacias.

Spines are widespread in dicotyledonous woody plants, and some herbaceous plants also have them. They are of leaf (cactus, barberry) and stem (hawthorn, wild apple, pear, blackthorn) origin. Spines instead of leaves usually appear in plants of arid habitats. This is primarily associated with adaptation to the economical use of water by reducing Transpiration. A typical example of such a modification is the spines of a cactus. They also perform a protective function, protecting the juicy stems of the cactus from being eaten by animals. Spines also appear in plants growing under conditions of sufficient moisture, having an exclusively protective significance (barberry).

The shoot nature of spines is indicated by their ability to branch (gleditsia), and to produce leaves, flowers, and fruits (blackthorn); they have very well-developed wood and mechanical tissues, due to which they acquire strength.

Aboveground stolons are short-lived creeping shoots formed mainly for spreading and vegetative reproduction. They have long internodes and green leaves. Adventitious roots are formed at the nodes on the lower side, and a shortened shoot (rosette) is formed from the apical bud, which after the death of the stolon continues an independent existence. Aboveground stolons that have lost the function of photosynthesis and perform mainly the function of vegetative reproduction are sometimes called runners (strawberries).

Tendrils are modifications of shoots or leaves that, in the process of prolonged apical growth, are capable of twisting around a support. Tendrils of shoot origin are characteristic of various species of grapes, passionflower, and other plants. In many legumes, the upper part of the compound leaf (the common axis, or rachis, and several leaflets) transforms into tendrils, as for example in vetch. In vetchling (Lathyrus), the entire leaf blade is modified into a branched tendril, while large green stipules take over the photosynthetic function. The leaf petiole (nasturtium, clematis, pitcher plant), as well as stipules (sarsaparilla) or the apex of a simple leaf (fritillary) can also transform into a tendril.

Aboveground tubers represent thickenings of the main stem (kohlrabi, succulent plants), or of lateral shoots in epiphytic tropical orchids. In some orchids, one or several internodes at the Base of the shoot thicken and form pseudobulbs—aboveground tubers containing nutrients. Small bulbils, which represent metamorphoses of leaves or flower buds and are formed in inflorescences (onions, garlic) and in leaf axils (lilies), are called bulbils.

Succulent shoots. Succulents are plants with juicy tissues specialized for water storage. Most of them grow in arid regions or simply under conditions of prolonged moisture deficiency. Stems, leaves, and buds can serve as water-storing organs.

Stem succulents are found among representatives of American cacti, African spurges, and swallow-worts. In these plants, leaves are metamorphosed into spines or disappear altogether, while juicy stems store water and carry out photosynthesis. Their stems become columnar or spherical, and as a result of convergent evolution under similar conditions, they look completely identical externally, although the plants belong to different families, i.e., they are not related.

In stem succulents, entire shoot systems undergo metamorphosis rather than just their component parts, as can be seen in the plants discussed above. Similar modified shoot systems are found in members of the goosefoot family (Chenopodiaceae). The individual succulent segments of saltwort stems represent shoot metameres.

Leaf succulents are characteristic of the stonecrop family (Crassulaceae), whose members possess fleshy, water-storing leaves. These include houseleeks, stonecrops, roseroot, and other plants native to sandy and mountainous regions. Several desert plants commonly cultivated indoors also belong to leaf succulents: agave, aloe, gasteria, and others. Another notable example is Kalanchoe daigremontiana, native to Madagascar and grown indoors under the common names "bryophyllum" or "Goethe plant".

The HEAD (or cabbage head) is a metamorphosed, enlarged bud of the cabbage plant that accumulates large amounts of moisture and reserve nutrients, primarily sugars, during its first year of life. This modified shoot evolved as an adaptive feature in the Mediterranean climate, where the water reserves stored in the head enable the plant to survive hot, dry summers.

Lignified stems (tree trunks and branches). After losing their assimilatory function, a portion of the tree shoots persists as the main skeletal axes, performing mechanical, conducting, and storage functions for decades. The entire shoot undergoes modification: the axis thickens due to cambial activity, forming lignified secondary wood, periderm, and bark. The buds transform into dormant buds.

Above-ground stolons and runners. Above-ground stolons are short-lived, creeping shoots that carry out photosynthesis and serve for territory expansion and vegetative reproduction. They can be observed in many plants, such as silverweed, stone bramble, yellow archangel, creeping bentgrass, and others. Above-ground stolons are also commonly referred to as runners. They are covered with small green leaves. The shoot of bugleweed lives for 3–4 years and terminates in flowering. Meanwhile, the runner itself claims new territory, photosynthesizes, and, by forming new rosettes, serves dispersal and vegetative reproduction. Prior to flowering, it breaks down and dies after fulfilling its role.

In strawberries, above-ground stolons are called runners. These stolons, unlike those in the other plants mentioned above, do not store nutrients. They are even more specialized for vegetative reproduction—thin and weak, with very long internodes and lacking green leaves, bearing only scale-like leaves from whose axils secondary stolons (runners with daughter rosettes) can grow. Almost immediately after the apical bud roots, the runners dry out and disintegrate, and the rosettes become independent either in the same year or the following spring. The rosette shoots are perennial; in strawberries, they flower and bear fruit for many consecutive years. Under cultivation, yields decline in older rosettes, which is why plantings are regularly renewed, thinned, and restocked with young rosettes.

Trapping devices are modified leaves of carnivorous plants adapted to catching small insects, which provides them with supplementary nitrogen nutrition (e.g., sundew, pitcher plant). They take the form of pitchers, urns, bladders, or rolling blades. Small insects that fall into them perish, are digested by Enzymes, and are absorbed by the plant.

In some plants, complete leaf reduction occurs in connection with a transition to a parasitic lifestyle (e.g., dodder, toothwort).