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

MAIN ORGANS OF VASCULAR PLANTS

The division of the multicellular body of higher plants into vegetative and Generative Organs is a characteristic feature.

An organ is a part of a plant Organism that has a specific Structure and origin, and is adapted to perform one or more Functions.

Vegetative organs are organs that functionally support the individual life of a plant, ensure its Nutrition and growth, mediate its interaction with the external environment, and also participate in vegetative propagation (the ROOT, stem, leaf, and their modifications).

Generative organs are organs associated with the Sexual reproduction of plants. They develop on vegetative organs at a specific phase of their development and ensure the propagation of individuals and the reproduction of the species. In angiosperms, the generative organs are the flower, fruit, and seed. Together with vegetative propagation organs, they belong to reproductive organs.

Reproductive organs are plant organs that perform the function of reproduction. In addition to the aforementioned organs, these include tubers, bulbs, rhizomes, root tubers, corm bulbs, stem tubers, etc.

In higher vascular plants, the axial organs are the stem and the root, which together maintain the unified axial System of the plant organism. The root collar serves as the transition point between the stem and the root, thereby dividing the axial system into an above-ground ascending part and an underground descending part.

Organogenesis in plants—namely, The formation of organs from meristematic regions—is studied by phytoembryology, whereas the morphological description of organisms is the domain of organography.

The process of organ formation has given rise to certain regularities. One of them is polarity, which is the morphophysiological Differentiation of the opposite ends of a Cell, tissue, organ, or plant as a whole. For instance, shoots invariably develop from the morphologically upper part of a cutting, while roots form from the lower part. The morphologically lower part exhibits positive geotropism, whereas the upper part exhibits positive heliotropism. The morphologically upper part of a plant is referred to as the apical region, and the lower part as the basal region.

Physiological differences between the poles of a plant manifest as Tropisms. A tropism is a growth response that causes a plant organ or part to bend toward an external stimulus that determines the direction of movement. Accordingly, tropisms can be positive or negative. In positive tropisms, the movement is directed toward the stimulus, whereas in negative tropism, it is directed away from it. Depending on The Nature of the stimulus, distinctions are made among geo-, photo-, thigmo-, hydro-, aero-, tropho-, helio-, and traumatropisms, among others.

Geotropism or gravitropism is a response to gravity that is clearly manifested in seedlings (the growth of the main root and main stem).

Phototropism refers to growth movements and bending of plant organs under METABOLISM/18.html">The Influence of unilateral illumination. The plant perceives not the direction of the light beam, but rather the difference in illumination (the light gradient) between the shaded and illuminated sides (such as The change in the orientation of a sunflower inflorescence depending on THE POSITION OF the sun).

Thigmotropism is a response to contact with a solid object (for example, the reaction of leaf or stem modifications such as tendrils, which coil around any object to use it as support).

Hydrotropism is the movement of a plant or its organ toward a more humid environment (characteristic of root systems).

Aerotropism is spatial orientation caused by an uneven distribution of oxygen (characteristic of root systems).

Trophotropism is the bending of a plant or its organs caused by the influence of nutrients.

Heliotropism is the daily orientation of a plant or its organs perpendicular or parallel to sun rays (for example, in sunflowers, lupines, and soybeans).

Traumatropism is the bending of a plant or its organ under the influence of injury.

Organs with a bilaterally symmetrical structure are capable of Nastic Movements. Nastic movements (nastiies) are growth movements of plant organs and parts triggered by various diffuse stimuli (such as changes in light intensity, Temperature, etc.). If rapid growth occurs on the morphologically upper side of an organ, this phenomenon is termed epinasty, resulting in The Development of buds and flower heads. Conversely, when rapid growth is observed on the lower side, the flower envelopes close—a phenomenon known as hyponanasty. In some plants, flowers open in the morning and close at night. A similar property is characteristic of the leaves of certain plants. These movements are known as the "Sleep of plants" or nyctinasty. The " sleep movements" of flowers and leaves, manifested as opening or closing in response to changes in light (photonasties; for example, dandelion flowers close at night and open in the morning) or temperature (thermonasties; for example, flowers of tulips and crocuses open when transferred from the cold to 20–25 °C), are classified as nastic because the external stimuli merely trigger them, whereas the direction of movement depends on internal factors.

Another regularity in the Formation of Plant organs is Symmetry, which occurs when one half of a given plant organ is a mirror image of the other. Symmetry is characteristic of both vegetative and generative organs. Distinctions are made regarding radial or multilateral symmetry—where three or more planes of intersection can be passed (stem, actinomorphic flower, root)—and bilateral symmetry, where the plant body or its organ can be conditionally divided into two identical halves (leaf).

When only a single plane of symmetry can be passed through a plant or its part, this type of structural Organization is termed monosymmetric. Such a type of symmetry is observed in doriventral structures. Dorsiventral organs are organs that possess distinct dorsal (back) and ventral (belly) sides.

No axis of symmetry can be passed through asymmetrical organs and parts (such as begonia leaves, canna flowers, etc.).

The third characteristic regularity of vegetative organs is metamerism. Longitudinal symmetry, or metamerism, is the repetition along the axis of an organ of a series of elements that constitute that organ (internode, leaf, axillary SHOOT bud, and node).

The subsequent characteristic regularities of vegetative organs are polymerization and oligomerization. Polymerization refers to the increase, and oligomerization to the decrease, in the number of identical organs or their parts during the course of evolution.

In plant Morphology, organs are classified as orthotropic or plagiotropic. Orthotropic organs are those that grow vertically relative to the substrate surface (e.g., primary roots, tree trunks, etc.). Such organs most commonly exhibit radial symmetry.

Plagiotropous organs (plagiotropic organs) are those oriented more or less horizontally relative to the ground surface (e.g., stems of knotweed and strawberries, lateral roots, rhizomes, etc.). They typically display monosymmetrical structure.

Lateral organs are those positioned to the side of the main axis (e.g., lateral branches, lateral roots, etc.). Lateral organs that originate On the surface of the plant body from the outer Tissues of an axial organ are called exogenous (lateral shoots), whereas those developing with the participation of internal tissues of axial organs are termed endogenous (lateral roots).

Morphologically, the upper end of an organ or its part is considered distal. Conversely, the morphologically lower end of an organ or its part is considered proximal (for instance, adventitious roots form at the proximal end of a cutting). The basal part is located near the base, i.e., at the morphologically lower end, while the proximal part lies closer to the center of the body or its median plane.

The position of an organ or its part relative to the shoot axis can be adaxial or abaxial. The adaxial side of an organ is that facing the shoot axis, whereas the abaxial side faces away from it.

The development of plant organs or their parts can proceed in various ways. Acropetal development refers to the development of parts of any organ from the base toward the apex, where younger elements are located closer to the apex and older ones closer to the base. For example, flowers develop in this manner in botryose inflorescences (spike, corymb, etc.), as do branches and leaves on the stems of most plants. Basipetal development involves the initiation of plant organ parts or lateral branches from the apex toward the base, with younger parts consistently positioned closer to the base. For instance, flowers in cymose inflorescences and stamens in plants of certain families develop in this way. This type of development is also observed in certain Algae and during the differentiation of leaf tissues in seed plants.

Notably, homologous and analogous organs are among the fundamental concepts in evolutionary morphology.

Homologous organs are plant organs that share a common evolutionary origin, although they may differ in form and function. Such organs develop from homologous embryonic rudiments. Examples include the potato tuber, tulip bulb, hawthorn thorn, Solomon's seal rhizome, and onion scape. The term «Homology» was introduced by the English botanist R. Owen (1843). Proving organ homology across different species requires three criteria: similarity in the morphological structural plan, their topographical position relative to other organs, and their morphogenesis.

Analogous organs are plant organs that exhibit similar external features and perform identical functions, yet differ in origin. For example, barberry spines are modified leaves, whereas hawthorn spines are modified shoots. Thus, while they have different origins, they perform a similar protective function. The term «analogy» was introduced by Aristotle and was first used in this specific context by R. Owen (1843).

Analogous organs arise through convergent evolution, whereby organisms belonging to different taxonomic groups acquire similar Structural and functional traits As a result of adapting to comparable environmental conditions. THE CONCEPT OF convergence is frequently applied to individual organs or structures.

Convergence refers to the similarity of traits in organisms of different origins resulting from their adaptation to identical environmental conditions. Examples include the morphological resemblance between cacti and certain euphorbias, specifically their vegetative organs (stems with photosynthetic tissue, reduced spine-like leaves). The term «convergence» was also coined by C. Darwin (1859).

Divergence is the evolutionary diversification of traits among organisms of the same taxonomic group, driven by adaptation to varying living conditions. The term «divergence of character» was introduced by the English scientist C. Darwin (1859), who considered intraspecific competition and natural Selection to be its primary drivers.

The phenomenon of mimicry is frequently observed in plants.

Mimicry is the resemblance of organs to environmental objects. In plants, mimicry serves to attract or deter animals and typically involves only specific organs rather than the organism as a whole. For instance, in certain orchids, flowers mimic the females of insect species whose males act as specialized pollinators; similarly, the trapping apparatus of carnivorous plants often resembles the bright flowers of other species, thereby luring insects to their demise within these «traps».

The uneven evolutionary advancement of various organs is termed heterobathmy. This phenomenon results in the coexistence of both primitive and progressive traits within a single species. For example, this phenomenon is sharply pronounced in the evolution of roots, stems, and leaves on the one hand, and flowers, fruits, and seeds on the other.

The occurrence of heterobathmy is explained by the fact that the organs and parts of any organism are functionally linked to varying degrees. For instance, a clear interdependence exists between the characteristics of leaves and the stem from which they arise, since the adaptive evolution of leaves inevitably induces modifications in the stem, causing the shoot to develop as an integrated whole. However, it should be noted that the adaptive evolution of flowers, fruits, and seeds can proceed independently of vegetative shoot evolution. The developmental rates of these organs can vary, allowing one organ to retain archaic features while another undergoes significant refinement.

The term «heterobathmy» was proposed in 1954 by A.L. Takhtajan.

Atavism refers to the reappearance in individual organisms of traits characteristic of distant plant ancestors. An example is the development of bipinnate leaves instead of phyllodes in certain Australian acacias. The term «atavism» was first introduced by the Dutch botanist H. de Vries (1901).

Neoteny is premature sexual maturity (a retardation of ontogeny) wherein reproductive organs appear during the Juvenile Stage of plant development. Examples include the precocious appearance of flowers and formation of fruits (acorns) on one-year-old oak seedlings.

In A number of plants, processes of structural simplification occur in parallel with the complexification and differentiation of other structures.

Reduction is the underdevelopment of an organ or its part resulting from the loss of its primary functions. Underdeveloped organs are termed vestigial or rudimentary. Examples include the leaf reduction in saxaul, root reduction in ghost orchid (Neottia nidus-avis), and eyes (buds) on potato tubers, etc.

The underlying cause of structural reduction is the loss of the functions these structures previously performed. Through reduction, simplification, and underdevelopment, an organ may transform into a vestige that has lost its functional significance. For instance, in the flowers of certain plants, vestigial stamens appear as small, sterile staminodes.

When organs are entirely lost, they are said to be aborted. Abortion refers to the complete or partial loss of an organ's functions as a result of reduction. For example, in flowers of the Scrophulariaceae family, only two stamens develop instead of the typical five.

The simplification of entire Tissue and organ systems is often associated with morphological regression resulting from a transition to different living conditions or parasitism. For example, parasites undergo reduction of roots and leaves, while submerged aquatic plants exhibit a simplified internal structure.