BOTANY WITH THE BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

VIII. BASICS OF PLANT MORPHOLOGY AND ANATOMY

Vegetative Plant Organs

As a result of evolution, the plant body has gradually grown more complex, leading to The Development of vegetative Organs. An organ is defined as a part of an Organism possessing a distinct Structure and origin, specialized for performing specific Functions. Their emergence and gradual refinement are closely tied to the transition of plants to land and their adaptation to terrestrial environments. Organs are divided into vegetative and generative. Vegetative organs include the ROOT, stem, and leaf. The stem and leaf are structurally and functionally interconnected, and are thus considered as a single complex organ known as the SHOOT. Vegetative organs ensure the vital activity of the plant organism.

Lower plants lack vegetative organs. Their body is represented by a thallus or frond. Primitive organisms inhabited an aquatic environment, absorbing Water and mineral nutrients through their entire body surface without any differentiation into vegetative organs, forming a homogenous thallus. With the migration of plants onto land, two distinct feeding poles gradually formed—the subterranean and the aerial—which drove the Differentiation of the vegetative body. The root was the first to appear, taking on the functions of anchoring the plant in the soil and supplying water and mineral nutrients. Aerial Nutrition (Photosynthesis) was initially carried out by the green stem, with leaves appearing later. The Emergence of leaves and stem branching multiplied the productive photosynthetic surface, which was of paramount importance for subsequent evolution. True vegetative organs make their first appearance in higher plants.

Plants exhibit polarity, which is the distinct difference between the morphologically upper and morphologically lower parts of a Cell, organ, or plant.

The refinement of vegetative organs is closely linked to The phenomenon of Symmetry. This refers to the spatial arrangement of a body where a line or plane divides it into two or more similar parts. If only a single axis of symmetry can be passed through a plant body, dividing it into two identical halves (right and left), this type of symmetry is termed bilateral, and the organ is called zygomorphic. The leaves of many plants are zygomorphic if the central vein divides them into two identical halves. If no axis of symmetry can be passed through a plant organ, it is described as asymmetric. Asymmetric structure is found in the leaves of elm trees and the flowers of cannas. When two or more axes of symmetry can be passed through a plant body, the structure is termed polysymmetric, and the type of symmetry is radial. This is characteristic of the stems of many plants. Polysymmetric flowers are called actinomorphic.

Subsequently, vegetative organs underwent further evolution: their number multiplied significantly, stems began to branch, and numerous homologous organs were produced. Their cumulative growth resulted in an expanded feeding area. The phenomenon of forming repeating organs and segments along the longitudinal axis is known as metamerism. Metamerism can be regarded as a special type of symmetry. The shoot possesses a typical metameric structure.

Conversely, a reverse process of gradual reduction of organs that had lost their function was also observed. This process is called reduction.

When studying vegetative organs, The concepts of homologous and analogous organs are essential. Homologous organs share a common evolutionary origin (i.e., they develop from similar embryonic primordia). The functions of these organs may be identical or entirely different. Significant modifications of homologous organs in Morphology are referred to as metamorphoses.

In contrast, analogous organs always perform similar functions and have a comparable structure, yet they evolve independently from different ancestral structures.

Tropisms are Changes in the orientation of an organ in response to a directional environmental stimulus. Depending on The Nature of this stimulus, several groups of tropisms are distinguished. Phototropism is related to the action of light, chemical stimuli induce chemotropism, and the force of gravity causes geotropism. It is believed that under METABOLISM/18.html">The Influence of these factors, an electrophysiological and subsequently hormonal reaction occurs within plant Tissues, determining A change in the direction of organ growth. A distinction is made between positive tropism, where the organ grows toward the stimulus, and negative tropism, where it grows in the opposite direction. For instance, a shoot exhibits positive phototropism, always growing toward the light, and negative geotropism, growing upward against the force of gravity.

Plants growing in conditions of abundant moisture display specific traits driven by these unique ecological habitats. Aquatic plants are characterized by a considerable development of their body surface and a mucilaginous coating. The Root System has lost its function of absorbing water and nutrients. There is a weak development of mechanical and Vascular Tissues, accompanied by a significant expansion of aerenchyma—a primary parenchyma adapted for storing large volumes of air. These plants lack adaptations that reduce water loss; their submerged leaves and stems possess no cuticle or epidermal appendages such as hairs, bristles, or similar structures. Epidermal Cells frequently contain Chloroplasts and are capable of photosynthesis.