# Support in Plants - **Evolution of Support:** * When life transitioned from water to land, organisms, including plants, developed mechanisms to gain *support* and *strength* to maintain their body shapes against gravity. - **Mechanisms of Plant Support:** * Plants employ a variety of mechanisms to achieve structural support: * *Turgor mechanism* in parenchyma cells. * *Mechanical tissues* (e.g., collenchyma and sclerenchyma). * *Arrangement of vascular bundles*. * *Secondary growth* (mentioned but not detailed in the provided text). ### 9.5.1 Role of Turgor Mechanism in Support - **Parenchyma Cells:** * Possess large *central vacuoles* filled with water. * When these cells are *turgid* (full of water), the water exerts *hydrostatic pressure* on the surrounding cell walls. - **Turgor Pressure:** * This pressure on the walls keeps the cells *stiff* and *hard*. * It provides essential *support* and *strength*, especially in *herbaceous plants* which may lack specialized supporting tissues. * A decrease in turgidity (loss of internal hydrostatic pressure) leads to wilting or collapse of the plant. ### 9.5.2 Role of Mechanical Tissues in Support - **Definition:** The term "mechanical tissues" refers to tissues composed of *thick-walled cells*, specifically *collenchyma* and *sclerenchyma*. #### Collenchyma - **Characteristics:** * Distinguished by extra *cellulose deposition* at the corners of its cells. * A *living tissue*, allowing it to grow and stretch freely as the plant develops. - **Function:** Provides mechanical support, particularly in: * Young plants. * Herbs. * Leaves. * Regions where *secondary growth does not occur*. - **Location:** Plays a significant role in support in *stems* and *petioles* due to its peripheral location near the *epidermis*. #### Sclerenchyma - **Characteristics:** * Solely dedicated to providing *support* and *mechanical strength* to plants. * Mature cells are *dead*. * Characterized by the formation of very thick *secondary lignified walls*. * *Lignin* is a macromolecule that provides *rigidity* and *tensile strength* to the wall. - **Types:** Sclerenchyma is divided into two main types: * ***Fibres:*** * Elongated cells. * Found in the *pericycle* of stems, forming solid rods of tissue. * Also present in *xylem* and *phloem* tissues. * ***Sclereids:*** * Roughly *spherical* or *variously shaped* cells. * Commonly found in *fruit walls* and *seed coats*. * Both types have heavily thickened, lignified walls with great tensile strength. ### 9.5.3 Arrangement of Vascular Bundles - **Dicot Plants:** * Vascular bundles are typically arranged in a *ring* in the stem. * This organized arrangement provides additional *support* to the plant body. - **Monocot Plants:** * In contrast, vascular bundles are *scattered* throughout the stem, offering less structural rigidity compared to the dicot arrangement.