# 9.6 Growth and Development in Plants **Development** is a programmed series of changes by which an organism is converted from a simpler form to a more complex one. **Growth** is an irreversible increase in the size and mass of an organism, which is a part of the development process. The four phases of plant growth and development are: 1. **Cell division:** Production of new cells. 2. **Cell elongation:** Enlargement of cell size. 3. **Cell maturation:** Cells reach their mature size. 4. **Differentiation:** Cells become structurally and functionally specialized to perform particular functions. **Open growth** is a unique pattern in plants where they continuously add new organs (e.g., branches, leaves, roots) throughout their life. --- ## Meristematic Tissues (Meristems) **Definition:** Specialized tissues found in specific growing regions (growing points) of higher plants, where cells remain forever young and divide actively throughout the plant's life. **Meristematic Cell Characteristics:** Generally small and cuboidal with large nuclei, small vacuoles, and thin walls. **Cell Division Process:** When a meristematic cell divides mitotically: * **Initial:** The new cell that remains within the meristem. * **Derivative:** The other new cell that is pushed away from the active division zone, subsequently stretching, enlarging, and differentiating into other tissue types. ### Major Kinds of Meristems #### Apical Meristems * **Location:** Located at the tips of roots and shoots, at opposite ends of the plant axis. * **Function:** Cell divisions and subsequent cellular enlargement in these areas are responsible for the lengthening of the above-ground and below-ground parts of the plant. * **Primary Meristems:** Termed as such because they are present in plants right from embryonic life. #### Lateral Meristems * **Location:** Cylinders of dividing cells located on the lateral sides in stems and roots of dicots and gymnosperms. * **Function:** Primarily responsible for increasing the plant's thickness and diameter. * **Secondary Meristems:** Termed as such because they are derived from apical meristems after embryonic life. **Types of Lateral Meristems:** * **Vascular Cambium:** Extends throughout the length of the plant; produces new tissues laterally, increasing plant diameter. * **Cork Cambium (Phellogen):** Found in the bark of roots and stems of woody plants, contributing to the width of the plant body. * **Intercalary Meristems:** Originate from apical meristems but are separated from them and located along the stems near the nodes. Growth in these tissues gives rise to side branches, leaves, and lateral buds that grow up to a certain size (*determinate growth*). **Tissues Produced:** Any tissues produced by the cell divisions of a lateral meristem are called *secondary tissues*. --- ## Types of Plant Growth Based on their origin, plant growth is categorized into two types: ### Primary Growth * **Definition:** The increase in length of the root and shoot. * **Initiation:** Begins when a plant starts its life after seed germination. * **Duration:** Continues throughout the life of a plant (*indeterminate growth*), though the rate may vary. * **Mechanism:** Carried out by cell division in the **apical meristems**. ### Secondary Growth * **Definition:** The increase in thickness and diameter of a plant. * **Characteristic:** Predominant in stems and roots of dicots and gymnosperms. * **Significance:** Contributes significantly to the structural support of the plant body. * **Tissues Terminology:** * **Secondary Tissues:** All tissues produced during secondary growth. * **Primary Tissues:** Tissues already present in the plant before the onset of secondary growth. --- ## Role of Lateral Meristems in Secondary Growth The two main lateral meristems involved in secondary growth are the cork cambium and vascular cambium. ### Role of Vascular Cambium * **Location:** Present in vascular bundles, situated between the primary xylem and primary phloem. * **Activity:** Its rate of cell division is faster than that of cork cambium, contributing more tissues to secondary growth. * **Products:** Adds two new tissues to the plant body: * **Secondary Xylem:** Produced towards its inner side (at a faster rate). * **Secondary Phloem:** Produced towards its outer side. #### Annual Rings (Growth Rings) * **Differential Growth:** The rate of secondary xylem production varies throughout the year due to environmental factors: * **Autumn and Winter:** Smaller sized cells are produced due to water deficiency. * **Spring and Summer:** Larger sized cells are produced, likely due to greater water availability. * **Formation:** This differential growth pattern forms distinct rings of smaller and larger cells, each representing one full year of growth. * **Age Determination:** The age of a plant can be determined by counting the number of these growth rings. ### Role of Cork Cambium (Phellogen) * **Origin:** Generally originates from the outermost layer of the primary cortex. * **Products:** Adds two new tissues to the plant body during secondary growth: * **Secondary Cortex:** Produced towards its inner side. * **Cork:** Produced towards its outer side, beneath the epidermis. * **Effect:** Cork cells eventually die, leading to the death of the epidermis. This process results in the rough appearance of the outer surface of mature trees. --- ## Wood and Bark ### Wood * **Definition:** The bulk of tissues added laterally (inner to the vascular cambium) during secondary growth, primarily consisting of secondary xylem. * **Heartwood:** The inner region of wood (comprising some secondary xylem, primary xylem, and the central pith). In later life, it becomes blocked by the deposition of wastes, giving it a dark appearance and making it non-functional for water transport. * **Sapwood:** The outer region of wood, consisting only of functional secondary xylem. It gives a lighter appearance and is responsible for water transport. ### Bark * **Definition:** The entire outer region of the stem, extending from the vascular cambium outwards to the epidermis. * **Inner Bark:** The portion of bark located inner to the cork cambium. It includes secondary phloem, primary phloem, pericycle, endodermis, primary cortex, and secondary cortex. * **Outer Bark:** The portion of bark located outer to the cork cambium. It primarily consists of cork and the dead epidermis. --- ## Growth Correlations * **Definition:** Interactions among different growing points within a plant where the growth rate of one part influences another. * **Types:** * **Positive Growth Correlation:** If the growth of one part promotes the growth of another part. * **Negative Growth Correlation:** If the growth of one part inhibits the growth of another part. ### Apical Dominance * **Definition:** A type of negative growth correlation where the presence of a growing apical bud inhibits the growth of lateral (axillary) buds. This phenomenon also occurs in roots, where the main root inhibits lateral root growth. * **Variation:** * **Strong Apical Dominance:** Plants exhibit tall height with short side branches. * **Weak Apical Dominance:** Plants show short height and dense growth of lateral branches. * **Hormonal Control:** * **Auxin:** A plant growth regulator released from the apical bud, which *inhibits* the growth of lateral buds. * **Cytokinin:** Another plant growth regulator produced by lateral buds, which *antagonizes* the auxins, thereby neutralizing the inhibitory influence of the apical bud on lateral buds. * **Balance:** * Plants with strong apical dominance produce more auxins from the apical bud and less cytokinin from lateral buds. * Plants with weak apical dominance produce less auxins from the apical bud and more cytokinin from lateral buds. --- ## Possible Questions and Answers **Q:** What is the primary function of apical meristems? **A:** To increase the length of the plant's roots and shoots, contributing to primary growth. **Q:** How do lateral meristems contribute to plant growth? **A:** They increase the thickness and diameter of stems and roots, primarily through secondary growth. **Q:** How can the age of a woody plant be determined? **A:** By counting the number of annual rings (growth rings) formed by the vascular cambium in its stem, as each ring represents one year's growth. **Q:** Explain the difference between heartwood and sapwood. **A:** *Heartwood* is the older, inner, non-functional wood, darkened by waste deposits. *Sapwood* is the younger, outer, functional wood responsible for water transport. **Q:** What is apical dominance and which plant hormones are involved? **A:** Apical dominance is a negative growth correlation where the apical bud inhibits lateral bud growth. It is primarily regulated by *auxins* (from apical bud, inhibiting) and *cytokinins* (from lateral buds, counteracting auxins).