Chapter 1 22 rise to phellem (cork) externally and phelloderm (secondary cortex) internally. The initiation of fibers and production of xylem indicates the start of secondary growth in hypocotyls (Taiz et al., 2010; Shana Kerr, 2017; Ragni and Greb, 2018). Vascular cambium, a closed ring of lateral meristematic tissue, is present in both roots and shoots, albeit with morphological differences. For instance, interfascicular cambium exists in shoots but is absent in roots, defined as cambium fusing to create a continuous ring in plant stems through a change in cell identity (Ragni and Greb, 2018). In plant development, primary growth, occurring in apical meristems, propels the vertical elongation of shoots and roots, contributing to an increase in height and the development of primary tissues like the epidermis, cortex, and vascular bundles. On the other hand, secondary growth, occurring in lateral meristems such as the vascular cambium and cork cambium, leads to the thickening of stems and roots. This process adds layers of secondary xylem and phloem, contributing to wood formation in dicots and enhancing mechanical support, water transport, and nutrient distribution. While primary growth facilitates upward and downward extension, secondary growth fortifies structural strength and adaptability, collectively shaping the overall form and function of plants. 1.3.3 Tomato stem structure identification in this thesis Tomato stems at four weeks of age are multi-layered structures that serve as a scaffold for plant growth and support (Figure 1.7). The stem consists of outer epidermis (light green layer), hypodermis (purple layer), 1-3 layers of collenchyma cells (blue-green layer), 1-3 layers of parenchyma cells (light orange layers), pith (yellow layer) and a vascular bundle and interfascicular cambium composed of tightly connected tiny cells (red layer). The outer epidermis layer provides a protective barrier against external stresses, while the hypodermis layer provides structural support to the stem. The collenchyma cells offer flexible support for the stem to grow and move, while the parenchyma cells are involved in photosynthesis, storage of water and nutrients, and transport of metabolites. The pith serves as storage tissue for water and nutrients, and the vascular bundle and interfascicular cambium provide the stem with transport tissue for water and nutrients. The tomato stem structure plays a crucial role in the overall growth and development of the plant (Esau, 1953; Evert and Esau, 2006).
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