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CHAPTER 24 STEMS AND SHOOTS As the shoot of a plant grows at the apical meristem it gives rise to different specialized tissues (see fig 24-9): Stems Buds Leaves Flowers LEAVES Leaves of a plant have two main parts: (see fig 24-9) blade petiole (the leaf stem) The structure of the leaf is designed in layers (see fig. 24-8). Top and bottom layers are epidermis Underside: stomata – adjustable opening, surrounded by a pair of guard cells, regulates the diffusion of carbon dioxide and water into and out of the leaf. Between these epidermal layers is the mesophyll (middle) Topside:loosely packed parenchyma cells - arranged in a row of column shaped palisade cells (contains chloroplast) . Bottom side: spongy cells with many air spaces. All of shoot structures come from small groups of cells which are left behind by the apical meristem . These groups of cells form leaf primordia (leaf producing group) and lateral buds (branch producing group). The cell groups are clustered at sites called nodes and are usually distributed at regular intervals along the stem. These intervals are known as internodes (the space between two nodes on the same side of the stem) The formation of branches from lateral buds is triggered by hormones (or the lack of certain hormones). The inside of the dicot stem contains the ring of vascular tissue and ground tissue. (see fig. 24-9) Pith - ground tissue that is inside the ring Cortex - ground tissue outside the ring is the (see fig. 24-9) The xylem and phloem in the ring is divided so that: xylem is toward the inside of the stem vascular cambium - between the xylem and phloem (lateral meristem is located here) (see fig 24-11) During
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secondary growth the cells of the VC divide and add to xylem and phloem. ( secondary xylem and secondary phloem). Amount of secondary growth varies during the yearly cycle: spring and early summer – faster growth late summer and fall - slower growth. This variation in growth rate leads to the production of annual rings . (See fig 24-12) phloem is toward the outside of the stem. THE ROOT SYSTEM primary root - first root to emerge as a seed begins to grow. in monocots–pr replaced by a fibrous root system –many roots equal in size. in dicots - pr turn into taproot system -one main root, other roots branch off. Roots grow by primary growth and the apical meristem cells are located underneath a layer of cell called the root cap - acts like a lubricating layer as the root pushes its way down through the soil. Cells of root cap secrete slippery slime layer and regularly slough off to make penetration easier. (see fig. 24-15). Epidermis of root – root is very permeable to water. - very thin and has no waxy cuticle. - forms root hairs to increase the surface area. The ground tissue system in the root forms two structures: - Cortex - mainly parenchyma cells – food storage (as starch, like potatoes).
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