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Test 4- Outline+3a

Test 4- Outline+3a - NOTES FOR BIOLOGY 1002 Outline 3A As...

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NOTES FOR BIOLOGY 1002 Outline 3A 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 of a plant have two main parts the blade and the petiole (the leaf stem) (see fig 24-9) The structure of the leaf is designed in layers (see fig. 24-8). The two outer layers (top and bottom) are epidermis and in the epidermal layer on the underside of the leaf there an opening called stomata . Between these epidermal layers is the mesophyll (middle). The cells here are parenchyma cells and they are arranged in a row of column shaped palisade cells toward the topside of the leaf. The bottom side of the mesophyll is 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. The ground tissue that is inside the ring is known as pith whereas the ground tissue outside the ring is the cortex (see fig. 24-9) The xylem and phloem in the ring of vascular bundles is divided so the xylem is toward the inside of the stem and the phloem is toward the outside of the stem. Between the xylem and phloem is the vascular cambium (this is where the lateral meristem is located) (see fig 24-11)
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During secondary growth the cells of the vascular cambium divide and add to both the xylem and the phloem. These additions are called secondary xylem and secondary phloem respectively. The amount of secondary growth varies during the yearly cycle, with growth occurring faster in the spring and early summer then slowing in late summer and fall. This variation in growth rate leads to the production of annual rings . (See fig 24-12) As a seed begins to grow the first root to emerge is the primary root . In monocots this primary root is replaced by a fibrous root system composed of many roots of about equal size. In dicots the primary roots turn into the taproot system , which has one main root that all the 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 (see fig. 24-15). This root cap acts like a lubricating layer as the root pushes its way down through the soil. The cells of the root cap secrete a slippery slime layer and regularly slough off to make penetration easier.
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