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Plant structure Leaf structure The leaf has three main sections to it. 1. Cuticle. 2. Epidermis. 3. Mesophyll. 1. Cuticle 2. Epidermis 3. Palisade mesophyll 4. Spongy mesophyll 5. Epidermis 6. Cuticle The cuticle: Consists of a waxy material which improves impermeability. It is produced by the epidermis cells. Epidermis: Consists of a thin layer of transparent cells that allow for as much light transfer as possible towards the palisade cells. It is also a protection. Mesophyll: Consists of two layers of cells with different characteristics and functions. The palisade cells (top layer): Consists of one or several layers of elongated, narrow cells with their long axes (perpendicular to the axis of the leaf) and situated below the epidermis. Spongy cells (bottom layer): Consists of irregularly shaped cells which are located under the palisade cells. They contain less chloroplasts. There may be many intercellular air spaces between the spongy cells which are interconnected and communicate with the stomata of the lower epidermis. It allows food producing cells from the leaf to access the gases needed for photosynthesis and respiration. Guard cells: These are cells surrounding each stoma. They help to regulate the rate of respiration by opening and closing the stomata. When the presence of potassium increases in the guard cells, water is sent to them to dilute the potassium. In doing so, the guard cells get “Fat” (turgid) and allow the stomata to open. As soon as the guard cells lose water, the cells become flaccid and the stomatal opening is then closed. This usually occurs when light levels drop and the use of CO2 in photosynthesis decreases. Adaptation of the leaves 1. Their elongated shape allows for a wider surface area of sunlight absorption. 2. They are thin, so gas exchange can happen quickly because there is light space to cover. 3. The large spaces between cells inside the leaf provide an easy passage of diffusion. 4. Many stomata in the lower surface of the leaf allows the exchange of CO2 and O2. 5. Higher concentration of chloroplast on the upper layer to insure photosynthesis. 6. Branching network of veins allows for a good water supply Plant nutrition: - Transport system - Transpiration - Photosynthesis Transport system in plants The transport of nutrients (water, minerals and other molecules) happens by means of two specialised cell tissues, Xylem and Phloem. Vital substances are moved from the source (wherever it is taken from or is produced) to sinks (where they are used or stored). The xylem and phloem are both grouped into vascular bundles. These bundles can be found in several places in leaves and stem. Xylem Is a tissue that consists of tubes and transport cells (vessels) that circulates water and dissolved minerals. These vessels are elongated tubes, joined end to end, with no content when mature. Their cell walls are thicker and contain lignin. Lignin helps improve the impermeability and strength of the tubes. In essence these cells are dead (the lignin does not allow for water diffusion into the cell). Phloem Consists of sieve tubes which form vertical columns. The ends of each cell is perforated to allow for products of photosynthesis to pass through. The walls are NOT lignified It transports food substances such as sugars away from the leaf to other parts of the plant. Vascular bundles Water travels up the stem in the xylem from the roots to the leaves. However, food travels in any direction, up or down the stem to where it is needed. In the leaves, they are arranged in a cylinder a little way in from the epidermis. Helps against wind movement In the roots, the vascular bundles are in the centre to resist against wind. Cortex and pith. The tissue between the vascular bundles and the epidermis is called the cortex. The central tissue in the Stem is called the pith. Xylem Phloem Cortex Endodermis Cross section of the root Roots The root has no distinct epidermis. The root tip (root cap) has cells that are continuously being replaced because they get damaged as they push through the soil. The root also has root hairs which increase the surface area for absorption. These are also renewed continuously. There are two types of roots. Tap root Fibrous root A single root grows vertically down into the soil. Several roots grow at the same time. There is not distinguishable root.