Movement of sugars in the plant
The product of photosynthesis in most plants is starch (some plants produce sugars only), which is stored temporarily in the chloroplast or moved in the phloem to be more permanently stored in the seed, the stem cortex or root, where specialized storage organs such as rhizomes and tubers may occur.
The movement or translocation of materials around the plant in the phloem and xylem is a complex operation. Phloem is principally responsible for the transport of the products of photosynthesis as soluble sugars, usually sucrose, which move under pressure to areas of need, such as roots, flowers or storage organs. Each phloem sieve-tube cell has a smaller companion cell that has a high metabolic rate. Energy is thus made available to the protoplasm at the end of each sieveplate, which is able to ‘pump’ dilute sugar solutions around the plant. The flow can be interrupted by the presence of disease organisms such as club root.
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