Stems function in the transportation of nutrients and water, the storage of nutrients, and the support of leaves. Sugars, some plant hormones, and other organic compounds are transported in the phloem. The movement of sugars occurs from where the sugars are made or have been stored, called a source. The place where they will be stored or used is called a sink. Botanists use the term translocation to refer to the movement of sugars through the plant. For example, most of the time, sugars move from the leaves to the roots, to the shoot apical meristems, and to the developing flowers or fruits. Sugars may be newly made in photosynthetic cells or may have been stored as starch in roots or other stems.
Movement of sugars in the phloem is explained by the pressure-flow hypothesis, which states that sugars are actively transported into sieve tubes. Look at Figure 29-12. As sugars enter the sieve tubes, water is also transported in by osmosis. Thus, a positive pressure builds up at the source end of the sieve tube. This is the pressure part of the pressure-flow hypothesis.
At the sink end of the sieve tube, this process is reversed. Sugars are actively transported out, water leaves the sieve tube by osmosis, and pressure is reduced at the sink. The difference in pressure causes water to flow from source to sink. This water is also carrying dissolved substances with it. Transport in the phloem can occur in different directions at different times.
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