Circulation In Vascular Plants
Water and dissolved minerals enter a plant's roots from the soil by means of diffusion and osmosis. These substances then travel upward in the plant in xylem vessels. The transpiration theory ascribes this ascending flow to a pull from above, caused by transpiration, the evaporation of water from leaves. The long water column stays intact due to the strong cohesion between water molecules. Carbohydrates, produced in leaves by photosynthesis, travel downward in plants in specialized tissue, phloem. This involves active transport of sugars into phloem cells and water pressure to force substances from cell to cell.
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