| MadSci Network: Botany |
Why questions are often difficult to answer especially in this case given that xylem and phloem function in very different ways. Thus, diameter of one is not necessarily related to the diameter of the other. One possible reason is that xylem vessels are dead at maturity so are somewhat like tiny pipes. The Poiseuille law, also called Hagen-Poiseuille law, for laminar flow in pipes states that the rate of flow is proportional to the fourth power of the pipe radius. That means that one large diameter pipe provides much more flow than several smaller diameter pipes with the same total cross sectional area as the larger pipe. Xylem vessels have thicker walls than phloem sieve tubes because they must be able to withstand substantial tensions. The xylem sap is usually under tension during the day according to the prevailing cohesion-tension theory. Phloem is not subject to such great stresses. The size of mineral nutrient ions would not have anything to do with it. They are all very small compared to sugars, phytohormones, and other organic molecules that move in phloem. References Hydraulic properties of individual xylem vessels of Fraxinus americana Calculating Velocities of Water Movement in the Xylem and in Living Cells Rice, Stanley A. and McArthur, John. 2004. Water Flow Through Xylem: An Investigation of a Fluid Dynamics Principle Applied to Plants. American Biology Teacher Volume: 66: 120-127
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