The Volume of Phloem Tissue in a Stem Is Typically

question 96

Multiple Choice

The volume of phloem tissue in a stem is typically much smaller than the volume of xylem tissue because:

• A)the pressure gradient driving water flow in the phloem is metabolically generated and, therefore, can be much larger than the environmentally generated xylem gradient.
• B)phloem conduits are much wider, and because flow is proportional to conduit radius raised to the fourth power, fewer phloem conduits are needed.
• C)xylem dies quickly and, therefore, needs to be replaced frequently.
• D)plants lose hundreds of H₂O molecules for each CO₂ gained.

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