Answer: pressure.
- A pressure
- B temperature
- C light level
- D gravity
Correct answer: A. pressure
Explanation: The pressure-flow mechanism moves sugar from high-pressure source to low-pressure sink.
Water can travel through cell walls (apoplast) or cell-to-cell through the cytoplasm (symplast) - but the Casparian strip at the endodermis blocks the apoplast route entirely, forcing everything through a selectively permeable membrane (symplast) before it can enter the xylem.
Concept context
Water and mineral absorption, the ascent of sap, transpiration, and phloem transport via the pressure flow hypothesis.