The correct path of water from soil to atmosphere is:

Question

BlackTom AI · Accepted Answer

Water enters root hairs and travels up the plant through continuous xylem vessels to the atmosphere via the substomatal cavity and stomata — so option a is correct.

1) Relevant knowledge points and concepts
- Uptake: Root hairs increase surface area and absorb soil water by osmosis. 
- Path across the root: Water moves through epidermis and cortex by apoplast or symplast pathways, passes the endodermis (Casparian strip forces symplastic entry) and is loaded into xylem vessels in the root.
- Long‑distance transport: Xylem (not phloem) conducts water upward through the stem via cohesion–tension (transpiration pull) and capillarity.
- Leaf exit: Water reaches leaf mesophyll, evaporates into the substomatal cavity, then diffuses out through open stomata into the atmosphere (transpiration).

2) Step‑by‑step reasoning
- Step 1: Soil water moves into root hairs because the root cell sap is usually more negative in water potential than the soil, so osmosis occurs into root hair cells.
- Step 2: Water travels across the root cortex (apoplast/symplast) and is forced through the endodermis into the pericycle, then into the xylem of the root.
- Step 3: Xylem vessels form a continuous column from root through stem to leaves; transpiration from leaves creates negative pressure that pulls this water column upward (cohesion of water molecules + adhesion to xylem walls).
- Step 4: In the leaves, water evaporates from mesophyll cell walls into the substomatal cavity (air space beneath stomata).
- Step 5: Water vapor diffuses from the substomatal cavity out through open stomata into the atmosphere.

3) Intermediate steps shown (root cortex → endodermis → root xylem → stem xylem → leaf mesophyll → substomatal cavity → stomata → atmosphere) — this sequence matches option a exactly.

4) Why the correct answer is correct
Option a lists the continuous anatomical and functional route (soil → root hairs → xylem of root → xylem of stem → leaves → sub‑stomatal cavity → atmosphere), matching physiological processes (absorption, xylem transport, evaporation into substomatal cavity, diffusion out via stomata).

5) Why other options are incorrect
- b omits xylem of root (skips the root xylem stage), which is anatomically and functionally required.
- c places water in the phloem; phloem transports sugars and some solutes, not the main transpiration stream of water upward.
- d misorders leaf and substomatal cavity (it puts substomatal cavity before leaves), which is anatomically incorrect since evaporation into the substomatal cavity occurs within the leaf.

Conclusion: The detailed anatomical and physiological pathway confirms option a is correct, matching the provided answer.

The correct path of water from soil to atmosphere is:单项选择题

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