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At night, when stomata shut and transpiration stops, what holds the water column in the stem and leaf?
AAdhesion to xylem walls and cohesion between water molecules
BAdhesion to phloem walls and repulsion between water molecules
CActive transport by companion cells lining the xylem vessels
DPositive pressure generated inside the leaf air spaces at night
Answer & Solution
Correct answer: A. Adhesion to xylem walls and cohesion between water molecules
1. By day, evaporation at the leaf surface creates tension that pulls the water column upward.
2. At night the stomata shut and transpiration stops, so that pull disappears.
3. The water is then held in the stem and leaf by adhesion of water to the cell walls of the xylem vessels and tracheids.
4. Cohesion of water molecules to each other holds the column together.
5. This pairing of adhesion and cohesion is the cohesion tension theory of sap ascent.
6. Naming the phloem or invoking companion cells shifts the explanation to the sugar transport tissue, which is not the water conducting route.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 30 "Plant Form and Physiology", section 30.5 Transport of Water and Solutes in Plants_
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