Capillarity is the movement of a liquid in a tube due to the adhesion of liquid

Question

Capillarity is the movement of a liquid in a tube due to the adhesion of liquid molecules to the sides of the tube and cohesive attraction of the liquid molecules to each other. The height that water may obtain capillary action alone is defined by this equation, h=14.87/r, where the radius (r) of the tube is in m and height (h) is in meters. So in small-diameter tubes such as xylem tracheids and vessels, water can move upwards by capillary action.

Part A:) Without doing any calculations, do you think capillary action is sufficient to move water to the tops of 100 m tall trees?

Part B:) Now get out your calculators. Given that tracheids are about 80 m and vessel elements about 200 m in diameter, how high (in meters) can water move in xylem by capillary action alone?

Explanation / Answer

Answer:

Part 1:

Only capillary action is not sufficient to pull water from the soil to the top of a 100 m tall tree. There are few more physical forces that works together. According to Scientist Dixon and Joly (1894) ascent of sap (water and minerals) in plants is caused by-

1) Transpiration pull: Water is evaporated by transpiration through the stomata of leaves. It produces suction pressure in the leaf. This pressure pulls the water upward.

2) Cohesion: The attraction force  among the water molecules is called cohesion. The cohesion force holds water molecules together. It produce a solid chain like column within the xylem tubes. Strong H-bonding between the water molecule are the reason of this cohesion force.

3) Tension: The pulling of water upward produces a tension in the xylem tubes. The transpiration provides the necessary energy . the water molecules produce this tension. the xylem water tension is much stronger. It can pull the water up to 200 meters in plants.

4) Adhesion: The attraction force between the water molecules and the cell wall of the xylem cells is called adhesion. The water molecules remains adhere to the xylem tissues by this attraction. This adhesion force keeps the column of water in the xylem tissues intact.

Part 1:   Given that tracheids are about 80 m and vessel elements about 200 m in diameter. and the height that water may obtain capillary action alone is defined by this equation, h=14.87/r (where the radius (r) of the tube is in m and height (h) is in meters).

For Tracheids, Diameter (d)=80m ; so Radius (r)=40m

So, height (in meters)water can move through tracheids, h=14.87/40 =0.37 m

For the vessel, Diameter (d)=200m ; so Radius (r)=100m

So, height (in meters)water can move through vessels, h=14.87/100=0.148 m.

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