Question

Trees need water to live, and they get much of the water they need from the ground through their roots. How does water flow up a tree trunk against the downward pull of gravity? Invoke scientific principles in your answer.

Answer 1

Yes, trees needs a lot of water and how a water is provided there upto this high level carrying >100 feets.

We will see different different principles that how it happens.

First of all, Come to the basic theories regarding to the ascent of sap or leveling up water in plant.

=>Vital forces theories like Godelwaski's relay pump theory, Bose pulsatory movement.

=>Root pressure theory

=> Physical forces theories like Imbibition theory, cohesion tension theory.

*Vital forces were due to the living cell that all water transport occuring due to rhythmic movement in cell somehow. Further we see ,

A. Godlewaski's theory:--  

Purposed by Godlewski in the year 1884

Causative factor:-- Rhythmic change in Xylem parenchyma and meduallart rays causes leveling up of water.

B. Bose pulsatory movement :-- In the year 1923,

Famous botanist Sir J.C. Bose showed that how the cells of innermost layer of cortex were showing pulses like movement.

Causative factor:-- According to Sir Bose these pulsation due to osmosis occuring cell by cell and causing water to level up.

*Root pressure theory:-- This is due to osmosis and occurs in small plants or herbaceous plants where they are only <1 feet.

Site of occurance :-- In the xylem of some vascular plants.

It occurs if transpiration is low and water level is high in soil.

Examples of root pressure :--

Guttation:-- You see many times that plant leaves have some droplets in the morning. This is due to low transpiration at night and high water in soil causes osmotic pressure factor to let happen this Guttation.

**But these theories given above could not clear that how water reaches upto this high level in gymnosperm , confers etc. Theories given above are suitable upto only few feets like (1 or 2 feet.)

* * What about 100 feet or >100 feet in Gymnosperms?

There are some theories which were based on physical forces and most accountable for ascent of sap in these Gymnosperms.

Physical forces theories :--

A. Imbibition theory:- Sachs in the year 1878 ,

According to him, Water gets imbibed to the cell wall material and ascent of sap occurs. It means plant wall is the factor of transporting water. And this theory seems weired but we know that water moves from lumen of vessel.

B. Cohesion tension theory (1894):- most accepted theory for the ascent of sap upto 100 feets in trees.

Purposed by Henry H. Dixon(Botanist), John Joly (Physicist)

This theory states that Tension (Transpiration pull) is due to Water cohesive property and adhesive property of cell wall both together causes ascent of cell. Here water water molecules hydrogen bonding and these adehsive forces to cell wall responsible for all this.

That's it.