Question

Cohesion and adhesion theory, is otherwise called
(a) Relay pump theory
(b) Pulsation theory
(c) Root pressure theory
(d) Transplantation pull theory

Answer 1

Hint: Due to continuous withdrawal of water from the xylem by transpiring the leaf the entire water column of the plant comes under tension/strain. This theory is additionally referred to as negative hydrostatic pressure & features a value upto 20 atm.

Complete step by step answer:
Cohesion or tensile and adhesion strength of water: Transpiration pull Theory
- Transpiration pull is unable to interrupt the continuity of the water column as water molecules have strong cohesive force (H- bonding) .
- Cohesive force is additionally called lastingness .
- There acts the force of adhesion between water molecules & walls of tracheary elements & walls are strong enough to resist collapsing when sap is under tension.
- The water column is pulled up passively from below to top of the plant, sort of like a rope.
- Tension of 1 atm is sufficient to tug water up to 10 m height; 10- 20 atm is sufficient to boost water to the tallest trees by overcoming all the resistances.
So, the correct answer is, ‘(d) Transpiration pull theory.’

Additional Information:
The force of cohesion is the result of attraction between molecules of an equivalent substance. this relies on various factors like relative molecular mass ,properties etc. The force of cohesion between molecules having polar nature are going to be quite between non polar molecules of comparable mass.
Cohesion transpiration pull mechanism:
- This theory was proposed by Dixon and Jolley, but in later years, the idea has been expanded and modified to elucidate certain observations that weren't explained before.
- Among the notable scientists who supported this idea are Curtis and Clark (1951) , Krammar, Hamel and Levit. This theory is additionally called as Suction force theory
Evidence in support of cohesion transpiration pull theory:
- Transpiration pull developed within the aerial regions at 50% RH within the air is quite 1000 bars.
- Cohesive force that holds the water molecules during a column as narrow as xylem vessels is extremely strong and that they withstand the opposing transpiration and gravitational pulls.
- Due to the opposing forces, tension develops within the water column. As a result, xylem elements become slightly narrow.
- The forces that operate cohesion transpiration pull are just passive forces and no metabolic energy is involved during this phenomenon, because the movement of water upwards.

Note:
Transpiration Pull theory is most generally accepted due to following reasons:
- Endodermis filters out gas bubbles from entering xylem.
- Plants possessing vessels even have tracheids for alternate pathways free from air bubbles.
- Plants with secondary growth produce new xylem channels per annum.
- Air bubbles get slowly re- absorbed in liquid water when tension is low in the dark.
- Positive root pressure also helps in shrinking air bubbles.
- Ascent of sap occurs even after overlapping cuts thanks to repair of cavitation.