Imagine a tree taller than a ten-storey building. Every leaf at the top needs water — not just a sip, but a constant supply, litre after litre, on a hot summer day. The roots are buried in soil, metres below. How does water climb that high, against gravity, without a pump?
That question is the ascent of sap — the upward movement of water and dissolved minerals through the xylem tissue from roots to leaves.
The intuitive picture
Think of a drinking straw. You suck at the top, and water rises. Now imagine a straw that is alive, continuous from root to leaf, and so narrow that water molecules stick to its walls and to each other with surprising strength. If you could pull water from the top — not by sucking, but by evaporation — the entire column would move upward as one, like a chain being lifted link by link.
That is exactly what happens. The leaves are the "mouth" that pulls. The xylem vessels are the straws. And the water column behaves like a continuous, stretchy rope.
The precise statement
The ascent of sap is explained by the cohesion-adhesion-transpiration pull theory (also called the transpiration pull theory). It has three linked ideas:
Transpiration pull — Water evaporates from the mesophyll cells of leaves through stomata. This creates a negative pressure (tension) at the top of the xylem, because the evaporating water molecules leave behind a "gap" that pulls on the water below.
Cohesion — Water molecules are strongly attracted to each other by hydrogen bonds. This cohesion makes the water column in the xylem behave like a continuous, unbroken thread. When the top is pulled, the entire column moves as one unit — it does not snap.
Adhesion — Water molecules are also attracted to the hydrophilic walls of xylem vessels and tracheids. This adhesion helps counteract gravity and keeps the water column from slipping back.
Transpiration pullcohesionupward movement of continuous water column
The tension at the top can be enormous — up to -20 to -30 atmospheres in tall trees. Yet the column does not break, because the cohesive force between water molecules is even stronger (theoretically enough to support a column over 100 m high).
Transpiration pull is affected whenever the xylem water column loses its continuity, e.g. when gas bubbles (embolism/cavitation) break the column, or when transpiration itself is reduced (closed stomata, high humidity, low light). …
Transpiration pull is affected whenever the xylem water column loses its continuity, e.g.
Transpiration pull depends on two things working together: an unbroken, cohesive column of water in the xylem, and active transpiration at the leaf surface to generate the suction. It is therefore affected under two kinds of conditions. First, if temperature fluctuations allow gas bubbles to enter and break the continuity of the water column (cavitation/embolism), the pull cannot be transmitted properly down to the roots. Second, whenever transpiration itself slows or stops — at night when stomata close, in conditions of high atmospheric humidity that reduce the vapour-pressure gradient, in low light, or when stomata are closed to conserve water under drought stress — there is little …
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2023Set ANNUAL1 markMCQ
Q.Match the following :
(1) Godlewski
(2) Stephen Hales
(3) J.C. Bose
(4) Dixon & Jolly --
(i) Pulsation theory
(ii) Relay - pump theory
(iii) Cohesion and Transpiration Pull theory
(iv) Root pressure
(a) (1)-(ii), (2)-(iv), (3)-(i), (4)-(iii)
(b) (1)-(i), (2)-(iii), (3)-(iv), (4)-(ii)
(c) (1)-(ii), (2)-(iv), (3)-(iii), (4)-(i)
(d) (1)-(iii), (2)-(iv), (3)-(i), (4)-(ii)
›Reveal solutionSolution
The correct matching is Godlewski-Relay pump, Stephen Hales-Root pressure, J.C. Bose-Pulsation, Dixon & Jolly-Cohesion and Transpiration Pull, which is option (a).
Several theories have been proposed over the years to explain the ascent of sap (water movement from roots to leaves) in tall plants:
Stephen Hales (1727) was the first to scientifically study and measure root pressure - the pressure developed in root xylem due to active absorption of water and ions, which pushes sap upward from below. This corresponds to (iv) Root pressure theory.
Godlewski proposed the relay-pump theory, suggesting that the living cells of the xylem (particularly ray parenchyma) act as a series of relay pumps that actively push water upward stage by stage. This corresponds to (ii) Relay-pump theory.
J.C. Bose proposed the pulsation theory, based on his observation of rhythmic pulsatory movements (changes in electrical potential and turgor) in the cortical cells of the stem, which he believed helped drive water upward. This corresponds to (i) Pulsation theory. …