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).
Why other theories failed
Before this theory was accepted, people proposed root pressure as the main driver. Root pressure does push water upward, but it is weak — typically 1–2 atmospheres, enough for a few metres at most. It cannot explain water rising 100 m in a redwood. Root pressure works in small plants at night or in early spring, but the primary force for tall trees is transpiration pull.
The key evidence
If you cut a stem and attach a pressure gauge, you can measure negative pressures (tension) in the xylem during the day.
If you introduce an air bubble into the xylem, the water column breaks and transport stops — proving the column must be continuous.
The rate of water movement matches the rate of transpiration, not root activity.
Watch out
A common mistake is to think that the xylem "sucks" water actively. It does not. The xylem is dead tissue at maturity — it is just a passive pipe. The pull comes entirely from the leaves.
The one-sentence takeaway
Water rises in plants because evaporation at the leaves creates a tension that pulls the cohesive water column upward through the xylem, with adhesion to the walls preventing collapse.
Ascent of Sap is a textbook-aligned concept from the Transport in Plants unit of the Class 11 NCERT/CBSE Biology syllabus, making it a frequent entry in "Ascent of Sap important questions" lists and a favourite for NEET Biology multiple-choice questions on plant physiology.
The cohesion-tension (transpiration pull) theory is the most widely accepted explanation for ascent of sap today.
✓Final answer
(d) transpiration pull theory
The cohesion-tension (transpiration pull) theory is the most widely accepted explanation for ascent of sap today.
Of the theories explaining ascent of sap, capillarity theory and root pressure theory both face serious physical objections (closed tracheid end-walls, a root-cortex barrier, near-zero root pressure in tall gymnosperms, and so on) and neither works for very tall trees on its own. Diffusion alone cannot move water fast enough over long vertical distances. The cohesion-tension theory (Dixon and Jolly, 1894) — in which cohesive/adhesive forces maintain a continuous xylem water column, and transpiration from the leaf creates a suction that pulls this column upward — is currently the most widely accepted explanation, since it best fits observations even in very tall trees.
✓Final answer
(d) transpiration pull theory
Recall which of the named ascent-of-sap theories is described in the textbook as the currently, widely accepted one.
Picking root pressure theory, which the text explicitly says fails for trees taller than 20 m.
Confusing 'diffusion' (a step in root absorption) with a theory of long-distance ascent of sap.
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.
Dixon and Jolly (1894) proposed the Cohesion-Tension (Cohesion and Transpiration Pull) theory, now the most widely accepted explanation: transpiration from leaves creates a pull (tension) that is transmitted down an unbroken, cohesive water column held together by hydrogen bonding (cohesion) between water molecules, right down to the roots. This corresponds to (iii) Cohesion and Transpiration Pull theory.
So the correct matching is (1)-(ii), (2)-(iv), (3)-(i), (4)-(iii), which is option (a).
✓Final answer
The correct option is (a): Godlewski-Relay pump (ii), Stephen Hales-Root pressure (iv), J.C. Bose-Pulsation (i), Dixon & Jolly-Cohesion and Transpiration Pull (iii).