it includes path of ascent of sap and various theories of ascent of sap under the heading vital theories, root pressure theory, Physical theories.

1. Transport of water/
Ascent of Sap in
plants
By: Dr. Sunita Sangwan
Assistant professor, Dept of Botany
Higher education Dept. Haryana

2. Ascent of sap
 The water and soluble mineral salts absorbed by
roots reach to the leaves through the roots, stem
and branches of the plant.
 This phenomenon of ascending of absorbed
water against gravitation through the vessels
and tracheids of xylem is called Ascent of Sap.

3. Path of Ascent of Sap
 The water ascends through the vessels and
tracheids of xylem. In other words, the path
of ascent of sap is xylem. Which can be
demonstrates by various experiments:

4. Experiments for Ascent of
sap
Experiment 1:
 That most of the water is trans-located in the
xylem can easily be demonstrated by a simple
experiment.
 In this experiment two small shoots of a woody
plant are taken. Their cut ends are so prepared
that in one shoot phloem is blocked with wax,
while in the other xylem is blocked.
 Now the cut ends are submerged in water . After
some time the shoot with its xylem blocked shows
wilting, but the shoot with its phloem blocked
remains unaffected.

5. Experiment 2:
 At the start of the ringing experiment, take two
plant twigs.
 In first twig section of the outer tissue (the
epidermis, cortex, endodermis, pericycle phloem)
except vessels and tracheids of xylem and pith is
removed around the complete circumference of
the woody stem at point and put it in water.
 In the second twig only centre portion containing
the xylem is removed and put this also in water.
 Leave the experiments for one day and observe
that the leaves of the first twig are still turgid but
the leaves of the second twig started wilting and
looks dropping leaves.

6. Experiment 3
 Cut the lower portion of white balasm stem
carefully in water so that air bubbles could
not enter into it.
 Place this portion vertically in eosin solution
and observe after a few hours that the stem
posses many minute vertical red linings due
to ascent of eosine solution.
 Now cut the TS of this stem and observe
that only the vessels and tracheids of xylem
become red whereas other tissues have no
effect of eosine and remain white.

7. Mechanism of Ascent of sap
 In small trees and herbaceous plants the ascent of
sap can be explained easily, but in tall trees like
Australian Eucalyptus, some conifers such as
mighty Sequoias (Sequoia, Sequoia Dendron,
Metasequoia are the tallest and thickest trees of
the present day flora, sometimes reaching a height
of 300-400′) where the water has to rise up to the
height of several hundred feet, the ascent of sap,
in fact, becomes a problem.
 Although the mechanism of ascent of sap is not
well understood, a number of theories have been
put forward to explain it.

8. Theories of
Ascent of
Sap
Vital
theories
Relay pump
theory
Pulsation
theory
Root
pressure
theory
Physical
theories
Imbibition
theories
Atmospheric
pressure theory
Capillary force
theory
Cohesion
theory of Dixon
& Jolly

9. Vital
theories
Relay pump
theory
Pulsation
theory

10. a. Relay Pump theory
 Godlewski (1884) proposed this theory.
According him the rhythmic changes in the
osmotic pressure of living cells of xylem
parenchyma are responsible for upward
movement of water.
 This theory seemed only hypothetical and
was discarded by strausburger and others
because ascent of sap continues in the
stem in which the living cells are killed by
poison.

11. b. Pulsation theory
 According to sir J. C. Bose (1923) ascent of
sap takes place due to pulsatory
mechanism of living cortical cells present
nearer to the vessels and tracheids of
xylem. By means of electric probe needle
he demonstrated the pulsatory mechanism
of living cells.
 This theory was also rejected because
many workers could not repeat the
experiment and there was no co relation
between pulsatory activity and the ascent of
sap.

12. 2. Root pressure theory
 Root pressure is defined as “A pressure developed in the
treachery elements of xylem as a result of metabolic
activities of roots”.
 If a well watered tomato plant is cut near its base, the
xylem sap is seen to flow out through the cut end with a
pressure. This phenomenon is quite common in many
herbaceous plants. The pressure of exudation can be
demonstrated by placing a vertical tube to the cut end of
the stem, a column of sap is seen to rise in it. This
pressure is actually the hydrostatic pressure developed in
the root system called root pressure. It is an active
process.

13. 2. Root pressure theory
It is believed that root pressure may be a factor of some
significance in the ascent of sap. But the theories discarded
because of the following points.
 Ascent of sap is observed in the plants in which roots are
removed.
 The magnitude of root pressure is hardly one to two atoms,
while a pressure of about 20 atoms is needed to raise water
to the tops of tall trees.
 In gymnosperms root pressure is rarely observed.

14. 3. Physical force theories
All those theories according to which dead cells are
responsible for ascent of sap are called physical
force theories. Under this category there are
several theories that are briefly discussed in the
next slides.
Physical
theories
a) Imbibition theories
b) Atmospheric pressure theory
c) Capillary force theory
d) Cohesion theory of Dixon & Jolly

15. a) Imbibition theories
 It was proposed (1868) and supported by
Sachs (1878). According to this theory the
ascent of sap occurs due to imbibitional
activity of the cell walls of the xylem
elements.
 This theory is discarded as it has been
experimentally found that water rises up
through the lumen of the xylem and not
through the walls.

16. b) Capillary force theory
This was proposed by bochm (1809). According to
this theory the tracheids and vessels of the xylem
behave as capillary tubes. Ascent of sap is partly
due to the capillarity of these tissues.
There are many objections to this theory:
i. For capillarity a free surface is required.
ii. The magnitude of capillary force is low.
iii. The taller plants should have narrow elements
while small plants have broader elements. But
anatomically it is not found to be correct.

17. c) Atmospheric pressure
theory
 It was proposed that atmospheric pressure must
be responsible for the ascent of sap. The water
transpires from the leaf which reduces the
pressure in the xylem cells and this gap is filled by
the water just below if due to atmospheric
pressure.
There are serious objection to this theory:
i. The atmospheric pressure can raise water only
upto 34 feet and not beyond it.
ii. Pressure of green surface at the lower end is
required for the atmospheric pressure to upper
end.

18. Transpiration pull
&
cohesion of water theory
 This theory was originally proposed by Dixon and holy
(1894) and was supported by Renner (1911), Curtis and
Clark (1951), Bonner and Galston (1952), Karmer and
Kozlowski (1960) and many others.
The theories based on the following three features:
1. Strong cohesive force of water molecules.
2. Continuity of the water column in the plant and
3. Tension on the water column due to transpiration pull.
The above system inside the xylem elements of the plant
satisfies all the above conditions, and thereby, offers a
strong support for the validity of the theory.

19.  Although H-bond is very weak, when they are present in
enormous numbers as in case of water; they develop a very
strong mutual force of attraction known as cohesion.
 The magnitude of this force is as high as 350 atoms. Therefore,
they can not break easily.
 Moreover, there is an attraction between molecules and the
wall of the xylem elements known as adhesion. The magnitude
of this force is around 50 atoms.
 Due to the cohesive and adhesive force a continuous water
column is formed in the xylem.
 Because of its strong cohesive force the column in not broken
by other force.
 Further air bubble does not block the system because the
continuity is maintained through the pits present in the walls of
tracheids and vessels.

20.  When transpiration take place from the mesophlly cells of
leaves, it results in diffusion pressure deficit inside them.
 The mesophyll cells than absorb water from the nearby
xylem elements.
 Loss of water from xylem elements results a similar
diffusion pressure deficit in them and ultimately a tension
is created.
 This tension acts as a pull which is called transpiration
pull.
 As a result of this pull the tension transmitted downloads
upto roots through the stem and upward movement of
water takes place which is analogous to pulling a bucket
of water from a well.

22. some objections of
transpiration pull theory
The transpiration pull theory inspite of some
objections of widely accepted.
1. There is correlation between the rate of
transpiration and ascent of sap.
2. In a branch cut from a rapidly transpiring
plant water snaps away from the roots.
3. With the help of depend graph Mac Dougal
observed that tree trunks show diurnal
contraction.

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