1. [Upward movement of water and other
components from roots to aerial parts of
the plant ]
1

2. Ascent of
water and
minerals….
•Upward
movement of
water and
minerals is
called as ascent
of sap or
translocation
of sap.
2

3. 3

4. Structures involved in the
process of ascent of sap …
• Water mainly
absorbed by root
hairs
• As xylem
elements are
present all over the
length that’s why
xylem is mainly
responsible for
ascent of sap
4

5. •Xylem cells
•Xylem vessels
•Xylem tracheids
•Pits
5

6. •Tracheids have large septal
perforations that provide
continuous system and do not
afford much resistance for the
smooth flow of water.
•The xylem parenchyma
provides access for the lateral
movement of water towards
cortical cells.
•It contains many inorganic and
organic components absorbed
by the root system.
•So the trachea and tracheids
with their large lumen and end to
end association act as excellent
pipelines for the movement of
water, minerals . 6

7. RATE OF MOVEMENT
Using dyes and radioactive
isotopes, it has been
determined that the
average rate of movement
of water in xylem is 60-75
cms/minute. This is under
normal transpiring
conditions. However, this
rate is not constant and it
varies depending upon the
environmental conditions,
particularly Relative
Humidity (RH) of the
atmosphere.
7

8. Mesophyll cells (transpiration occurs
Increase in DPD
Absorption of H2O from near by cells
Leads to increase in DPD from cell to cell
Leads to pulling of water through xylem vessels of leaf
Xylem midrib
xylem of root
Continuous intake of water from root hairs 8

9. •vital theories
•Root pressure theory
•Physical theories
Vital theories are stated here ..
•Godlewski’s relay pump theory
•Pulsation theory
•Root pressure theory
9

11. • Due to rythmatic
change in OP of
xylem elements
•Living cells absorb
water by osmosis
•And finally water is
pumped into xylem
vessel due to
lowering of pressure
in living cells
•Thus staircase type
of movement occurs 11

12. •Living cells of innermost layer
of cortex had rhythmic
pulsations.
•These pulsations are
responsible for the upward
movement of water.
•Fine needle attached to
galvanometer was inserted in
plant stem
•Galvanometer shows slow
oscillations but when it
touched cortex cells it turned
violent indicating that the
cells were pulsating.
12

13. •When watered plant is cut near
base xylem sap flow out at the
end with a pressure.
•Mainly observed in herbaceous
plants
•Water oozes out due to bleeding
•This is due to hydrostatic
pressure inside root c/a root
pressure
•Root pressure appear due to
osmotic pressure.
•If root is supplied with isotonic
or hypertonic solution root
pressure disappears.
13

14. 1)Capillarity :water rises in narrow tubes due to
the force of surface tension
• It stated that it is the capillary action of narrow
xylem tubes and it is further supported by
atmospheric pressure
2) Imbibitional theory : ascent of sap takes
place due to imbibitional forces through walls of
xylem cells.
•Magnitude of imbibitional force may be up to 1000
atm,
•But discarded due to fact that water moves in
lumen of vessels rather than through the walls.
14

15. 15

16. COHESION TENSION THEORY…….
•Theory was
proposed by Dixon
and Jolley, but the
theory has been
expanded .
•This theory is also
called as Suction
force theory.
16

17. COHESIVE FORCES
When fine water columns are
subjected to a transpiration
pull as great as 1000 bars or
more, the water column is
likely to snap, but it does not
because inter molecular
forces that hold the water
molecules together are
greater than the opposing
forces,
.
17

18. Such forces are called cohesive forces; they are nothing but
hydrogen bonds between the water molecules. The overall
strength of water column in such narrow xylem elements
has been estimated to be many folds higher than the
transpiration pull and the gravitational pull put together
18

19. •Transpirational pull develops a negative pressure
or tension in xylem sap which is transferred to the
roots.
•DPD of water in mesophyll cells increases.
•So water is absorbed by cell wall from protoplasm
•DPD of xylem cells increases.
•When transpiration is rapid but absorption is
slow, water in xylem cells is passed into the state of
tension.
•
19

20. •Leaves loose water by stomata by the process
of transpiration
•Water transpired through the mesophyll cell
spaces also
•DPD increases and water potential decreases
in mesophyll cells.
•Due to this water moves along concentration
gradient.
•Now the water is continuously pulled by
cohesive and adhesive forces
•This upward force is called as transpirational
pull. 20

21. Flow chart showing cohesion tension
theory of Dixon
Strong force of attraction in water molecules
{cohesion}
Tensile strength of water + adhesive force of
water
Water molecules remain adhere to xylem
vessel walls
21

22. EVIDENCE IN SUPPORT OF COHESION
...
1. Transpiration pull developed in the aerial regions
in the air is more than 1000 bars.
2. Cohesive force that holds the water molecules in a
column as narrow as xylem vessels is very strong and
they withstand the opposing transpiration and
gravitational pulls.
3. Because of the opposing forces, tension develops
in the water column. As a result, xylem elements become
slightly narrow. This results in the contraction and
expansion of the stem which has been demonstrated by
using dendrometer. The diurnal behavior of rhythmic
contraction and expansion is a good evidence for the
water column to be in tension when the transpiration is
22
rapid or not.

23. 4. The forces that operate cohesion
transpiration pull are just passive forces and no
metabolic energy is involved in this
phenomenon, because present the movement of
water upwards.
23

24. Guttation :
•While transpiration
loss of water in the
form of liquid is c/a
guttation
•Such plants have pore
in the epidermis near
xylem elements c/a
hydathode.
•Thus rate of
expiration increases.
24