Sap is with dissolved ingredients (minerals) .The upward movement of water
from roots towards the tips of stem branches and their leaves is called
Ascent Of Sap.
It occurs through the tracheary elements of xylem. Ascent Of Sap can be
studied under two broad categories-
1. Path of Ascent of sap.
2.Mechanism Of Ascent of sap.
Path of Ascent of sap
a. Stain Test
Take a leafy shoot freshly cut under water . Dip the cut end of
the shoot in an Eosin solution contained in a beaker .
Hold the leafy shoot erect by means of a stand. After sometime
veins of the leaves will become red. Flowers develop the same
color.
Even stem may look reddish. Cut thin transverse section of the
stem and leaves. Observe them under the microscope. The walls of
the tracheid's and vessels will be found colored.
Result
The veins of the leaf consists of vascular bundles. The appearance
of red color in the vein of leaf shows that the colored water
travels through the vascular bundles.
The microscopic study of the vascular bundle proves that the
colored water moves through vessels and tracheid's of xylem.
b. Ringing Experiment
Cut two leafy shoots under water . Keep their lower ends dipping in
water. in one shoot remove 2-4 cm long ring of bark roughly in the
middle region of the shoot.
Remove the pith of the stem of this or basal 4-6 cm long region by
means of a needle.
Remove the xylem in the middle of the second shoot. Fix the shoot
to stand and allow the apparatus as such for 1- 2 days.
The leaves of the first shoot will remain turgid while those of
second shoot would get wilted.
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| Ascent Of Sap - Transportation Of Water In Plants |
In the first leaves remain turgid even after 24 hours showing
clearly that water continues to rise upwards the leafy shoot despite
removal of bark and pith.
Removal of bark breaks the continuity of epidermis, cortex , phloem
and pith do not take part in transport of sap or water .The only
tissue left intact is xylem.
Xylem therefore must be the tissue taking part in the transport of
water. This is confirmed from the wilting of the second shoot in
which xylem has been discontinued.
Theories related to Mechanism Of Ascent of sap
Water or sap is lifted from near the root tip to the shoot tip
against the force of gravity , sometimes to height of 100 m. The
rate of translocation is 25-27 cm/min. Several theories related to
Ascent of Sap are-
1.Vital Force Theory
A common vital force theory about the ascent of sap was put
forwarded by J.C Bose. It is also called as Pulsation theory
.
This theory believes that the innermost cortical cells of the root
absorb water from the outer side and pump the same into xylem
channels.
However , living cells do not seem to be involved in the ascent of
sap as water continues to rise upward in the plant in which roots
have been cut or the living cells of the stem are killed by poison
and heat.
The theory was put forwarded by Priestly . Root pressure is
positive pressure that develops in the xylem sap of the root of some
plants. It is a manifestation of active water absorption
.
Root pressure is observed in certain seasons which favor optimum
metabolic activity and reduce transpiration. It is maximum during
rainy season in the tropical region and during spring in temperate
regions.
The amount of root pressure is commonly 1-2 atm or bar and may vary
up to 10 atm or bar occasionally. Root pressure is retarded or
becomes absent under condition of starvation , low temperature and
low oxygen.
Merits of Root Pressure Theory
i.Osmotic - Tracheary elements of xylem accumulate salts and sugars leads to
the increase in solute concentration.
High solute concentration causes withdrawal of water from the
surrounding cells as well as from the normal pathway of water
absorption.
As a result a positive pressure develops in the sap of xylem.
ii. Electro- Osmotic-
A bio-electric potential exists between the xylem channels and
surrounding cells which favor the passage of water into
them.
iii. Non- Osmotic - Differentiating xylem elements produce hormones that function as
metabolic sinks and cause movement of water towards
them.
The living cells surrounding xylem can actively pump water into
them.
Demerits Of Root Pressure Theory
i. Root pressure is not present in all types of plants.No or
little root pressure is found in Gymnosperm which includes some of
the tallest trees of the world.
ii. Root pressure is only during the favorable conditions like
rainy and spring season.
iii. The normally observed root pressure is generally low,
which unable to raise the sap to the top of the trees.
iv. Water continues to raise up even in the absent of
roots.
v. The rapidly transpiring plants do not show any root pressure
instead a negative pressure is observed.
vi. The amount of exudation due to root pressure is quite low
as compare to the rate of passage through the xylem.
vii. Absorption in detopped plants is quite low as compare to
the intact plants.
viii. Root pressure disappears in unfavorable conditions while
Ascent of sap continues uninterrupted.
ix. Root pressure is generally observed at night when
evaporation is low.
It may be helpful in establishing continues water chains in
xylem which often break under enormous tension created by
transpiration.
Water rises in tubes of small diameter , kept in vessel having
water due to force of surface tension or adhesion or
cohesion.
Water similarly rises up in the walls of xylem channels due to
adhesion . Cohesive force present among water molecules pulls the
water upward through the xylem channels.
The upward movement will continue till the forces of adhesion and
cohesion are balanced by the downward pull of gravity.
Demerits of Capillarity Theory
i. The value of capillarity is very small and able to raise water
only up to a height of 1m in vessels of normal
diameter.(0.03mm).So it is only use full for small plants.
ii. Capillarity occurs only when base of the tubes dips in the
container of water. Xylem vessels are not directly connected
with soil water.
iii. Rise due to capillarity will increases when lumen of the
vessel is less. So, the tall plants should have the high
capillarity value as they have smaller lumen compare to small
plants. But the truth is reverse.
iv. Capillarity theory is not applicable in plants having
tracheids ,due to presence of end walls.
4.Cohesion Tension / Transpiration Pull Theory
This theory was put forwarded by Dixon and Joly.
Merits Of Cohesion Tension Theory
i.Continuous water column -
There is a continuous column of water from root through the stem
into the leaves.
The column of the water doesn't fall down under the impact of
gravity because transpirational force provide the required energy
and pull.
Cohesion , adhesion and surface tension keep the water in
place.
ii. Cohesion and tensile strength -
Water molecule remain attached to one another by a strong
mutual force of attraction called cohesion force.
The mutual attraction is due to Hydrogen bonds formed among
the water molecules.
Due to this force of water column bear a pull of 100 atm.
Water Column further doesn't break it's connection from the
tracheary elements because of an another force called
adhesion, between the their walls and the water
molecules.
The attraction force is more in water molecules compare to
gaseous state which produces surface tension that leads to
high capillarity through tracheids and vessels.
iii.Development of Tension Or transpirational Pull-
Intercellular space present among mesophyll cells of the leaves are
always saturated with water vapors. The latter come from the wet
walls of mesophyll cells.
The intercellular spaces of mesophyll are connected to the outside
air through stomata. outside air is seldom saturated with water
vapors.
It has lower water potential than the moist air present inside the
leaf. Therefore , water vapors diffuse out of the leaves. The
mesophyll cells continue to lose water to the intercellular
space.
As a result curvature of meniscus holding water increases resulting
in increase in surface tension and decrease in water potential ,
sometimes up to 30 bars.
The mesophyll cells withdraw water from the deeper cells as it's
molecules are held together by hydrogen bonds.
Deeper cells in turn obtain water from the tracheary elements. The
water in the tracheary elements would , therefore, come under
tension .
A similar tension is felt in millions of tracheary elements lying
adjacent to the transpiring cells.
It causes the whole water column to come under tension. As the
tension develops due to transpiration, it is also called
transpiration pull.
This pull overcomes all the downward forces and raise the water up
to 130m .
Demerits Of Cohesion Tension Theory
i. The gases dissolved in sap shall form air bubbles under
tension and temperature. Air bubbles break the continuity of
water column and stop ascent of sap due to transpiration
pull.
ii. A tension up to 100 atm has been reported in xylem sap
while the cohesive force of sap can be as low as 45 atm.
iii. Overlapping cuts do no stop ascent of sap though they
break the continuity of water column.
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