7. Discuss about Theories of acent of SAP in Plants.
Ans. Here are the following theories which
explain the ascent of sap in plants:
(a) Root pressure
(b) Capillarity
(c) Vital theory and
(d) Cohesion - tension theory.
(a) ROOT PRESSURE
The hydrostatic pressure generated in the root which
forces the water upward in the stem is called root pressure. Root
pressure forces the water up from below. But it is believed that root
pressure does not play much role in the ascent of sap due to the following reasons:
1. The magnitude of root pressure is Mitch lower.
. 1 - hus this pressure is insufficient to raise, Water to much
height. It rarely exceeds Jam. 13tuapressure of about 20atm.
is required to raise the sap to the tops of tall trees.
2 . Root pressure shows sensonal fluctuations.
New lca% es are coming out and transpiration isslowinspring.
Sctitishighest in spring. But transpiration is very rapid in
summer. Soithas. lower value in summer. Therefore, it does
not play any role in ascentlsap in summer,
3. Water continues torise up in the absence of root
pressure. 'Vitus when acutshoot is placed in water, it does not wilt.
4. Root pressure has not been obsened in g.
mnosperms.
(b) CAPILLARITY
A glass tube withanamns bore is called a capillary
tube. If one end of the capillary tube is dipped in water. the
level of water rises in the tube ltoccurs due to property ofadhesion of
water. Adhesion is the airection of molecules of water vl / 4 ith the
glass, the phenomenon is called capillarity. The height of water in a
capillary tube is' aveniely proportional to diameter of the bore. It
means smaller the capillary bore. more water is mised. x. )
lem ducts acts as narrow capillary tubes. But capillarity. does not
play much role in the ascent of sap due to the following reasons:
1. 1.11c capillary bore ofxy lem essels and trachcids is
about 40m in diameter. lts capillary force can raise yater on iv up to 40
cm,
2. Capillarity worlessonlyifone of the open endsofthe
capillary tube is dipped in water. But this situation does not exist in
nature. The xy iem ducts of roots are not in contact with liquid water of
the soil.
(c) VITAL THEORIES
There were IWO iews about vital theory:
1. Vital theory was first proposed by Godiewski. According
thisthpory the movement of water Iakes place due to the pumping activitvofthe
cellsofwood parenchyma and Medullary. rava File periodic chanee
intheirasmatic nnessure causes pumping action. There isan increarn otosmotic
pressure in these cells. It withdraws water from the bordering vessels, then
their Osmotic pressure is decreased. Itpumps water into the above essel. In
this way water moved up step by step. . 2. According to IC. Bose (1923)
the ascent of sap is due to the pulsator) actii. it of the cortical cells
of theslew outside the endoderm is.
Strasburgher disapproyed this theory. lleshowed that
water continued torise in plants after death of their cells.
(d) COHESION TENSION THEORY
his theory %s as enunciated by Dixon (1910). This
theory states that the negative pressure or pull or suction is created in the
leaves as a result of transpiration pulls the water upward. According to
this theory lollowing factors play role in the ascent of sap.
1. Transpiration pull:
Water is evaporated from the stomala by
transpiration. It produces negative pressure or suction pressure in the
leaf. This pressure pulls the water upward.
2. Cohesion:
The force of: attraction among the water molecules is
called cohesion. The cohesion holds water together. It formsasolid
chain like column within the xylem tubes. - [here ishydrogen bonding
between the molecules of water.
3. Tension:
The pullingofwliter upward produces tension in the xylem
tubes. The transpiration provides the necessary energy or three.
The hydrogen bonds between the water molecules 200 meters (more than NO feet)
in plants. . produce this tension. the xylemi Yater tension
is much stronger. t. can pull the 'Yater up to S
4. Adhesion:
The attraction between the water molecules and the cell
wall of the xylem cells iscalled adhesion. The water
moleculesremainsadhere to the xylem tissues. So the columin of water in
the xylem tissues does not break. The composition of the cell wall
provides this necessary adhesion for the water molecules. It helps the
water molecules to creep tip. The cellulose of the cell wall has special
affinity with water. Cellulose can imbibe water.
Strongxylem wall: The xylem walls have high tensile
strength. So they do not buckleinwards. Thelignin and cellulose provide
strength to the cell wall of the xylem tissues.
Mechanism of transpiration pull
1. Msophyll cells:
Water eyaporates from the intercellular spaces of the
leaves into the air, Twithdraws water from the mesophyll celk. The water
vapours pass out through the stomata. More water evaporates from the
satunted walls of the mesophyll cells. The cell walls withdraw water from
the vacuoles of cells. Hence the osmotic pressure of inesophall cell.
is increased. and their turgor pressure falli, it increases their
suction pressure. Therefore. these cells ithdraw water from the deeper
inesoph) ll cell. In thisagradient of suction pressure is established.
2. Xylems:
This suction pressure reaches the parent)) 'flaunts cells.
'Mese cells draw out ksziter from the xylem vessels. The greater
the%% quer loss by transpiration the greater will be the magnitude of the
force. Thisforce then pulls up the water in the xylem vessels of the stem
from above.
3.Root:
Transpiration pull produces tension in the xylem. This
tensionistransmitted down the column of water in the xylem elements through the
stem. It reaches the absorbing region of the rooLAs a result the whole
column of water is lifted up.
4. Water column:
Cohesive forces are present between the water molecules.
These forces maintain the continuity of% voter column in the xylem. The
magnitudes of this cohesive force are very large. It may be 350 am.
This means that a force greater than 350 atm. is required to
breakacolumnofwater. Nese enormous. cohesive holds water molecules
together like a cord or wire. The rising column of water is called
transpiration stream. ObjectionsCohesion - tension theorlk is the most
accepted theory. But there are a fen objections tocohesion - tension
theory.
The water column in a glass tube would
breakonshaking. The plants are bent severely in a strong w Md. It
can break the column of water. This does not happen. The xylem
ducts ha% every narrow bore. Therefore there is greater adhesion between
water and the xy lem mails. n does notal low the water column to break,
Dissolved gases may appear and form babbles inside the
xylem ducts due to change in temperature. These bubbles can break the
continuity of water column in the xilem. But there ishigh tension in
thexsilent. This tension dissolves am nair bubble
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