Ascent of Sap
• The upward movement of water from roots to
aerial parts of plants through xylem is called
ascent of sap.
• Balsam plant experiment to demonstrate xylem
is the pathway for ascent of sap.
• A beaker, Water, Eosin stain (red ink),
stand, White balsam plant with roots.
• Put few drops of eosin stain in a
beaker containing water.
• Insert a balsam plant into it. Fix to
stand. Keep it for few hours.
• Red lines can be seen in the
transparent stem and in the veins of
• When a section of the stem is
observed under the microscope, it
shows that only the xylem vessels are
• This expt confirms xylem is the
pathway for ascent of sap.
Mechanism of ascent of sap.
Transpiration pull and cohesion theory:
• The theory was proposed by Dixon and Jolly (1894).
This theory is based on
1. Cohesive and adhesive properties of water.
2. Transpiration pull force developed in leaves by
• Cohesion is the phenomenon of attraction between
similar molecules. The water molecules remain
attracted by the cohesive force.
• There is attraction between water molecules and
the inner wall of xylem by adhesive property.
• Due to this water forms a continuous column in
• During day time mesophyll cells
looses water due to
• As a result turgor pressure and
water potential of mesophyll
• At the same time water potential
of xylem vessels of leaf is high.
• The mesophyll cells sucks the
water from xylem vessels.
• Due to this –ve force called
transpiration pull force develops.
• This force moves to the xylem
vessels and pulls water on
experiment to demonstrate ascent of
sap due to transpiration pull force
• When water lost from aerial
part of the plant by
transpiration, the transpiration
pull force develops. This force
is responsible for ascent of sap.
• Simple potometer, water,
mercury, beaker, plant twig,
stand, and wax.
• Fill the potometer completely
• Fix the cut end of plant twig
to the upper end of
potometer using single holed
• Immerse the lower end of
potometer in a beaker
• Make the setup air tight by
• Leave the apparatus in
sunlight for few min and
• Observation: After
few min raise in
mercury level is seen
in the tube.
• Inference: the raise in
mercury indicates the
force in potometer is
responsible for ascent