This transpirational pull occurs because of 1 the

from the leaves to the roots. This transpirational pull occurs
because of (1) the cohesion of water molecules to one another
due to hydrogen bond formation, and (2) by adhesion of water
molecules to the walls of the xylem cells which aids in offsetting
the downward pull of gravity.The upward transpirational pull on
the fluid in the xylem causes a tension (negative pressure) to
form in the xylem, pulling the walls of the xylem inward. The
tension also contributes to the lowering of water potential in the
xylem. This decrease in water potential, transmitted all the way
from the leaf to the roots, causes water to move inward from the
soil, across the cortex of the root, and into the xylem.Plants lose
as much as 90% of the water that enters the roots by the process
of transpiration. Evaporation through the open stomates is
clearly a major route of water loss in plants.However, the
stomates must open to allow the entry of CO
2
used in
photosynthesis. Therefore, a balance must be maintained
between the gain of CO
2
and the loss of water by regulating the
opening and closing of the stomata on the leaf surface. Many
environmental conditions influence the opening and closing of
stomates and also affect the rate of transpiration.Temperature,
light intensity, air currents, and humidity are some of these
factors. Different plants also vary in the rate of transpiration and
in the regulation of stomatal opening. In this lab, you will
measure transpiration under various laboratory conditions using
a potometer. Hypothesis : - “If all of the plants in this experiment

will lose water through transpiration, than those affected by the
heat sink and the fan will lose a larger amount of water due to
the environmental conditions, because transpiration will pull
water from the potometer into the plant with the structure and
the cell types of a stem cross-section can be observed under a
microscope”.
Purpose : - The purpose of this lab was to measure the rate of
transpiration in a plant under various conditions in a controlled
experiment. We than d iscover ed the effect that different
environmental factors have on transpiration rate. The
environmental factor we chose to test was amount of light.
V ariables: - 1. - Independent variable: - The light, what is being
changed . 2. Dependent variable: - The plant, what is changing
due to the independent variable. 3. - Control Variables : -
Amount of light.
Materials & Procedure : -
1. Place the tip of a 0.1 mL pipette into a 16 -inch piece of clear
plastic tubing.
2. Submerge the tubing and the pipette in a shallow tray of
water. Draw water through the tubing until all the air bubbles
are eliminated.
3. Carefully cut your plant stem under water. This step is very
important, because no air bubbles must be introduced into the
xylem.
4. While your plant and tubing are submerged, insert the freshly