FREE ESSAY ON TRANSPIRATION LAB

TRANSPIRATION LAB

Introduction
Water is essential to plants in many ways. It first provides the major substance for
living, to keep cells from shriveling up and dying. The second major function is to keep
the plants rigidity. As plant cells become turgid, full of water, the cells expand,
filling the extent of their cell walls, which are kept taught with turgor pressure. If
the cells lose water, two problems occur. First, the cells dehydrate, causing the
organism to die. Second, turgor pressure is lost as cells become flaccid, limp and
unfilled, causing a loss of support for the plants structure which makes it appear
wilted. 
As aquatic plants evolved into large complex land plants, an adaptation occurred in the
center of plants to allow full growth without the problem of water loss. A system of
vascular bundles extending from the tips of the furthest leaves to the deepest roots of
each plant developed, carrying water in xylem sap and sugar in phloem. While phloem can
transport sugar in any direction within the plant, xylem can only move water up, from
root to leaf. Once in the leaf, the water evaporates through stomata-tiny gaps in the
lower epidermis of each leaf, which are regulated by guard cells-a process called
transpiration
The movement of water into and out of the xylem involves water pressure factors in
different sections of the plant. As water slips into the roots through osmosis, a
positive water pressure gently pushes the water into the plants roots and supplies a
jumpstart for the water's journey up the vascular bundle. However, it is not this
pressure that supplies a great force towards the upward movement of water; it is the
evaporation of water from the stomata that pulls water upward and out. When the stomata
are open to take in carbon dioxide for carbohydrate production, water begins to evaporate
and seep out of the tiny holes in each leaf. With a constant pull of water outward, other
water molecules are pulled up to replace it. The pull is provided by the cohesive
properties of water molecules as each leaving molecule pulls on another molecule which is
hydrogen bonded to it. The process continues as a series of movements until all the water
molecules in the xylem sap are being pulled upward by their hydrogen bonds to the water
molecules ahead of them. Thus the slight negative pressure occurs.
Different environmental factors can have impacts on the intensity of water evaporation,
and thus the rate of plant transpiration. Just like water in an open environment, a dry
environment would increase the evaporation of water, and the rate of transpiration. A hot
or very bright environment would do the likewise. Conversely, moist, dark, or cool
environments would allow for a slower rate of transpiration because water would not be as
readily evaporative. When testing the rate of transpiration for any given plant, I
hypothesize that plants exposed to copious quantities of light will transpire more
rapidly than those in a regular environment. 
Methods
We selected a bean plant on which to test varied environmental factors on transpiration.
The different environments included excessive sunlight-a floodlight one meter from the
plant, wind/dry air-a stationary fan approximately one meter away from the plant on low
speed, humid/rainy climate-leaves misted, then covered with a clear plastic bag (open at
the bottom for air exchange). Normal room conditions were also tested for the control.
One bean plant was used for each simulated environment. 
To set up the experiment, four pieces of Tygon clear plastic tubing were cut to sixteen
inches. Inside each was placed the tip of a 0.1-mL pipette. Taking four ring stands, one
paired with each tube/pipette set, each end of the tubing was clamped, so that the tubing
made a "U" shape. Next the tubing was filled with water so that no air bubbles were
present and that water completely filled the tubing and pipette. The four bean plants
were each placed into the open end of their respective tubing, then sealed with petroleum
jelly around the sides (to prevent accidental water evaporation). 
The plants were allowed to sit for ten minutes before the initial reading was made, to
allow for equilibration. After recording levels of water for all plant environment
simulations, readings were made in ten minute increments until thirty minutes elapsed.
After this, the leaves were cut off of each plant to be weighed and measured. With these
figures, we found the total surface area of each plant, after which we could calculate
the rate of transpiration for each climate.
Results
To determine the rate of transpiration for each tested bean plant, the cumulative water
loss (in milliliters) was divided by the leaf surface area of each plant (in meters
squared). This rate was figured for each time increment: initial, ten minutes, twenty
minutes, and thirty minutes. Table 1 shows these calculations for the control, group a,
floodlight, b, fan, c, and mist, d. The relationship among the data is shown on Figure 1.
The lines for test plants b and c both show high rates for transpiration, while control
plant a is at a moderate rate of transpiration and test plant d has a relatively low rate
of transpiration compared to the other plants.
Conclusion
As Figure 1 shows, the plants tested in dryer climates, b and c, showed higher rates of
transpiration. This is due to the greater potential for evaporation in their
environments. The extra exposure to light adds heat which dries up water vapor around the
plant and inside the leaves, as it leaves through the stomata. The water in the tube was
then pulled by the negative pressure created by the evaporation of water, increasing the
transpiration rate. With plant c, the fan dried water vapor around the plant and in the
leaves, causing the area to be dry, thus creating a negative pressure for water in this
plant as well. Plant d had a very low rate of transpiration because its environment was
very moist. Water was very unlikely to evaporate in the misted enclosure, therefore
causing the plant only to need to replace the water which it used to maintain its turgor
pressure. The environment for plant a provided a normal room climate. Although
evaporation was likely, it did not seem to be a large factor in the plant's functions.
So, as water did escape from the stomata of the plant's leaves, the slow rate created
enough negative pressure to replace the water being lost to the air and being used by the
plant, which wasn't very much. 
When this experiment was initially done in our classroom, many faults occurred. Without
prior experience handling plants and petroleum jelly, the experiment is difficult. While
it is a good idea to see the experiment in order to understand it, the book provided the
best data.