AP Biology
Transpiration Lab
Background: Leaves constantly lose water and cause plants to dry out. Terrestrial plants
must move water from the soil into their roots; from the root tissue up through their stems
to their leaf tissue; and finally out through the stomata in leaves into the atmosphere. This
process is driven by evaporation, since loss of water in the leaves “pulls” on chains of
water molecules in surrounding tissue (xylem) all the way down to the roots.
The cohesive force created by hydrogen bonds between water molecules, and the
adhesive attraction of water molecules to xylem walls maintains these unbroken chains
and keeps the tension from breaking the chains of water molecules.
Evaporation lowers water potential in leaf tissuepulls water by osmosis from
surrounding tissuepulls on water in stemspulls on water in rootsroots pull in water
by osmosis from soil.
The loss of water from leaves is called transpiration, and is probably the greatest
movement of mass produced by living things on the planet. Large trees can transpire tons
of water a day in the summer. Transpiration creates morning clouds over tropical
rainforests that then produce rain each afternoon.
Plants can regulate the rate of transpiration by closing stomates. However once closed,
stomates block the uptake of CO2 and this interferes with photosynthesis. Many plants
have evolved metabolic ways of reducing transpiration without having to shut down
photosynthesis – remember CAM and C3 plants. Others have evolved structural features
to reduce transpiration, like thicker leaves, needle-shaped leaves, or no leaves at all.
Some have evolved ways of storing extra water in special tissues to get through droughts.
Aquatic ancestors of land plants, probably algae, colonized the land by gradually
adapting to drier terrestrial environments. Vascular tissues to carry water, stomates to
regulate transpiration, and waterproofing cuticle on leaves evolved in plants beginning in
the Silurian Period 438 million years ago. The adaptive radiation of plant taxa reached its
peak in the Mesozoic Era.
Purpose:
Measure the effect of three different environmental conditions on the
transpiration rate of plants by graphing % change in mass over time.
Hypothesis:
If bedding plants are exposed to different environmental conditions, then
their transpiration rates will change.
Experimental groups: windy, sunny, humid
Control group: ambient classroom atmosphere and light
Materials:
Four bedding plants
(each group) sandwich bags
Twist ties/string
Balance
Procedure:
Light bank
Fan
Atomizer
1. Carefully remove each bedding plant from its plastic pot pack without
damaging leaves or crushing stem.
2. Place a sandwich bag over each plant’s rootball and close gently but
tightly string
3. Create a data table and mass each plant. Record initial mass.
4. Place each plant in its environment:
Control: classroom location that receives normal light and air
Windy: classroom location with fan
Humid: Mist leaves with water and place leaves inside a second
plastic bag, secure with string/twist tie. Place in classroom location
Sunny: lightbank table.
5. Record each plant’s mass over five days, at the same time each day.
6. Calculate the % change in mass each day for five days for each plant.
1. Graph “The effect of environmental conditions on % change in Mass”
2. Write a detailed conclusion explaining how you believe each
environment affected the rate of transpiration. Be sure to describe the
environments used, document constants, explain any sources of error, and
note unexpected mishaps that might have effected your data.
*3. Using your text, research how succulents and cacti have evolved ways
of surviving in arid environments, and summarize your discoveries in a
well-organized paragraph.
*4. Explain why grass sometimes forms tiny droplets of water at the tip of
each blade in the early morning, even when there is no dewfall. Hint: look
up guttation). Summarize your discoveries.
*5. Growing plants increase in mass but do not consume food. Explain
three sources of this mass.
*Complete as pre-lab questions. Due tomorrow before lab.
Results:
Mass Data Table
Control (g)
Windy (g)
Humid (g)
Bright (g)
Windy
Humid
Bright
Day 1
Day 2
Day 3
% change in Mass Data Table
Control
Day 1
0
0
0
Day 2
Day 3
% change in mass = (ending mass – initial mass/ initial mass)100
Group Members:
Modified from: AP Biology Summer Institute, NC State University
Kathryn Weatherhead, Hilton Head High School
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