Transpiration Lab (#9A, College Board lab Manual)
Objectives:
• To understand how water moves from roots to leaves in terms of the physical and chemical
properties of water and the forces provided by differences in water potential.
• To understand the role of transpiration in the transport of water within a plant.
• To understand the structures used by plants to transport water and regulate water movement.
• To test the effects of environmental variables on rates of transpiration using a controlled
experiment.
Introduction:
The amount of water needed daily by plants for the growth and maintenance of tissues is small in
comparison to the amount that is lost through the process of transpiration (the evaporation of water
from the plant surface) and guttation (the loss of liquids from the ends of vascular tissues at the margins
of leaves). If this water is not replaced, the plant will wilt and die.
The transport of water up from the roots in the xylem is governed by differences in water
potential (the potential energy of water molecules). These differences account for water movement from
cell to cell and over long distances in the plant. Gravity, pressure, and solute concentration all contribute
to water potential, and water always moves from an area of high water potential to an area of low water
potential. The movement itself is facilitated by osmosis, root pressure, and adhesion and cohesion of
water molecules.
The Overall Process: Minerals actively transported into the root accumulate in the xylem,
increasing solute concentration and decreasing water potential. Water moves in by osmosis. As water
enters the xylem, it forces fluid up the xylem due to hydrostatic root pressure. But this pressure can
