Cambridge International A Level Biology Revision Guide

Chapter 7: Transport in plants
Measuring the surface area of the whole leaf using
grids
It is useful to be able to measure the rate of transpiration per unit
leaf area. This allows valid comparisons to be made between
different species, different plants of the same species or different
leaves of the same plant. In order to do this, leaf area must be
measured. This can be done after the experiment. A method for
doing this is described below.
Grid paper is required (or graph paper) with 2 cm, 1.5 cm,
1.0 cm or 0.5 cm spacing. The smaller the grid size, the more
accurate the determination of the area, but the more time
consuming. Think about the best size to use before choosing.
The outline of the leaf is drawn on the grid (Figure
7.20a). Whole squares are identified first (Figure 7.20b)
and their combined area calculated. Squares only partly
occupied by the leaf can be treated in various ways. For
example, only count it if the leaf occupies more than half
the square, or find how many squares contain part of
the leaf and divide this by two.
a
b
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Figure 7.20 Measuring the surface area of a leaf.
QUESTION
7.6 a State three variables that must be controlled as
far as possible if comparing transpiration rates
from two different plants.
b Suggest how you could measure the volume of
water lost from the leaves during an experiment
to measure the rate of transpiration.