Preface to the Second Edition
are used. Also, molar units are becoming widely accepted in biological
science disciplines for excellent scientific reasons (e.g., photosynthetic
light reactions clearly are driven by photons of light and molar units are
required to describe this process.) A coherent view of the connectedness
of biological organisms and their environment is facilitated by a uniform
system of units. A third reason for using molar units comes from the
fact that, when difisive conductances are expressed in molar units, the
numerical values are virtually independent of temperature and pressure.
Temperature and pressure effects are large enough in the old system to
require adjustments for changes in temperature and pressure. These temperature and pressure effects were not explicitly acknowledged in the
first edition, making that approach look simpler; but students who delved
more deeply into the problem found that, to do the calculations correctly,
a lot of additional work was required. A fourth consideration is that use
of a molar unit immediately raises the question "moles of what?" The
dependence of the numerical value of conductance on the quantity that
is diffusing is more obvious than when units of m/s are used. This helps
students to avoid using a diffusive conductance for water vapor when
estimating a flux of carbon dioxide, which would result in a 60 percent
error in the calculation. We have found that students adapt readily to the
consistent use of molar units because of the simpler equations and explicit
dependencies on environmental factors. The only disadvantage to using
molar units is the temporary effort required by those familiar with other
units to become familiar with "typical values" in molar units.
A second convention in this book that is somewhat different from the
first edition is the predominant use of conductance rather that resistance.
Whether one uses resistance or conductance is a matter of preference,
but predominant use of one throughout a book is desirable to avoid confusion. We chose conductance because it is directly proportional to flux,
which aids in the development of an intuitive understanding of transport processes in complex systems such as plant canopies. This avoids
some confusion, such as the common error of averaging leaf resistances
to obtain a canopy resistance. Resistances are discussed and occasionally used, but generally to avoid unnecessarily complicated equations in
special cases.
A third convention that is different from the fist edition is the use of
surface area instead of "projected area." This first appears in the discussion
of the leaf energy budget and the use of "view factors." Because many biophysicists work only with flat leaves, the energy exchange equations for
leaves usually are expressed in terms of the "one-sided" leaf area; this is
the usual way to characterize the area of flat objects. If the energy balance
is generalized to nonflat objects, such as animal bodies or appendages,
tree trunks or branches, or conifer needles, then this "one-side" area is
subject to various interpretations and serious confusion can result. Errors
of a factor of two frequently occur and the most experienced biophysicist has encountered difficulty at one time or another with this problem.
We believe that using element surface area and radiation ''view factors"
are used. Also, molar units are becoming widely accepted in biological
science disciplines for excellent scientific reasons (e.g., photosynthetic
light reactions clearly are driven by photons of light and molar units are
required to describe this process.) A coherent view of the connectedness
of biological organisms and their environment is facilitated by a uniform
system of units. A third reason for using molar units comes from the
fact that, when difisive conductances are expressed in molar units, the
numerical values are virtually independent of temperature and pressure.
Temperature and pressure effects are large enough in the old system to
require adjustments for changes in temperature and pressure. These temperature and pressure effects were not explicitly acknowledged in the
first edition, making that approach look simpler; but students who delved
more deeply into the problem found that, to do the calculations correctly,
a lot of additional work was required. A fourth consideration is that use
of a molar unit immediately raises the question "moles of what?" The
dependence of the numerical value of conductance on the quantity that
is diffusing is more obvious than when units of m/s are used. This helps
students to avoid using a diffusive conductance for water vapor when
estimating a flux of carbon dioxide, which would result in a 60 percent
error in the calculation. We have found that students adapt readily to the
consistent use of molar units because of the simpler equations and explicit
dependencies on environmental factors. The only disadvantage to using
molar units is the temporary effort required by those familiar with other
units to become familiar with "typical values" in molar units.
A second convention in this book that is somewhat different from the
first edition is the predominant use of conductance rather that resistance.
Whether one uses resistance or conductance is a matter of preference,
but predominant use of one throughout a book is desirable to avoid confusion. We chose conductance because it is directly proportional to flux,
which aids in the development of an intuitive understanding of transport processes in complex systems such as plant canopies. This avoids
some confusion, such as the common error of averaging leaf resistances
to obtain a canopy resistance. Resistances are discussed and occasionally used, but generally to avoid unnecessarily complicated equations in
special cases.
A third convention that is different from the fist edition is the use of
surface area instead of "projected area." This first appears in the discussion
of the leaf energy budget and the use of "view factors." Because many biophysicists work only with flat leaves, the energy exchange equations for
leaves usually are expressed in terms of the "one-sided" leaf area; this is
the usual way to characterize the area of flat objects. If the energy balance
is generalized to nonflat objects, such as animal bodies or appendages,
tree trunks or branches, or conifer needles, then this "one-side" area is
subject to various interpretations and serious confusion can result. Errors
of a factor of two frequently occur and the most experienced biophysicist has encountered difficulty at one time or another with this problem.
We believe that using element surface area and radiation ''view factors"