Download biology, I certainly believe that plu- ralism is

Books
biology, I certainly believe that pluralism is appropriate when assessing
textbooks. There are many different
virtues that one might aspire to in
an introductory text, including brevity, clarity, insightfulness, detailed
examples, and rigorous philosophical argumentation. Godfrey-Smith’s
text rates very highly on the first
three desiderata: It provides a clear
and concise introduction to the philosophical issues that arise in connection with biology. Readers will
encounter stimulating and fresh perspectives on central topics, delivered
by an author with a remarkable command of the field. But those who
hunger for detailed examples and a
richer assessment of arguments may
be better served by other texts.
TODD GRANTHAM
Todd Grantham (granthamt@cofc.
edu) is a professor and chair of the
philosophy department at the College
of Charleston, in Charleston, South
Carolina.
doi:10.1093/biosci/biu206
WHY PLANT PHYSICS?
Plant Physics. Karl Niklas and
Hans-Christof Spatz. University of
­
Chicago Press, 2012. 448 pp., illus.
$60.00 (ISBN 9780226586328 cloth).
K
arl Niklas and Hans-Christof
Spatz, both highly respected when
it comes to the structural analysis of
plant tissues, have brought forward a
significant addition to the literature on
plant structure. Plant Physics presents
a comprehensive overview of the physics relevant to the structural economy
of land plants. The topics covered
include a concise introduction to the
principles of structural mechanics,
fluid dynamics, and electrophysiology,
with in-depth coverage of plant–water
relations and environmental biophysics. The topics are amply referenced,
and the book concludes with chapters
http://bioscience.oxfordjournals.org
on experimental and theoretical tools
and a glossary.
But why should you have a book
on plant physics on your shelf? Isn’t
plant structure simply the downstream manifestation of complex but
familiar cellular and molecular mechanisms working within a particular
set of physical constraints, or is there
something more fundamental that sets
plants apart?
In animals, the emergence of biological form during embryogenesis
and growth is hugely complex. Signal
processing, rapid nerve transmission,
and muscular control must all be coordinated during development—never
mind the various kinds of social interaction and consciousness that animals
might enjoy. The prospect of articulating a developmental narrative that
can encompass all of these processes
is probably still unattainable, which
forces us to deal with each subsystem individually, as its own discipline
(Niklas 2012). But in the plant kingdom, development is more transparent. This is because of the constraints
that the cellulosic cell wall has placed
on the evolution of morphospace. In
the land plants, all somatic cells are
permanently trapped within a continuous fabric of rigid ­cellulosic cell walls,
which thereby eliminates any possibility of cell migration. One consequence
of the universal presence of the cell
wall is that land plants, rooted in what
is essentially a freshwater environment, routinely develop cellular turgor
pressures that can only be considered extreme by metazoan standards.
Pressures of up to 1.0 megapascal are
commonplace in expanding plant cells.
The plant body can be regarded as a
collection of pressure vessels embedded in a more or less rigid matrix,
whose properties have to be finely
tuned in order to control growth. Cell
­division becomes restricted to the terminal meristems where new cells are
formed, just as adding new floors at
the top of a skyscraper creates new
living space. Construction of the plant
body becomes something of an engineering problem.
Furthermore, although plants
and animals have similar cytoskeletal mechanisms for chromosomal
assortment, cell division in the plant
kingdom is functionally very different
from animal cell division. In the plant
kingdom, the products of mitosis are
permanently bonded to one another,
with the whole plant forming a single
mechanically continuous structure.
Cells divide by precisely orienting new
partition walls, much as an architect
would erect a new partition across an
existing room, establishing a spatial
relationship between daughter cells
that can never change. Animal cell
mitosis, however, involves a cell essentially pinching in two, after which the
two “daughter” cells are, at least in
principle, free to move independently
to create new neighborhoods and to
foster functional relationships in ways
that are unavailable to plant cells.
Morphogenesis can involve cell migration and flow. But in the land plants, in
which cells are pressurized and frozen
in place, the development of form
becomes an architectural process,
in which the permanent installation
of precisely oriented partition walls
is necessary to support the turgordependent tissue stresses that radiate
throughout the growing tissues.
So while evolutionary constraints
have resulted in a developmental paradigm that seems simpler than that of
animals, they have also endowed plants
with a suite of mechanical and biophysical tools that are largely unavailable to
animals. This amounts to a unique evolutionary context, a structurally based
information system that can inform
developmental events with a spatial
February 2015 / Vol. 65 No. 2 • BioScience 215
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and temporal resolution accessible only
to physical and mechanical systems. It
should come as no surprise that since
the emergence of plants onto the land
surface perhaps a billion years ago, the
evolution of plant morphospace has
been shaped by physical mechanics and
the nanostructural behavior of the wall
itself. Plant growth and morphogenesis
is increasingly being regarded as being
coordinated and regulated by physical
and mechanical feedbacks working at
the tissue level. (Sampathkumar et al.
2014).
Physics and physical mechanics are
abundantly evident everywhere we
look in the plant kingdom—whether
in the tallest trees, rising more than
100 meters above the land surface, or
in the energy-storage and vibrationdamping characteristics of a finely
crafted split bamboo fly rod. At the
level of basic research, we are beginning to appreciate the full extent to
which the behavior of plant cells and
tissues is intimately tied to the biomechanics and biophysics of the cell wall
(Niklas 2014). It is becoming increasingly clear that the evolution of plant
form has been driven by innovation in
the m
­ aterial properties of the cell wall
at the u
­ ltra- and n
­ anostructural levels.
Plant developmental ontogeny turns
out to be profoundly different from
that of animals and should be regarded
as a sui generis morphogenetic paradigm that must be understood on
its own terms. It is possible, in fact,
that the land plants may offer us our
first opportunity to articulate a comprehensive narrative that encompasses
molecular and genetic subsystems
while framing the flow of morphogenesis within a clear biophysical context
(Lintilhac 2014).
Niklas and Spatz approach the
material as experts who have dealt
with most of the salient biomechanical issues of plant structure, hydrodynamics, and aerodynamics. They
explore the physics of gas exchange,
transpiration, fluid dynamics, structural mechanics, membrane electrophysiology, the strength and acoustic
properties of wood, and more. But this
is not just a textbook. For the plant
biologist, this book will be an invaluable reference for many years. For the
physicist, the attraction of this book
may be the breadth of topics covered
and the implication that physics lies at
the very heart of plant developmental
biology. As biologists, we have coveted
what physics takes for granted, which
is a theoretical continuity between the
symmetries of matter and energy at
the subatomic level and our growing
understanding of the organization of
the cosmos as a whole. Can we contemplate a time when a similar continuity might emerge in the biological
sciences?
References cited
Niklas KJ. 2014. The evolutionary-developmental origins of multicellularity. American
Journal of Botany 101: 6–25.
Sampathkumar A, Yan A, Krupinski P,
Meyerowitz EM. 2014. Physical forces regulate plant development and morphogenesis.
Current Biology 24: R475–R483.
Lintilhac PM. 2014. The problem of morphogenesis: Unscripted biophysical control systems in plants. Protoplasma 251: 25–36.
PHILIP LINTILHAC
Philip Lintilhac (philip.lintilhac@uvm.
edu) is a research associate professor in
the Department of Plant Biology at the
University of Vermont, in Burlington.
doi:10.1093/biosci/biu191
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216 BioScience • February 2015 / Vol. 65 No. 2
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