Consider the energetics of an isolated
tree or a red germanium plant in a window. Why does the tree grow toward
the sun and have a symmetrical shape? Why do the germanium leaves
press hard against the glass? The obvious answer is to capture sunlight-energy
and turn it into biomass and seed to ensure the plant's continuance
as a species. The seeds grow and produce seed to be more fit. Fitness
in Darwinian jargon is a measure of reproductive success that includes
such factors as differential mortality, viability, mating drive and
success, and differential fertility and fecundity.
It is obvious that the tendency to grow and go
to seed is seated deeply within the genetics of the angiosperm or flowering
plant and carried forward from the past with all the verve of a telos, a goal. The architecture and color of leaves, the shape of trees, the
combined canopy of trees—all
these common features of woods look as if designed to capture as much
sunlight as possible. Go outside and look at the trees in your neighborhood.
Each species has a distinct shape. Each organism adapts to its local
environmental constraints. Most trees grow symmetrically, but pruning
by wind and accident may change their shape. Even with dramatic exogenous
shaping by the wind, the basic symmetrical shape of a tree can be seen
through its twisted branches. Most trees are symmetrical to their trunk,
but often two or more main leaders of a tree will grow together as one.
Often groups of trees of differing species will grow with symmetrical
shapes as they share solar resources. The tree-sun relationship is perhaps
the strongest, simplest, and most pertinent example of our thermodynamic
paradigm. Trees "reaching" for the sun and optimally capturing
and degrading the gradient between the sun and frigid outer space seems
to graphically incarnate our vision of the thermodynamic part of the
biological world. Go out and observe trees, and you will see living dissipative
systems stretching skyward to capture available solar energy.
One can imagine, and imagine you must, because you cannot see this process
with your naked eye, that our tree in the middle of a field is a giant
dissipative structure capturing high-exergy sunlight and degrading most
of that energy as respiration and low-grade latent heat via transpiration.
It is like a giant water fountain spewing water in the form of latent
heat. It is like a candle burning high-exergy waxes (the flame burns
high-exergy chemical bonds) and degrading that high-exergy fuel into
low-grade heat that you cannot feel across the dinner table. It seems
that 1% of the plant's energy goes into the tiny photosynthetic engine
that controls these immense dissipative systems. In spite of the lopsided
ratio favoring dissipation over growth, we rarely think of an oak tree,
with its hard wood, leaves, and acorns, as a physical structure designed
to perform a photosynthetic process. In reality, however, a tree is best
understood as a giant degrader of energy. Its imposing structure is,
comparatively, secondary to its solar degradation activities.
Trees actively send out their roots and leaves
to capture energy and water, two ingredients needed for increased dissipation
by the tree. If kinetic and or dynamic conditions
permit, organizational processes are to be expected. Each new leaf, each new phototrophic
rearrangement, is a new opportunity for energy degradation. In short,
the Cartesian statement "I think therefore I am" becomes "I am because
I dissipate." The leaf arrangement of individual plants is a statement
as to the teleomatic drive to capture and degrade energy. Even more amazing
is the arrangement of limbs, branches, and leaves of different species
within a forest. They seem to have followed a choreographer's instructions.
Collectively they seem arranged into groups to gather the most energy
for all of them. On dense forest floors, plants with wide leaves collect
the last remnant of sunlight that filters to the forest floor. Growth
in plants is a thermodynamic phenomenon as well as a Darwinian process.
Back to top >
|
 |
Part
III: The living
11.
Thermodynamics and Life
12.
Brimstone Beginnings
13.
Blue Planet Blues
14.
Regress under Stress
15.
The Secret of Trees
16.
Into the Cool
17.
Trends in Evolution
These trees are species with differing structure and canopy, and differing strategies
for optimizing the capture of solar energy. The symmetrical shape of single trees
bears witness to this solar strategy and is a manifestation of the second law
of thermodynamics. Genetic information endows each species with its basic shape,
but environmental factors like wind, crowding, and altitude prune and alter its
original character.
During daylight hours solar radiation impinges on the Earth's surface at a rate
of about 800 watts per square meter. Only 1% of the radiation that hits a tree
is turned into biomass like wood and leaf. Eighteen percent of the energy hitting
plants is converted to sensible heat, and 15% is reflected. By far most of the
energy expended by plants, about 66%, goes toward evapotranspiration, the conversion
of water into latent heat. A well-watered, typical twenty-meter-tall deciduous
tree will transpire one hundred kilograms of water a day. This process requires
58 million calories. Trees are giant dissipative systems that take in high-quality
energy, ultraviolet and visible radiation, and release most of that energy in
low-quality energy, latent heat.
|
