Hello Wayne and Ecologers,
My sense is that the quote is "sort of true" but that isn't
saying
much because it is couched in the most general terms. It talks about all
plants, but duckweed, saguaros, and oaks have such different environments
and body plans that many generalities will break down when applied to
them.
Another problem is the use of the word "precision." The very
organised appearance of many plant shoots, with their very regular
phyllotaxis, certainly seems precise, and the root systems I have seen
have
nothing like this geometric regularity. That however, doesn't mean that
the
placement of roots is not "precise" when analysed from the point of view
of
those roots fulfilling their function.
Also, the quote seems to assume that getting water for the plant
is
the only thing that roots do. However, they also, in many cases, serve as
anchors, as storage organs, and for absorption of nutrients, and these
functions may result in deployments in which water absorption is not
optimised. (There are even roots modified into defensive thorns.)
Your point number 2, that plants don't "place" their leaves, seems
to me to be more an issue of semantics than of biology. Sure, gravity and
wind act on plants and influence the placement of leaves. However, plant
genetic systems and anatomy have evolved in the presence to those forces,
in
a sense "counting on" those forces to interact in a certain way with the
plant's growth program. The human skeleton would probably develop very
differently if a baby were raised in the gravityless environment of the
space station, but I wouldn't use that to justify the statement that
skeletal development is not under control of the organism.
Lastly, I would point out that the deployment of leaves on a plant
is not something that is controlled at the level of the individual leaf,
nor
at the level of the individual shoot, but in the case of trees is subject
to
higher-level controls and interaction with the local (from the shoot's
point
of view) environment. Consider, for instance, the very different
deployment
of leaves on say, a maple tree, in low, horizontal-reaching twigs
(plagiotropic growth) and vertical twigs high in the crown (orthotropic
growth).
If each shoot in a tree crown grew in such a way as to maximise
photosynthetic return for *that shoot*, the result would be a crown that
grew less and less optimal at maximising photosynthesis for the whole
crown. For instance, simply "growing toward the light" would result in an
elongated crown pointing toward the light source, with the incoming light
running parallel to the long axis of the crown, and therefore shooting
right
past the sides of the crown. In this case, growing at right-angles to the
light source would result in broad umbrella-like crown that captured much
more light. Of course, the geometry of light-distribution in the
environment is quite complex, but consider the tall, thin spires of boreal
conifers, where much of the light comes from low in the sky, verse the
spreading crowns of savannah trees, where more light comes from high in
the
sky.
I go into this discourse about crown development because of the
comparison between roots and leaves made in the original quote, and to say
that such factors may also be at work in root systems. Deep in the soil,
perhaps near the water table, simply following a gradient toward
increasing
wetness may be a good program to follow to maximise water intake.
However,
near the soil surface, where water soaks down from above, the situation is
more analogous to leaves and light: here a shallow, outward-spreading root
system (growing at right angles to the resource flux) can absorb
downward-soaking rainwater before deeper roots have a chance, just as
higher
leaves can absorb light before it reaches lower leaves.
In summary, I think the weakness of the quote lies in its
generality, but if you went into the kind of detail I just did, how may
televiewers would still be watching? (How may of you ecologists are still
reading?)
Thanks, Wayne, for you stimulating contribution to the listserv.
Martin M. Meiss
2010/11/20 Wayne Tyson <landr...@cox.net>
Ecolog:
As Kent has pointed out, the quote apparently is from David
Attenborough's
book; I found it on a website concerning "biomimicry."
http://www.asknature.org/strategy/2386af151126ef7579cf5c506b6dc7de
I have also received some off-list comments that lead me to be more
direct,
although I did not want to add my own opinions initially, trying to keep
it
simple and short, without bias. So here's my take on the paragraph, and I
would like to stand corrected where I am wrong or off-base. Thanks to all
who have responded thus far.
First, Attenborough has done a lot to foster appreciation of Nature via
film and print, and I do not want this to appear disrespectful. On the
other
hand, if people are being mislead about how plants function, perhaps
there
are Ecolog subscribers who have both the competence and the status, not
to
mention the courage, to set us all straight, including, if necessary,
Attenborough.
1. It is my understanding that plants do not "find" water, nor do they
"seek" it; they develop where conditions conducive to root development
exist--where such conditions do not exist, they do not develop. For
example,
if my understanding is correct, the most basic requirements for plant
root
development are the presence of available water in sufficient quantity
and
pore spaces at least equal to the diameter of the root tip. I don't
remember
(if I ever knew) how much energy a root tip can exert when growing
forward,
but have a dim memory that it was relatively slight. Of course roots seem
to
be able to exert very high pressures laterally, but I don't have recent
research on either. I am acutely sensitive to the fact that some of the
"facts" I "learned" in years past, or those which are simply wrong, may
constitute a flawed personal paradigm, so I hope y'all will fill me in on
the latest references.
2. It is also my understanding that plants do not "position" their leaves
"for" optimal absorption of light, but that photochemical reactions,
gravity, and other forces are involved in complex processes that result
in
what an observer, unaware of those conditions, might attribute to perhaps
an
interesting bit of "cleverness" on the part of the plant.
3. I am not aware of any mechanism possessed by any plant that can cause
it
to "drive a tap root" deep into the ground "to reach" THE water[-]table.
I
have investigated profiles where roots (of phreatophytes) were confined
to
the upper soil profile even though a water-table existed at a greater
depth
and had not developed through an intervening part of the profile of
relatively dry soil after many years of light rainfall seasons which were
not sufficient to produce a wetted profile to the depth of the capillary
fringe of the water table. Of course, observations are not controlled
experiments. The third sentence seems to comport with the theory I have
learned, with the exception of the implication that they "had" to develop
root systems "far deeper than they are tall." Though this seems quite
true,
the implication that there might or should be some relationship between
height or spread of the above-ground part of the plant seems odd, since I
know of no reason why there should be a connection--both above-ground and
below-ground parts of plants develop according to the conditions of their
environment, hence both above-ground and below-ground environments that
are
quite different. Similarly, the areal extent of root systems and their
depth
beneath the surface are a function of the presence of conditions
favorable
to root growth, not an expression of some "competitive strategy" on the
part
of the plant.
This is not to pick on Attenborough, but it does serve as an example of
science writing that may go a bit too far with conclusions, when a
discussion of the actual facts of how plants work would not have only
been
more informative but more interesting. There is no need to write down to
"the general public," and science writers have a responsibility to make
sure
that their writing does not "lead them down the garden path," as it were,
especially as the general population contains a fraction who may go
beyond
entertainment and with whom the opportunity exists to lay a firm and
factual
foundation for their growth into scientists.
So there, as briefly as possible, is my own take on this particular
paragraph. It has caused me to pay closer attention to science writing in
general and to Attenborough in particular, because he is so widely
respected
in the field. I recently watched one of his nature films in which he
advanced the idea that species "advanced" through evolution. That left me
wondering how many people think that evolution is a process by which a
species "improves" or "progresses" over time rather than adapts to
changing
environments--a concept which seems to be common amongst non-evolutionary
biologists. It seems to me that getting it right is not the
responsibility
of the public, but the responsibility of scientists and science writers.
At
least such writing should not be misleading. And, it seems of vital
importance in interpreting ecology to the world at large.
I look forward to your corrections.
WT
----- Original Message ----- From: "Wayne Tyson" <landr...@cox.net>
To: <ECOLOG-L@LISTSERV.UMD.EDU>
Sent: Thursday, November 18, 2010 12:15 PM
Subject: [ECOLOG-L] Roots find water
Ecolog:
Please advise whether or not the following quote is completely valid; if
not, what specifically is invalid and why :
"To find water, a plant has to position its roots with just as much
precision as it arranges its leaves. If moisture is in very short supply,
then a plant may have to drive a tap root deep into the ground to reach
the
water table. Some desert plants have had to develop root systems that are
far deeper than they are tall and extend laterally a very long way beyond
the furthest extent of their foliage. Even if the environment is
well-watered, a plant may still need to compete with others for this
essential commodity, so it positions a network of roots within a few
inches
of the soil surface, where it can gather the rain water before others
can."
WT
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