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 <[email protected]>
> 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" <[email protected]>
> To: <[email protected]>
> 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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