Saturday, May 19, 2012


date: Thu, 29 Mar 2007 21:54:47 -0400
from: Frank Telewski <telewskiatXYZxyz.MSU.EDU>
subject: Re: [ITRDBFOR] divergence versus convergence


With all due respect, your understanding of stomatal function is very
simplistic. Water availability is not the only regulatory factor and is
not directly responsible for the turgor of guard cells. The turgor
pressure of the guard cells is regulated by alteration of the osmotic
potential of the cytoplasm (H and Ca ion transport across the plasma
membrane) in response to stimuli including ABA in terms of drought stress
or the internal CO2 concentration, resulting in the changes in Water Use
Efficiency, which is increased in elevated CO2 environments. Even
mechanical stress, such as that induced by wind sway, can result in a short
term closure of stomata. Mutants in the production of ABA or sensitivity
to ABA have very altered stomatal responses. This implies a genetic
component to stomatal function, so I'd be hard pressed to generalize that
"all guard cells work in the same way" with out really investigating each
tree one samples to verify that 'hypothesis'.


At 08:58 PM 3/29/2007, you wrote:
>You show me ONE erosional or depositional process that occurs at the same
>rate in all streams at all times I'll buy your argument. It doesn't happen,
>thus by your logic, uniformitarianism cannot be applied to most studies of
>geomorphology. I don't think Hutton or Lyell would have argued that.
>Quartz is made of silica. That much is a constant. But the size of the
>grains, the degree of cementation, the velocity of the water and the
>sediment content of the water all vary, thus rates of erosion and deposition
>WILL VARY from place to place and from time to time. The nature of the
>processes of erosion and deposition, however, do not.
>Guard cells do work in the same way -- whether they open or close the
>stomatal opening is dependent upon turgor pressure of the cells. Individual
>genetic variation may affect the speed and or intensity of response to
>changing moisture levels in the leaf, but it does not alter the nature --
>mechanics -- of the response to changing moisture levels in the leaf.
> David M. Lawrence | Home: (804) 559-9786
> 7471 Brook Way Court | Fax: (804) 559-9787
> Mechanicsville, VA 23111 | Email:
> USA | http:
>"We have met the enemy and he is us." -- Pogo
>"No trespassing
> 4/17 of a haiku" -- Richard Brautigan
>-----Original Message-----
>From: ITRDB Dendrochronology Forum [mailto:ITRDBFOR@LISTSERV.ARIZONA.EDU] On
>Behalf Of Hilary Stuart-Williams
>Sent: Thursday, March 29, 2007 8:42 PM
>Subject: Re: [ITRDBFOR] divergence versus convergence
>Nope, I don't think so. You have found yourself forced to resort to
>generalisations: stomata respond. Uniformatarianism is exact. Not
>"nearly the same" but "the same". If you could guarantee that the
>biochemistry and the genetics of the plant species were exactly the
>same, then I would agree. But you can't assert that. Quartz sand IS
>exactly the same over time - no genetic variation.
>David M. Lawrence wrote:
> > Uniformitarianism is perfectly appropriate here. Just because there are
> > individual variations in a process, whether in stomatal response to water
> > stress or in erosion and deposition rates of sandbars, doesn't mean that
> > cannot make generalizations of how the process works for all from
> > observations of how the process works in some. For example, stomates tend
> > to close in response to water stress as a result of loss of turgor
> > in the guard cells. In some individuals such closure will happen sooner
> > than in others, but the basic process works in much the same way -- and
> > probably has worked in much the same way since guard cells as we know them
> > first evolved.
> >
> > Dave
> >
> > ------------------------------------------------------
> > David M. Lawrence | Home: (804) 559-9786
> > 7471 Brook Way Court | Fax: (804) 559-9787
> > Mechanicsville, VA 23111 | Email:
> > USA | http:
> > ------------------------------------------------------
> >
> > "We have met the enemy and he is us." -- Pogo
> >
> > "No trespassing
> > 4/17 of a haiku" -- Richard Brautigan
> >
> > -----Original Message-----
> > From: ITRDB Dendrochronology Forum [mailto:ITRDBFOR@LISTSERV.ARIZONA.EDU]
> > Behalf Of Hilary Stuart-Williams
> > Sent: Thursday, March 29, 2007 7:19 PM
> > Subject: Re: [ITRDBFOR] divergence versus convergence
> >
> > Hi
> >
> > I'm not sure that I agree with the use of uniformatarianism here. As a
> > geologist (in a plant physiology group) I take it to mean that the same
> > physical and chemical systems will react in the same way in the past as
> > in the present. This is fine for sand bars and rivers, or even for
> > speleothem isotopes, but I am not so sure about organic systems. You
> > have to start by making the pure assumption that the systems are
> > identical. We know that two wheats that look identical when growing,
> > and certainly would look the same as fossils, have genetic differences
> > causing variation in their stomatal response and water use efficiency.
> > They are NOT the same and you cannot extrapolate the precise responses
> > of one from the other.
> >
> > I know that this is not a new question, and I am not attempting to
> > undermine dendro (or I wouldn't subscribe to this group) but I don't
> > think that uniformatarianism is the right word (or concept).
> >
> > Hil
> >
> >
>Hilary Stuart-Williams PhD
>Research Officer - Stable Isotopes
>Environmental Biology Group
>Research School of Biological Sciences
>The Australian National University
>ACT 0200 Australia
>Tel 02 6125 2099
>Fax 02 6125 4919
>Mobile 0421 905 478

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