Jon Greenberg jongreen at bluemarble.net
Sun Jan 28 14:00:20 EST 2001

Thank you for your respnse, David. I'm a bit confused by all this. The question
about this lab activity was actualy raised by someone who had seen your paper.
In light of all this, I am still left with some basic questions:

1) How should we interpret experiments in which roots seem to grow toward water
in soil, or at least to grow more in wetter soil? Is this hydrotropism or isn't

2) If roots simply grow more in wetter soil because they can better maintian
turgor there that they need for growth, does it follow that hydrotropism is not
a true tropism any more than a growth repopnse to, say, N or other nutrients, or
is it the case on the other hand that this is merely the mechanistic explanation
of the response, but knowing how it works makes it no less a tropism?

(Please respond to my other e-mail address (and the list, if you like)--

jon.greenberg at phschool.com

Many thanks,

Jon Greenberg

David Hershey wrote:

> The recent research on hydrotropism that I have seen has been done with
> roots
> grown in humid air rather than in soil, thus "in humid air" must become a
> part
> of the modern definition of hydrotropism. Even at close to the soil
> permanent
> wilting point of -1.5 MegaPascals water potential, the soil relative
> humidity
> is about 99%. Based on the humid air hydrotropism research, it seems that
> there would not be a large enough relative humidity gradient in soil to
> induce
> a hydrotropic response. Therefore, hydrotropism seems more of a laboratory
> phenomenon rather than one important under natural conditions. It may have
> application in microgravity conditions of outer space.
> I wrote an article examining some of the classic precollege teaching
> experiments on hydrotropism:
> Hershey, D.R. 1992. Is hydrotropism all wet? Science Activities 29(2):20-24.
> >===== Original Message From jongreen at bluemarble.net (Jon Greenberg) =====
> >Hello again, plant folks.
> >
> >I am editing a high school biology lab manual that is under revision,
> >and came across an activity to demonstrate hydrotropism. I recall
> >learning in grad school about 20 years ago that roots grow toward water.
> >However, I understand that this has been questioned and some do not
> >agree that there is such a thing as hydrotropism.
> >
> >Can anyone enlighten me on this point/
> >
> >
> >---
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