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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