Thigmotropism ("thigmo" meaning "touch" and "tropism" from the Greek tropi,
meaning "turn") is a term used to describe a plant's response to contact with
a solid surface, such as when climbing plants (like beanstalks or vines) grow
around poles. It sounds like your student observed an event which has been
described as "thigmomorphogenesis" (morphogenesis means development, from the
Greek "morpho" meaning "form" and "genesis" meaning "origin".) A plant
biologist named Mordecai Jaffe investigated this in the early 1970s by noting
the effects of touch on plants. He noticed that most plants responded to
repeated rubbing by developing shorter and thicker stems, resulting in plants
that are 40-60% as tall as plants that haven't been rubbed. In nature and in
agriculture, this can also occur when wind causes repeated flexing of stems.
More recently, some genes have been found in the commonly used research plant
Arabidopisis which "turn on" or are strongly expressed in response to touch.
(Email me if you want more information on this.)
As references, you might want to check out pg. 414-416 of the college-level
textbook "Plant Physiology, 4th ed" by Salisbury and Ross. If you have access
to scientific journals, you may want to refer your student to:
Jaffe, Mordecai J. (1973) Thigmomorphogenesis: The response of plant growth
and development to mechanical stimulation. Planta 114:143-157.
Jaffe, M.J. (1980) Morphogenetic responses of plants to mechanical stimuli or
stress. BioScience 30:239-243
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