How do plants turn towards light?

Cormac Shaw cshaw at
Tue Feb 4 07:02:35 EST 1997

Dear netters,
     I was aware of the facts of phototropism and how plants GROW
toward light. However, is there not the case of plants turning their
leaves to follow the sun as it tracks across the sky. Out of
curiosity, could someone explain the mechanism of this related
phenomenon? Perhaps the original poster would be interested, too.

(A microbiologist).

On Tue, 04 Feb 1997 09:00:48 +0000 (GMT)
 marder at (Jonathan B. Marder) wrote:

>In article <32F6120F.7CEA at>, Fredrik Loenn <f92fl at> wrote:
>>Plants turn towards the sun (or other illumination), because the cells
>>not exposed to the light, split more quickly. This makes the plant bend
>>against the light.
>Did you know that Darwin worked on this. The response is called 
>"phototropism". The version Darwin characterized was phototropism of cereal 
>coleoptiles (the coleoptile is the sheath which protects the emerging first 
>leaves). This bends towards the light because of unequal *elongation* of 
>cells on opposite sides. Light (prinicipally blue) is perceived in the apex
>and causes unequal transport of the plant "hormone" auxin down the coleoptile, 
>so that more auxin reaches the non-lit side. This causes them to elongate more
>rapidly. The details of this phototropic response are described in 
>undergraduate Plant Physiology textbooks.
>Jonathan B. Marder             ,      Department of Agricultural Botany
>E-mail: MARDER at |      The Hebrew University of Jerusalem
>Phone: (08 or +9728) 9481918   | /\/  Faculty of Agriculture
>Fax:   (08 or +9728) 9467763   |/  \  P.O.Box 12, Rehovot 76100, ISRAEL
Cormac Shaw, BSc,                 |
Dept. of Industrial Microbiology, |  e-mail: cshaw at
University College Dublin,        |   phone: +353 (1) 706 1796
Dublin 4, Ireland                 |     fax: +353 (1) 706 1183
"Under the most rigorously controlled conditions of pressure,
temperature, volume, humidity, nutrients and other variables,
the organism will do as it pleases."

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