Function of heliocentric positioning of flowers

Thomas E. Yancey tey7004 at GEOPSUN.TAMU.EDU
Tue May 13 10:04:25 CDT 1997

I would like to solicit thoughts from listmembers about the functional purpose
of heliocentric positioning of flowerheads.

As I walked to school this morning (a very pleasant, post-rain, high pressure
day with bright sunshine) I noticed that a composite wildflower which is common
in our area shows a consistent orientation of flowerheads towards the sun, or,
if in a shaded area, towards the brightest portion of the sky. The flower has a
bright yellow coloration. Only the flowerhead and immediately underlying part of
the stem are involved in heliocentric positioning. Some unopened flowerheads
also show the heliocentric positioning, but post-bloom heads housing developing
seeds do not. Other portions of the plant orient straight up. Other annual
flowers growing in the same area do not show heliocentric orientation.

Inasmuch as the major chlorophyll-bearing portions of the plant (food producing
portions) do not engage in heliocentric positioning, why do the open flowerheads
do this? Despite the prevalence of yellow pigment, is photosynthesis taking
place in the flowerhead to the extent that orientation towards the sun is
important? Is this a mechanism for increasing reflectivity, to attract
pollinators? Can anyone suggest a functional explanation for this. The
"behavior" also occurs in sunflowers, so there should be some understanding of
it. I was surprised at how mysterious this aspect of plant growth seems.

T. Yancey

Thomas E. Yancey                                            _______
Department of Geology and Geophysics                       |   |   |
Texas A&M University                                        _  |  _
College Station, TX 77843-3115                             |-| | |||
Voice: 409 845 0643    Fax: 409 845 6162
email: tyancey at

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