Monique Reed monique at
Mon Nov 17 09:28:43 EST 1997

>At 1:31 PM -0500 11/15/97, Carl Pike wrote:
>>In our class discussion of the photoperiodic control of flowering, after I
>>presented the experiments showing (by means of night interruption) that
>>plants are timing the length of the night, rather than the length of the
>>day, a student asked **why** (in an evolutionary sense) it is the night
>>that is timed.

Oh, heck.  That is going waaaay back in plant physiology.  If I remember 
correctly, it has to do with a light-sensitive chemical (phytochrome?) that 
is switched from one form (red) to another (far-red) in the light and which 
slowly changes back in the dark.  To trigger flowering, the right amount has 
got to change back.  The change from dark form to light form, though, can be 
very rapid.  This is why a flash of light at night will inhibit flowering in 
"short-day" (actually, long-night) plants like poinsettias.

I guess this is useful to the plant because while your day might be 
interrupted, once it's night, it's dark until morning.

M. Reed
(trying frantically now to remember how plants deal with moonlight....)

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