In article <1993Aug23.165518.23577 at Virginia.EDU> mgk2r at Virginia.EDU (Michael G. Kurilla) writes:
>I recall from my high school biology, that light sensitivity is
>the reason for leaves changing colors. Temperature is too
>subject to variation throughout the summer and fall. If it
>were temperature, then a cold August would cause trees to
>change colors early and a warm fall would slow the process. The
>amount of light is less subject to variation since only the
>presence of lots of clouds could effect a slight change.
>>Exactly what are the feedback mechanisms that allow trees to
>measure the amount of sunlight they receive? That is how do
>they integrate the light intensity? Do they measure the amount
>of CHO that they produce and below a certain thresehold decide
>fall is in the air?
There has been plenty of attention to efficient causes,
but little (to my surprise, considering the usual evolu-
tionary bent sci.bio takes) to final ones. My favorite
just-so story involving fall foliage is that it's a co-
evolved trait, one that's supposed to signal "EAT HERE"
to the great migratory flocks of seed-dispersers. A
cousin of mine did some slick calculations demonstrating
that there hasn't been enough time since the last glaci-
ations to have allowed for the fixation of such a
phenotype; I haven't been able to work around his ob-
jection yet. Anyone know the latest from the specialists
in this sort of thing?
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