Why do beets invest in pigments buried underground where there
is no light, no photosynthesis etc? I guess we could ask the same thing
about carrots, radishes...
This is a fine example of a scientific concept often assumed without
knowledge (i.e. data). Don't be offended, we all often make the same
erroneous assumptions. The error is assuming that "correlation" is the
same relation between two variables as "causation". Plant pigments
perform other functions in the plant other than in the photosynthetic
cycle. This confusion that "correlation" is equivalent to "causation"
is so widespread we are often "tricked" before we know what the "real"
cause is. Examples are everywhere. For example, back in the early
1950's, before polio vaccines, the conventional wisdom was that polio
was "caused" by public swimming pools. In many communities public
pools were closed during the summer. As the actual viral "cause" of
polio was identified, it became evident that it was the close contact
between susceptible people. The swimming pool could have been any
other place where a large population, most of whom by coincidence were
young people, gathered in the summer time. There was a correlation,
but that correlation was not the cause. A good science exercise for a
class in science would be to search for examples where the two
concepts "correlation" and "causation" are confounded and confused.
The manner in which you ask the question suggests a conscious purpose
to the pigment, which I do not think is a valid view of the situation.
One could also ask why do we humans invest in pigment in our blood and
bile when we don't get to see them in the normal course of events
Many pigments serve more than one purpose - bile for instance, while
it is a dark green pigment solution, is actually there as part of our
digestive processes - it's color is an irrelevant side effect.
Hemoglobin is red, but its main purpose is the transport of oxygen.
While the colors in carrots and beets are impressive, they may have a
purpose other than just their color - and it may well be that they
actually serve to deter some insects and other animals from eating
them because of their TASTE - not their color. Even the green color of
leaves is not there to show off - its purpose is to trap light energy
for photosynthesis, so it is an error to think of the plant as
'investing' in pigments for their color.
Tennant Creek High School
Anthony R. Brach
Missouri Botanical Garden
c/o Harvard University Herbaria
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Update: June 2012