MadSci Network: Botany
Query:

Re: Why are plants green?

Date: Mon Apr 12 13:09:04 1999
Posted By: Hurley Shepherd, Agricultural Research, USDA Southern Regional Center
Area of science: Botany
ID: 921712558.Bt
Message:

Trying to "outthink" natural selection is a dangerous proposition, because you never know what was going on at the time of selective processes which were going on millions of years ago. We look at it from our perspective and say "I could do better than that!" Why go through dinosaurs to get to birds? Why do some aquatic creatures spew vast amounts of their sperm into the water in hopes of fertilizing an egg? The answer is "Because it works that way". Evolution does not always end up with what we perceive as the best way for organisms to do something. It just proceeds one step at a time using the available resources (both environmental and genetic) to end up with survival, not just of the organism but of the offspring. The bottom line is survival, as you pointed out.

For plants, I can imagine reasons for starting with the pigment systems they started with. Perhaps the chlorophyll molecules were easy to make, since they were related to the cytochrome molecules which were already being made (and are found in all organisms), so only a few changes needed to be made to the end of the pathway to get chlorophyll instead of a heme- containing molecule, instead of a whole new pathway to evolve. And since they were probably aquatic organisms, and blue light penetrates water better than green, that was an added bonus. Then when the plants became terrestrial, another bonus--the pigment absorbs in the red range , too!

Once they were terrestrial, getting enough light was not usually a problem since most plants do not even use all of the blue and red light they receive. When they got crowded and started to colonize more marginal areas, natural selection took over to favor variants which optimize light resources. It is interesting to note that the same type of response--leaf movement--is used to get more light in some environments (leaves which turn to face the sun) and less light in others (leaves which turn their sides to the sun). But in general only plants which live beneath other plants (understory plants) have trouble getting enough light. Their adaptations include larger leaves, more chlorophyll, and anatomical changes.

Okay. That's my guess at what the plant was "thinking". Remember that evolution is a process and not a destination. What defines fitness is everchanging for an organism and we don't know all of the conditions that existed when the light harvesting systems were arising. Maybe plants will someday start to utilize more of the green wavelengths. I wouldn't be surprised.

For a good discussion on current evolutionary biology ideas, visit the Evolution site at the Virtual Library.


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