Answer: Most strong-tasting or unusual chemicals contained within plant tissues seem to be related to pests. That is, the plant, in the course of its evolution, manufactured or concentrated certain substances within its tissues that had the effect of repelling certain pests. The plants with these substances succeeded better than the others, and therefore went on to reproduce.
Think of fiery hot peppers, or strong-flavored garlic, or even the nicotine in tobacco--all these have been used in one form or another by gardeners to repel insects, so it's a reasonable conclusion that the chemicals responsible for these substances had the effect of repelling pests for the plant too. I'm not sure that caffeine is one of these chemicals, but I wouldn't be surprised. I'm not sure you can give a definitive statement in your paper, but you might be able to suggest this as a possible reason that plants developed this substance.
Hope this helps.
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