Dear Straight Dope:
I am in a quandary. Why are citrus fruits are segmented? No one I have asked knows. This question is starting to really bug me. Can you help?
Each segment represents a separate functional ovary. Technically each segment is called a “locule,” and the entire combined structure is called an ovary, but that’s just botanical jargon. Many flowering plants have fruit, or more typically pods, divided this way at least internally (or something similar), but this particular “design” is under-represented in those fruits we eat most commonly. Many of the ones we’re most familiar with have the ovaries fused to various degrees–cut an apple in half and you can see it has five ovaries, but the partitions between them persist only near the center of the fruit where the seeds are. In citrus, the partitions are complete, is all.
As for asking why citrus fruit are different from other fruit, that’s like asking why ducks and chickens aren’t the same. They evolved along different paths, by a combination of accident and selection over millions of years. Artificial selection by humans in the last few thousand years hasn’t changed them all that much from their wild state, compared to the differences they had already acquired on their own.
The most unusual citrus variant developed so far is one called the "Buddha’s Hand," in which the locules are only in contact basally, and each "wedge" of the fruit is therefore nearly independent of the others, including having its own rind (it looks like a bunch of leathery yellow splayed fingers: http://florawww.eeb.uconn.edu/acc_num/199200352.html).
Maybe in a few hundred years someone will find and then begin to breed a mutant citrus with no segments. It just hasn’t happened yet, and may not ever.
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