Why is the elixir of the octogenarians called “prune juice” instead of “plum juice”? I mean, we don’t call grape juice raisin juice. Or, is it that prune juice doesn’t come from plums, but is actually somehow extracted from already dried prunes?
Many people think that dried prunes are actually dried plums. This is because the stores don’t generally sell prunes that aren’t dried, at least not under that name. I guess that’s because they juice them instead.
Prunes are a closely related fruit, but they are not plums. We had a prune tree in the yard when I was a kid, and we usually ate them fresh, rather than drying them. Yummy.
I really think that’s incorrect. I know there’s a particular variety of plum that is ideal for use as prunes, but my dictionaries confirm that prunes are dried plums.
http://www.stapleton-spence.com/CustomerService/pjmfgr.htm
The web page goes on to describe the actual juice making process. Basically the prunes are heated in water until done.
(sung to “Wild Thing”)
Prune juice!
You make my bowels loose!
You make EV-rything pass THRU me!
Prune juice!
Prune juice … I think I love you.
Because prune juice is made from prunes. Plum juice would be made from plums (I’ve never actually seen it, and it might also be called “plum nectar”).
But prunes are plums, you say. Correct, but the juice is only made from the dried fruit. There’s no “raisin juice” because juice is not made from the dried grapes, it’s made from grapes that haven’t been dried.
Speaking as a gardener, there are at least a dozen different kinds of plums. Check this out.
http://www.johnmortonltd.co.nz/plums1.html
This isn’t quite the same thing as saying there are different kinds of apples. Granny Smiths are pretty much the same as a Gala, just green, and more sour.
But the different kinds of plums, golly. Don’t go by the extremely limited selection you see at the grocery store. Plums come in all different sizes and colors, green and yellow and red and purple and blue, some maturing earlier, some later, some for eating out of hand and some for drying and turning into prunes.
So, yes, prunes are made from plums–a very specific kind of plum.
Saltire, your tree was a “prune” tree. It was the kind of plum tree whose fruits are well-suited for drying, but you can eat them fresh, too.
Check this out.
http://fpms.ucdavis.edu/Tree/PlumPruneVariety.html
That “707 Prune” must be good, because they’ve kept it around since 1972. And the “Italian Prune” sounds interesting. But oh, that “French Prune”! Ooh la la! 
From T.E.'s prune juice link:
FTR, prunes are now officially being called “dried plums” by the California Prune Board (which, incidentally, is being changed to the California Dried Plum Board).
Mark your calendars accordingly.
Well, now I know. Thanks to everyone who enlightened and entertained me/us.