Thanks, Mangetout, but the shell doesn’t really address the internal cavity. I’m thinking gasses have got to play a role here. What the hell is the vapor pressure of ethylene? Is the inside of hollow fruits filled with gaseous plant hormones? Could be. Don’t know. Might kinda make sense.
There’s empty spaces around seeds in ripe beans and many peas. Ditto peanuts.
A lot of tree nuts have spaces. E.g., walnuts, pecans, and filberts a little bit. Sweetgum balls are quite hollowish.
Another squash with a lot of space when fully developed are loofahs. At the end they are basically papery skin, some seeds and, well, a loofah.
Again, with the bell pepper, you are seeing the result of hundreds to thousands of years of human selective breeding, not natural evolution. The ancestors of all peppers would have been small fruits with small spaces around the seed–it is only because of human breeding that the hollow space has became huge as the fruit becomes huge becases the “flesh” of the fruit doesn’t grow in thickness in proportion to the fruit growing in volume. That’s likely the thing with pumpkins and peppers–there are genes to work with to make the fruit huge, but not genes to make the flesh proportionaly thick. (Volume grows by 10,000%, flesh thickness grows by 1,000%, result much bigger hollow.) If it were possible to grow huge but solid peppers and pumpkins (without modern gene-level editing tools) I’m sure they would have shown up in breeding sooner or later and kept in cultivation. As for why that happens, who knows? Just some quirk in the genetics of the wild ancestor.
This is somewhat like dogs and cats–look at the huge variety of relative body part proportions in dogs–relative size of skulls, length/width of snouts, length of legs, etc., not to mention behavior. Show multiple breeds of dogs to an alien and they might think that they are different species with millions of years between them. On the other hand, look at cats. As many breeds as they are, they all look and act more or less pretty much the same–no greyhound cat, no dachshund cat, no “pointer” cat. Sure, some of that may be because nobody has tried to breed a cat with a long snout with long spindly legs, but a big part also has to be in genetic plasticity.
They’re bred by humans to be suitable for carving.
Calabashes are another hollow squash. People use them to make canteens, bowls, and other storage devices.
OK, let go over the quesrtion one more time. Regardless of human agriculture, why are some fruits hollow? Why are their spaces in some fruits around the seeds? “Breeding” is a lazy answer. Why did the wild plants have spaces around the seeds?
Honestly, people. This is like if I asked why canines have a good sense of smell and all you could talk about is how bloodhounds are bred from wolves.
But why shouldn’t they have spaces around the seeds? It may simply be because of the way the fruit grows. There doesn’t have to be a positive adaptive reason why there are spaces around the seeds.
In the classic paper The Spandrels of San Marco and the Panglossian Paradigm Stephen Jay Gould and Robert Lewontin pointed out the fallacy of looking for an adaptive reason for every feature of an organism. The spandrels of St. Marcos Cathedral are features that appear to have been deliberately designed for their own sake but are simply a side result of the architectural structure. Similarly the growth pattern of a fruit may result in empty spaces, even though there is no adaptive reason for them (and no selective reason against them).
My favorite example of such a “spandrel” is the human butt-crack. You may well ask the adaptive reason why we have such a prominent crease in our backsides. But really, the adaptive feature is our hypertrophied gluteus maximus muscles, which enable us to stand erect. The butt-crack is simply a result of having such large expanded muscles side by side, and has no adaptive reason itself.
Insulation against fluctuations in temperature. Like I said - the pumpkin is a seed bank for the pumpkin seeds.
Full paper here (PDF.)
Breeding is entirely relevant–the naturally evolved precursor plants have small spaces that are filled entirely with seeds and pulp and are not superior or inferior to having that space filled with seeds and flesh. It is only human selective breeding that has blown the small spaces up to be a large percent of the fruit’s volume. And other vegetable varities derivedby humans from the same wild plant don’t have giant hollow spaces. Pumpkins have giant hollow spaces because they have been bred by humans to grow vastly larger than they evolved to grow and grow that way in the same few months that the tiny natural forms grew, and nobody put any priority on producing breeds of pumpkins that produced enough useless pulp to pack the interior full. (I have cut open other types of cultivated vegetables that have had a void in the center because of rapid growth–watermelons, tomatoes, and potatoes at least come to mind.)
(And I just thought of another edible wild fruit that has non-flesh-filled seed cavities that is native to my area–the maypop. (For the plants that survive ritual flower mutilation.))
An interesting curiosity here is the Osage Orange - a tree that produces large lumpy green fruits sometimes known as “hedge apples”.
What’s curious is that the fruits just sit there, on the ground near the tree - nothing eats them. It seems a strange evolutionary strategy to produce seed-laden fruits that no animal wants to eat (and thus disperse).
The apparent solution to the mystery is that these fruits were once eaten by now-extinct megafauna, such as ground sloths and mammoths. When these disappeared, the tree’s outlook was grim.
But the wood of this tree is superior for bows, which made it of interest to various Indian tribes, creating a motive to conserve it. It later became popular as a natural fence tree, and for its rot-resistant wood. It’s now common well outside its original range (eastern TX and southeastern OK).
I thought that was sexually selected among Homo sapiens plumberensis.
Speaking of hedge apples, by the definitions above apples are hollow too. Pears too. And so on. Pea pods are hollow. Peanuts - underground - are hollow. Nature may abhor a vacuum, but is quite willing to work with air.
The float hypothesis sounds pretty logical… you can bob for apples, for example. So does the insulation idea. I’ll add another, that if a seed is separate from the pulp, it also helps when the fruit begins to rot that the mold or whatever is less likely to impinge on the seed itself.
I dunno. The plumbers I know look like they reproduce by binary fission.
There are plenty of plants with hollow stems, including bamboo. It’s really not that unusual in nature.
Could be that gourds gain an advantage from being able to roll also - as plants, they are scrambling climbers - so if they happen to scramble up to the top of a ridge and fruit there, the fruit has a chance of rolling down the other side, perhaps quite a distance from the parent plant - and although there are no completely reliable generalisations in evolution, it does seem to be fairly true that distributing seeds to a distance from the parent is often a positive selection.
Ha! Nice.
Note that carving pumpkins and pie pumpkins differ quite a bit in how hollow they are. Pie pumpkins have a much thicker rind and thus a smaller hollow. But that makes them harder to carve, hence carving pumpkins. Humans have been playing these breeding games a long time with squash.
Heh! I like that. Also Thanks to Collibri, md2000, and Mangetout for realizing that “humans did it” isn’t really an answer. Breeding will exploit a trait and make it much more profound, but the precursor trait had to be there to begin with. I like the idea that air is an insulator for temperature control. Makes a bit of sense, because it’s a better insulator than a wet substrate. I also like the warning that, hey, let’s be sure to not jump to conclusions regarding the purposes of evolutionary structure. I like that especially, because it points out that things are usually more complicated than they seem on the surface.
But at risk of falling into that fallacy, TokyoBayer mentioned bamboo. I would posit that the stems on that plant and others like it are hollow to provide maximal support for height with minimal expenditure in mass. A hollow cylinder is pretty good for structural support.
Also thanks to dtilque for the bad joke and Xema for the slightly off topic but interesting factoids on hedge apples.
One thing I want to get back to I touched on earlier but hasn’t been addressed:
I honestly don’t know, but plants use ethylene to encourage the ripening of the fruit. At normal atmospheric pressures ethylene is a gas. Is it possible that seed pods are hollow to take advantage of that?
I think air space inside of fruit and veg may help with hardening the seed shells as well. I can’t see how the seeds would protect themselves if they were kept really wet. If you need for your seeds to survive the trip through an animal’s digestive tract, I’d imagine that drying space makes a world of difference.
What I want to know is, how does the air get in there? Think about the formation of the fruit. The interior is surrounded at all times by the skin and stem. And although the plant does respire, it seems like an awfully big flesh/air ration for just respiration. Especially in the case of bell peppers.