Out of my gourd

My, it’s certainly been a while since we looked at botany. Oh, we’ve gone all kinds of interesting places since we looked at the Rutaceae, but to me, nothing quite has the same zing as examining another plant family. We’ve got a good one today, too. Yes, it’s time to meet the Cucurbitaceae, your one-stop-shop for all things melony and gourd-like.

The Cucurbitaceae are yet another commercially important plant family, and a very diverse family. You can find members of the family lurking in your crisper (cucumbers), chilling on your porch wearing crudely carved grins (pumpkins), growing to monstrous size in North Carolina (watermelons), and hanging out in the shower (loofah squash).

There are over 800 species of the family out there, mostly in the tropics and subtropics (like so many other plant families). Curcurbits have a long history of cultivation throughout the world. Squash was a vital component of indigenous agriculture in the Americas - look at the role it played in Iroquois “Three Sisters” cultivation, or the similar crop combination grown in the American Southwest. Ancient Egyptians enjoyed their melons , and cucumbers were name-checked in both the Bible (repeatedly) and in the Epic of Gilgamesh. Like many other plant families, it has been a long and fruitful association - in addition to delicious melons and pumpkins, cucurbits have provided us with the aforementioned loofah sponges, gourds that can be used as containers and musical instruments, and even medicinal herbs - one of the temperate cucurbits, bryony, is native to northern Europe and was historically much used as a purgative, sunburn treatment, and even whooping cough cure.

One interesting feature of a number of cucurbits is the monstrous size the fruits can grow to. I alluded to this earlier with the giant watermelon of Sevier County, North Carolina. That 291 pound behemoth is not alone - it is joined by a 1486.6 pound squash in Ontario, and a 1818.5 pound pumpkin in Quebec. True, those are the results of special seed and careful care and fertilizing, and true, they are extraordinary even by the standards of competitive jumbo-sized cucurbit breeding. But still, those are some big fruits.

There's a Charlie Brown joke in here somewhere

What exactly is at play here? How is it possible to get the fruits to be that big? Well, obviously, breeding two plants that produce large fruit will produce a plant that produces large fruit - keep doing it long enough, selecting plants with unusually large fruit, and you’ll get monster fruit. But there has to be something special, here. A wide array of fruits can be grown to large sizes, but there is a difference between a lemon the size of your head and a pumpkin you can put to sea in.

                                                    
               
There is some difference between these two oversized fruit specimens

There are a few factors at work, starting with genetics. The genetic basis for absurdly large cucurbits is a sadly neglected branch of science, but a study on gene expression in watermelon growth pointed in some interesting directions. Something to keep in mind is that there are far more forms of plant life out there than there are fully funded labs to study them - for this reason, the bulk of research has historically focused on model organisms - much in the same way that a great deal of animal science focuses on mice, a great deal of plant science focuses on species like Arabidopsis thaliana. Model organisms, however, can’t quite match the diversity of life out there. Cucurbits (their fruit in particular) grow and ripen differently than most of the model organisms out there, a fact that is only just being addressed in research. However, we now know that, just as the growth forms of cucurbits are different from those of many other plants, the genetic basis of those growth forms also differs. Of the gene sequences involved in watermelon growth, something like a third of them are unknown in any of the model plant organisms. At least some of these genes allow watermelons to reach the size they do by coding for rapid development of the plant’s vein system. This allows nutrients to be more rapidly routed to the growing fruit, and for the fruit to grow bigger.

Then there is the question of just how the fruits can support their massive weight. Theoretically, there comes a point when the shape of a fruit can no longer support its weight - after that, any addition of weight will break the fruit. Pumpkins, however, have a high level of growth plasticity - pumpkin shape is capable of changing in order to support added weight. Massive pumpkins have a flattened shape very different from the near-spherical perfection of smaller pumpkins, a flattened shape that allows the pumpkin to deal with the increasingly great internal stresses of weighing over 1,000 pounds.

Besides the super-sized versions of common cucurbits, there are the other, somewhat lesser-known members of the family. Unlike the ground-hugging pumpkins and squash, calabash gourds grow directly out of trees. Calabash gourds are generally neat - they are bat-pollinated, and may once have been dispersed by now extinct North American megafauna (remember last week’s post about the need to bring those guys back?). Among the more unusual members of the family is the too-weird-to-be-made-up squirting cucumber. Rather than waiting on a seed disperser, the squirting cucumber takes matters into its own hands - as a part of the ripening process, the fruit fills with juice, until the internal pressure is sufficient to explode the fruit off the fine, and shoot it as far as 6 feet. The fruit squirts out seeds as it goes along its merry way. A video of the fruit in action can be seen here (apologies for the somewhat subpar subtitles).

Old school version seen here

Yes, the Cucurbitaceae are a family of wonder. Bat-pollinated, north of a ton, exploding wonder. And, I promise you, there are equally entertaining things to be found elsewhere in botany. See why I like the topic so much?