Throwback Thursday Thanksgiving feast

We’ve got several posts in the pipeline – and this year we are contributing to Advent Botany – but meanwhile, we bring you posts from the past to nerd-up your kitchen as you cook. Don’t forget, nothing deflects from an awkward personal revelation or a heated political conversation like a well-placed observation about plant morphology.

We wish you a happy, healthy Thanksgiving!

Click a caption to serve up the whole post.

Convolvulaceae: sweet potatoes whole

Whole sweet potatoes (NOT yams!)






How giant pumpkins got so big: A Q&A with Jessica Savage

Biologist Jessica Savage answers a few of our questions about her research on the physiology behind giant pumpkin size.

In October 2014, a giant pumpkin grown by Beni Meier of Switzerland tipped the scales at 1056 kilograms (2323 pounds) and set a new world record for the heaviest pumpkin ever weighed. Modern competitive pumpkin growers have been imposing very strong selection on pumpkin size for decades. Pumpkin fruit size keeps climbing, and old records are broken every year or two (Savage et al. 2015).

Beni Meier with his 2014 record-winning 2323-pound pumpkin, presumably a specimen of the Atlantic Giant variety of Cucurbita maxima. Photo from here.

The giant pumpkins breaking records are probably not the same kinds of squashes that are destined to be Halloween jack-o-lanterns or pumpkin pie. The behemoths are primarily fruit specimens of the Atlantic Giant variety of the species Cucurbita maxima (Savage et al. 2015). C. maxima varieties are collectively known as the hubbard squashes (see our post on squash diversity for information on the several squash species that earn the name “pumpkin”). Many hubbard squash varieties are grown as food and have been under selection for delicious, sweet, firm fruit. If you buy canned “pumpkin” for pie, you’re probably eating one of those sublime hubbard varieties.

Amish pie pumpkin, a large and delicious variety of hubbard squash, Cucurbita maxima. That's a 12-inch chef's knife for scale.

Amish pie pumpkin, a large and delicious variety of hubbard squash, Cucurbita maxima. That’s a 12-inch chef’s knife for scale.

The edible hubbard varieties do produce impressively big fruit, but they are nowhere near the size of the Atlantic Giants (Savage et al. 2015). “These fruits really present an interesting study system because they have been under strong selection from humans for years for their size,” says Jessica Savage, a biologist who studies the evolution and physiology of giant pumpkins. Savage continues: “While it is desirable to breed many fruits to be large, in the case of edible fruits, there are other things to consider including taste. This is not a concern for the giant pumpkin growers. In fact, giant pumpkins tend to be fibrous and are not considered palatable to many. All that matters is size.”

The 2014 Topsfield Fair giant pumpkin weigh-in, Topsfield, MA (photo by J. Savage)

The 2014 Topsfield Fair giant pumpkin weigh-in, Topsfield, MA (photo by J. Savage)

So, how did giant pumpkins get to be so large? What physiologically separates the giant pumpkin varieties from their brethren destined for pumpkin spice lattes? The short answer is that giant pumpkin plants produce more phloem than do plants of other squash varieties (Savage et al. 2015). Phloem is the component of a plant’s vascular system that moves solutes, mostly sugar, around the plant. Xylem is the other main component of plant vasculature, and its primary job is to move water and nutrients from the soil through the plant. We discussed xylem function in our post on maple syrup. Jessica Savage and her colleagues studied the anatomy and physiology of both phloem and xylem and a variety of other components of squash plants’ carbon acquisition and allocation in an effort to determine how Atlantic Giant squashes achieve giant size (Savage et al. 2015).

Large specimens of edible varieties of Cucurbita maxima and Cucurbita moschata, similar to the edible hubbard used in Savage's research

Large specimens of edible varieties of Cucurbita maxima and Cucurbita moschata, similar to the edible hubbard used in Savage’s research

Savage and her colleagues grew an edible hubbard squash variety and Atlantic Giants under identical conditions in a greenhouse and measured several aspects of the plants’ carbon (sugar) supply chain, from the carbon source in the leaves, through the vascular transport system, to the carbon sink: the fruit. Squashes and other species in the squash family (Cucurbitaceae) are particularly suited to studies of plant vasculature, as Jessica Savage explains below in a special interview post to explain phloem function and how it helps us understand the evolution of size in giant pumpkins.

Q&A with Jessica Savage about phloem and giant pumpkin physiology

Question 1: How do you like to explain phloem and its function to a general audience? “With pictures!” is a good beginning to an answer here.

Jessica Savage: “Plants are able to “feed themselves” by producing sugar in their leaves through photosynthesis but they need to be able to move this sugar to different parts of their body to sustain growth in various organs like roots and fruits/flowers. They achieve this using part of their vascular tissue, the phloem, which is specialized in transporting sugars. However, plants, different than animals do not have a heart to drive circulation of fluid in their vascular system, instead, they rely on mostly passive processes.”

Phloem function schematic diagram by J. Savage

Phloem function schematic diagram by J. Savage

“In the phloem, transport is driven by the movement of water into and out of the tissue in different parts of the plant. In their leaves, which we refer to as “source tissue” because it is the source of sugar, the phloem contains a high amount of sugar. This sugar draws extra water into the cells making them full and swollen. Meanwhile, other locations in the plant act as “sink tissues” (for example the roots or fruits) because they pull sugar out of the phloem to use for growth and other processes. In these locations, sugar concentrations are low inside the phloem. This causes water to leave the cells making them more limp. Because the source and sink tissue are connected, water moves from the full cells in the source to the more limp cells in the sink tissue. This “pressure-driven” pump allows for plants to move sugars from where it is plentiful to where it is more scarce.”

Question 2: Why do pumpkins (and other cucurbits) lend themselves to the study of plant vasculature?

Jessica Savage: “Pumpkins and cucurbits are great for studying sugar transport because the cells in their phloem are wide and have large pores connecting them. This makes them easy to see using a microscope. They also have some great properties which help us study their physiology, for example, the cells in their leaves are very connected. This seemingly simple characteristic is very important because if we want to put dye into the phloem to watch phloem transport in living plants, we can put the dye in any of the cells in the leaves and it will eventually get into the phloem.”

Question 3: On the other side of that coin, how does understanding phloem function and its evolution help us understand how giant pumpkins got so giant?

Jessica Savage: “The size of giant pumpkins to me is intriguing because it presents what I often refer to as a “traffic control problem”. In normal sized pumpkins, there are many small fruit and sugar is transport in different directions to feed them all. This is similar to traffic in the country where everyone is going to work in different directions and there is not too much traffic on any one road. However, in giant pumpkins a large amount of sugar is going into one fruit. This is more like the situation where you have a big city and everyone is going into this city for work in the morning. As a result, there needs to be either more roads or roads with a higher capacity to support the high amount of traffic entering the city. The same is true for giant pumpkins, either they need more phloem or phloem with a higher transport capacity to sustain these large fruits, especially considering that giant pumpkins can move a couple pounds of sugar into their fruit in a day.”

“When we look at giant pumpkins, we find that the solution that pumpkins have to this problem is producing more roads, or more phloem. When you compare different varieties of Cucurbita maxima, the giant varieties that have been bred for pumpkin competitions all produce more phloem. This allows them to move more carbon into individual organs than smaller fruited varieties. This changes is likely important in supporting the rapid growth rates we see in these fruits.”

Figuring out the mystery of how Atlantic Giants produce massive fruit

Excerpt from Figure 2 from Savage et al. (2015) of microscope slides of vascular anatomy of C. maxima. From their caption:

Excerpt from Figure 2 from Savage et al. (2015) of microscope slides of vascular anatomy of C. maxima. From their caption: “Vascular bundles of (a) Atlantic Giant and (b) Hubbard squash varieties. Bars are 250 micrometers. Pedicel cross sections of (c) Atlantic Giant and (d) Hubbard squash varieties. Bars are 500 micrometers. (e) Irregular vascular bundle in pedicel of an Atlantic Giant. Bar is 500 micrometers. Labels are as follows: EP and IP are external and internal fascicular phloem, respectively, F is fibres, and X is xylem.” Atlantic Giant have noticeably more phloem than does the hubbard.

To figure out how giant pumpkin size was achieved, Savage and her colleagues traced the anatomy of sugar production, transport and consumption from leaf to fruit. In their greenhouse study, Savage and her colleagues (2015) reported that the leaves from both varieties photosynthesized at about the same rate, so each square centimeter of leaf brought similar amounts of sugar into the plant in a given amount of time. And the hubbards actually produced slightly more leaf area than did the Atlantic Giants, so the total sugar inputs were similar or tipped slightly in favor of hubbards. The vascular system of both types had similar amounts of xylem (the amount of area in cross section occupied by xylem). Both the aboveground and belowground vegetative parts of the plants grew at roughly the same rate. The Atlantic Giant, though, made more flowers, and their fruits grew much faster than the hubbards. How? The vascular transport system—composed of stems, the petioles connecting the leaf blades to the stem, and the pedicels connecting the fruit to the stem (the “handle” of a ripe pumpkin)–of Atlantic Giants contained much more phloem than did the petiole-stem-pedicel pipe system of hubbards (Savage et al. 2015). The structure and function of the phloem itself was very similar for the different squash varieties. Only the amount of phloem changed, providing the capacity to transport the high volume of sugar needed to grow the giant fruit.

An Atlantic Giant pedicel, the narrow

An Atlantic Giant pedicel, the narrow “stem” connecting the fruit to the main stem of the plant. Photo by J. Savage

Additionally, Atlantic Giants seem to take a substantially longer time to develop the ovary (the immature fruit prior to pollination, as well explained in Katherine’s post about watermelon, a close relative of pumpkin) and mature the fruit, with the longer maturation time translating to larger overall size (Savage et al. 2015). Further, Atlantic Giants have a thinner, softer outer fruit wall (exocarp) than do edible hubbards, which may increase the ability of Atlantic Giants to expand to large size (Savage et al. 2015). I imagine the softer, more elastic outer fruit wall would decrease its storage capacity, but that’s not important for these competitively grown pumpkins, unlike in the hubbards you expect to store for months to eat through the winter and spring.

Understanding how squashes have achieved the feat of large fruit size is interesting in and by itself, but the work of Savage and her colleagues also has strong implications for understanding the limits of agricultural yield and fruit and seed production strategies of wild plants (Savage et al. 2015).

Many thanks to Jessica Savage for helping us understand phloem function and her outstanding research on the physiology and evolution of giant pumpkins. We wish everyone a happy Halloween!


Savage, J. A., D. F. Haines, and N. M. Holbrook. 2015. The making of giant pumpkins: how selective breeding changed the phloem of Cucurbita maxima from source to sink. Plant, Cell and Environment 38:1543-1554.

Triple threat watermelon

Will seedless watermelons make us superhuman or turn our children into giants?  Hardly, but they do give home cooks the power to count chromosomes without a microscope.   Just a knife or a hard thunk on the sidewalk are enough to get a watermelon to spill its genetic guts.

If you were reading a Hearst Corporation newspaper in late 1937, you might have thought humanity would eventually be swallowed up by giant carnivorous plants, unwittingly unleashed by uncontrolled biotechnology.  The San Francisco Examiner reported on November 21st of that year that the discovery of an “elixir of growth,” meant that “…science may at last have a grip on the steering wheel of evolution, and be able to produce at will almost any kind of species…”  including “…a plague of man-eating ones.”  In 1937 Americans had much more important things to worry about, just as we do now.  Still, that discovery may in fact have threatened one cherished aspect of the American way of life by triggering the slow demise of late summer state fair watermelon seed spitting contests.  It doubtlessly paved the way for seedless watermelons, and in 2014 the total harvest of seedless watermelons on American farms – nearly 700 thousand tons – outweighed the seeded watermelon harvest more than 13 to 1 (USDA National Watermelon Report). A similar pattern is emerging this year.  Is there no stopping the attack of the seedless watermelons?

Image from microfilm of an actual page in the San Francisco Examiner, published Sunday November 21, 1937. Found in the Media and Microtext Center of Stanford University Libraries.

CLICK to read. Image from microfilm of an actual page in the San Francisco Examiner, published Sunday November 21, 1937. Found in the Media and Microtext Center of Stanford University Libraries.

And more important, how is it even possible to get seedless fruit from an annual plant?  From a plant whose only mode of reproduction is through those very seeds?  From a plant that cannot make suckers as bananas do and cannot be perpetuated endlessly through grafts like fruit trees and vines?   Such is the challenge posed every single year by watermelons, but thanks to the “elixir of growth” discovered by Albert Blakeslee and subsequent work by Hitoshi Kihara, one of the most prominent agricultural geneticists of the 20th century, the world has an elegant solution. Breeders continually improve the varieties available, and consumer demand keeps growing, yet seedless watermelon production methods have remained essentially unchanged for three quarters of a century. Continue reading

The new apples: an explosion of crisp pink honey sweet snow white candy crunch

What’s in a name?  An apple with an old fashioned name could taste as sweet, but it might not sell.  The most sought after branded varieties reveal what people look for in an apple: sweet and crunchy and bright white inside.  Do the fruits live up to their names?  Are Honeycrisp apples crunchier than others?  Do Arctics actually stay white?  We zoom in on the cells to find out.

Some of you will remember the era when the Superbowl halftime show repeatedly featured Up With People.  That was around the time when Granny Smiths arrived in our supermarkets and finally gave Americans a third apple, a tart and crunchy alternative to red and golden delicious.  Those were simple days.  Continue reading

Taking advantage of convergent terpene evolution in the kitchen

The Cooks Illustrated recipe masters recently added nutmeg and orange zest to a pepper-crusted steak to replace two flavorful terpenes, pinene and limonene, lost from black pepper when simmered in oil. In doing so they take advantage of convergent evolution of terpenoids, the most diverse group of chemical products produced by plants. Nutmeg and orange zest, though, were hardly their only options.

The terpene swap

Black pepper (Piper nigrum) growing in Cambodia (photo by L. Osnas)

Black pepper growing (photo by L. Osnas)

To develop satisfying crunch, the Cooks Illustrated recipe for pepper-crusted beef tenderloin requires a prodigious quantity of coarsely ground black pepper (Piper nigrum; family Piperaceae). If applied to the meat raw, however, in the recipe authors’ view, this heap of pepper generates an unwelcome amount of spicy heat. To mellow it, the recipe authors recommend simmering the pepper in oil and straining it out of the oil before adding it to the dry rub. The hot oil draws out the alkaloid piperine, which makes black pepper taste hot, from the cracked black pepper fruits (peppercorns).

Nutmeg seed showing brown seed coat folded within the ruminate endosperm

Nutmeg seed

To their dismay, however, the recipe authors discovered that the hot oil also removes flavorful compounds from the cracked pepper, in particular the terpenes pinene and limonene. To rectify this flavor problem, the recipe authors added pinene-rich nutmeg (Myristica fragrans; Myristicaceae) and limonene-rich orange (Citrus x sinensis; Rutaceae) zest to the dry rub, along with the simmered black pepper. In doing so they take advantage of widespread and diverse array of terpenoids in the plant kingdom. Continue reading


The rapunzel plant (Campanula rapunculus; Campanulaceae). Photo from Wikipedia.

The rapunzel plant (Campanula rapunculus; Campanulaceae). Photo from Wikipedia.

I never suspected that I’d learn something about edible botany by indulging my 3-year-old’s princess obsession, but I have. According to the Brothers Grimm, Princess Rapunzel is named after the cultivated  vegetable of the same name, growing in a witch’s garden. The wording of the story suggested to me that the Grimms’ contemporaries would be familiar with the plant as a vegetable, that it wasn’t a fantastical invented thing. Apparently rapunzel was a popular vegetable in the Grimm’s Europe. Continue reading

Apples: the ultimate everyday accessory

Infinity scarves? No. They won’t keep doctors away. Apples are the ultimate everyday accessory (fruit). Katherine explains where the star in the apple comes from. Could it be due to a random doubling of chromosomes? We also give readers the chance to test their apple knowledge with a video quiz.

Although apples are not particularly American – nor is apple pie – they color our landscape from New York City to Washington State, all thanks to Johnny Appleseed. Or so goes the legend. Everyone already knows a lot about apples, and for those wanting more, there are many engaging and beautifully written stories of their cultural history, diversity, and uses. See the reference list below for some good ones. There is no way I could cover the same ground, so instead I’ll keep this post short and sweet (or crisp and tart) by focusing on apple fruit structure and some interesting new studies that shed light on it.
Of course if you do want to learn more about apple history but have only 5 minutes, or if you want to test your current knowledge, take our video quiz! It’s at the bottom of this page. Continue reading

Alliums, Brimstone Tart, and the raison d’etre of spices

If it smells like onion or garlic, it’s in the genus Allium, and it smells that way because of an ancient arms raceThose alliaceous aromas have a lot of sulfur in them, like their counterparts in the crucifers. You can combine them into a Brimstone Tart, if you can get past the tears.

The alliums


garlic curing

The genus Allium is one of the largest genera on the planet, boasting (probably) over 800 species (Friesen et al. 2006, Hirschegger et al. 2009, Mashayehki and Columbus 2014), with most species clustered around central Asia or western North America. Like all of the very speciose genera, Allium includes tremendous variation and internal evolutionary diversification within the genus, and 15 monophyletic (derived from a single common ancestor) subgenera within Allium are currently recognized (Friesen et al. 2006). Only a few have commonly cultivated (or wildharvested by me) species, however, shown on the phylogeny below. Continue reading

The Extreme Monocots

Coconut palms grow some of the biggest seeds on the planet (coconuts), and the tiny black specks in very good real vanilla ice cream are clumps of some of the smallest, seeds from the fruit of the vanilla orchid (the vanilla “bean”). Both palms and orchids are in the large clade of plants called monocots. About a sixth of flowering plant species are monocots, and among them are several noteworthy botanical record-holders and important food plants, all subject to biological factors pushing the size of their seeds to the extremes. Continue reading

Walnut nostalgia

Walnuts may not seem like summer fruits, but they are – as long as you have the right recipe.  Katherine takes you to the heart of French walnut country for green walnut season.

France 1154 Eng newAnnotation fullRes 2

Public domain, via wikimedia commons

English walnuts do not come from England. The English walnut came to American shores from England, but the English got them from the French. The (now) French adopted walnut cultivation from the Romans two millennia ago, back when they were still citizens of Gallia Aquitania. Some people call this common walnut species “Persian walnut,” a slightly better name, as it does seem to have evolved originally somewhere east of the Mediterranean. But the most accurate name for the common walnut is Juglans regia, which means something like “Jove’s kingly nuts.” I think of them as queenly nuts, in honor of Eleanor of Aquitaine, because if any queen had nuts, she certainly did. During her lifetime the Aquitaine region of France became a major exporter of walnuts and walnut oil to northern Europe, and it remains so more than 800 years later. Continue reading