Wednesday, November 18, 2015

Give Thanks For The Cranberry

Biology concepts – epigynous berries, seed dispersion, scarification, drupe, endocarp


Ocean Spray alone sells 86.4 million cans of jellied cranberry
sauce each year. No matter which sauce you prefer, I bet it
has a lot of added sugar. Cranberries alone are tart enough
to shrink your head.
Cranberry sauce is a Thanksgiving staple, but it’s a lot like fruitcake at Christmas – you either love it or hate it. Let me give you some reasons to love it.

Cranberry (Vaccinium macrocarpon) is one of very few commercially grown fruits native to North America. The vine needs cool temperatures and acidic, sandy soil conditions, so New England, Southern Canada and the Pacific Northwest are prime growing locations. Similar latitudes in Europe also support growth of cranberries (Vaccinium oxycoccus) in their bogs. We have previously talked about bogs where the acid conditions preserve human remains and produce bog mummies.

But there is an exception in the Southern Hemisphere – Chile in South America. In the northern part of Southern Chile, volcanic ash soils mimic the sandy soils of peat bogs, both in consistency and acidity. Runoff from the Andes Mountains allows for water, and the temperatures are similar to those in Washington and Oregon - perfect for cranberry growing.

The Ocean Spray Company harvests berries in North America in autumn, but it needs berries in the summer too. In January of 2013, Ocean Spray bought the cranberry processing interests in Chile. The harvesting period in Chile is March to May, just in time to supplement Ocean Spray’s dwindling supplies.

Cranberries are tart compared to other fruits; they have five times as much acid as their close cousins, the blueberries. Why? It may be the acidic soils they grow in. In terms of evolution, growing in peat bogs was a good choice. Not many things can grow in a bog, so competition is low. Competition for what is the question – there is very little nitrogen in the soil of a bog, and the water is acidic too.

Plants need fresh water and nitrogen to survive, so the cranberry evolved better nitrogen tapping mechanisms, as well as leaves and stems that can retain their fresh water very well. Not many other organisms have adapted to these conditions, but the cranberry thrives, transferring the acids to its leaves, stems and fruits.

This is the bog copper butterfly (Lycaena epixanthe) that
lives its entire life on a cranberry vine. It not only survives
the acidic condition of the plant – it eats it up. It lays its eggs
on the under side of the leaf, and the pupa and the larva can
survive a flood that covers the plant for months.

This acidity is also a help when it comes to pests. Several acidic compounds have been isolated from V. macrocarpon that stop insects from eating the leaves and stems. I’m guessing insects don’t like Sour Patch Kids. The exception is the butterfly Lycaena epixanthe; it spends its entire life feeding on the cranberry plant.

The second reason for the high acid content of the cranberry is that it doesn’t need to be sweet. The blueberry is much sweeter, but it  has to be. Blueberry bushes spread their seeds by having birds, rodents, or humans eat them one place and excrete them in their feces somewhere else; sweetness promotes consumption.

Seed dispersal is the most basic reason for any plant producing a fruit. If a seed falls directly beneath the parent plant, no one wins. Both patent and child will require the same nutrients, and they will end up competing for everything. Things would also get very crowded.

Several mechanisms of seed dispersal have evolved. Wind is a popular way to disperse seeds. You’ve seen those helicopter seeds from Maple trees – they catch the air and twirl down vertically, but also move horizontally. Sycamore trees have tufts on their seeds to catch the wind as well.

Fruiting is also a way to disperse seeds. Animals need carbohydrates, and fruits are an important source for many animals. When they eat the fruit, they also eat the seeds. Later on, the animal grabs a copy of Sports Illustrated, locks the door, and deposit the seeds somewhere else.


These are some of the types of fruits. The peach is a drupe. It
has an edible mesocarp. The coconut is also a drupe, but its
mesocarp is more fibrous (flake coconut). The tomato is a true
berry. It’s pericarp and locules or all edible. The raspberry is an
aggregate fruit, many ovules and mesocarps held together. The
raspberry is also a drupe, which you know when you get those
seeds stuck in your teeth. Each little fruit is a druplet.
In fact, some seeds must pass through the digestive tract of an animal in order to germinate. Some seeds, like those of drupes (drupa = overripe olive), have a hard endocarp (seed coat), derived from the ovary wall. In fact, that’s what makes a drupe a drupe. Fruits like peaches, almonds, coconuts, olives, are considered drupes and each little part of a blackberry or raspberry is a druplet.

The germinating embryonic plant isn’t strong enough to break through the drupe endocarp on its own. Something must be done to weaken the endocarp. The weakening (scarification) may come from scratching the surface, freeze/thaw, fire (for the Ponderosa Pine), or perhaps from the digestive enzymes of an animal. Many berries, like blackberries, currants, and raspberries require digestive scarification in order to germinate. But the cranberry isn’t one of these berries.

Why don’t cranberries need to be eaten for seed dispersal? Because they float! When the bog (or similar sandy wetland) floods, the berries are carried away from the parent plant, away to some far off place that may or may not be suitable for cranberry vine growth. That’s the problem with floating; you gotta go with the flow.

Cranberries float because they have air pockets trapped within them. Floating fruit isn’t that exceptional, apples float too. It’s a good thing; think how may lives this has saved during bobbing for apples season!

On top we see the coconut – it’s a drupe with a tough exocarp.
You can see the germinating plant coming through one of the
eyes. Seed dispersal for the coconut is shown on top right. We
don’t know where palms come from originally, because they
could spread around the world in just one generation. The
cranberry also floats, because of the air pockets shown on the
bottom right. The frog is just a bonus – cute, huh?

Given their bouyancy, it amazes me that it wasn’t until the 1960’s that someone thought of flooding the bogs in order to harvest the cranberries. They have machines that shake the vines and release the ripe berries.

Cranberry plants grow very low to the ground, they have long runners (rhizomes), that can extend six or more feet from the parent vines, and these can sink roots to become new plants. Because of their short stature, it only takes about 18 inches of water to flood a cranberry bog for the wet harvest. So those commercials with the two goobers standing waist high in water in their waders are a bit of a stretch.

The cranberry was probably at the first Thanksgiving; they are hearty and ready to be harvested just about the time we are sitting down to our turkey and stuffing.  But, the pilgrims misled us – the cranberry isn’t a real berry! And don’t say it was because the pilgrims were from across the ocean. The cranberry is closely related to the European lingonberry, so the mistake had already been made.

The cranberry is a false berry, also called an epigynous berry (epi = in addition to, and gynous = ovary). A berry is a fleshy fruit derived from a single ovary. False berries develop from an inferior ovule and contain tissues from parts of the flower other than the ovary, while true berries develop from superior ovary tissue only (see picture). Other examples of epigynous berry-producing plants are bananas, coffee and cucumbers.


Here is one difference between real and false berries. All true
berries are hypogynous, where the ovary (in red) is above
where the petals and pistil come out. False berries have an
inferior ovary. Another difference is that the true berry is
made from only the ovary, while the false berry incorporates
other parts of the flower. Below on the left is the red currant,
and on the right is the cranberry. As a berry, the currant is true
and the cranberry is false. But really, can you tell the difference?
The V. macrocarpon false berry fruit is indispensible as a Thanksgiving sauce, but medicine has found other uses for cranberry compounds. In the first 10 months of 2013 alone there were 86 papers published on the merits of cranberry compounds.

Most people who know about medicinal cranberries have had a urinary tract infection (UTI). For a hundred years or so, old wives (and young wives) have espoused the virtues of cranberry juice in preventing or treating UTIs.

Recent years have seen many studies try to validate the home remedy. As for if cranberries work, there is evidence on both sides. Hundreds of published reports say it’s the best thing since sliced bread, and hundreds say it doesn’t do a darn thing. Such is science – and that’s a good thing. Argue away so we know we get it right in the end.

One 2013 study found that sweetened dried cranberries added to the diet made a real difference in women who were susceptible to UTIs. Half the women in the study didn’t have even one UTI while on the study, and they all had reduced numbers of incidents.

As for why caranberries may work, scientists first thought it was the acid that killed the UTI-causing bacteria. Then it was believed that cranberry compounds prevented the attachment of the bacteria to the wall of the urogenitial epithelium via the bacterial fimbriae (appendages for attachment). This may actually be true, but other actions are also possible.

Another 2013 study showed that for the UTI causative agent Proteus mirabilis, eating powdered cranberry was very effective for preventing UTI. In this experiment, the researchers found that the organisms did not swim well or swarm when exposed to cranberry compounds. In fact, the gene that expresses proteins for their flagella (for motility) were inhibited by cranberry powder.

In addition, their urease virulence factor was also suppressed. A virulence factor is any molecule that helps an infectious organism to colonize and/or obtain nutrition from a host, or helps it to evade or suppress the host immune system.

This is a dividing bacterium showing the fimbriae that help it
attach to surfaces. You can see the difference between these
and the flagella that help in the motility of the organism. It
may be that cranberry compounds mess with both to
prevent UTIs.

Not to be a downer, but a different group carried out a meta-analysis (an organized compilation of many studies involving a lot of statistical math) of many cranberry/UTI studies in 2013 and determined that cranberry compounds have no effect on the prevention or treatment of UTIs. So, all that talk about just how cranberry molecules suppress UTIs (fimbriae, acid, down regulation of host molecules) can be ignored if you don't believe they work.

The news is better on other fronts. In obese men, cranberry juice was able to inhibit the stiffening of blood vessels, an important factor in development of cardiovascular disease (CVD). The effect was greatest in men with metabolic syndrome – a combination of high blood pressure, blood glucose, and cholesterol, as well as obesity.

A second study confirmed this by showing that 1 cup of cranberry juice each day reduces blood glucose levels and CVD risk in men with type II diabetes. And this is just the beginning; 2013 studies also show how cranberry compounds may help you age well – this makes sense, some vines have been producing cranberries since before the American Civil War. Other studies show that cranberry is a potent anti-viral agent as well as preventing bacterial UTIs. Respect the berry – uh, false berry!

Next week, let’s talk about another symbol of Thanksgiving, the indian corn that you think is just decorative is actually a fascinating story of discovery.



Burleigh AE, Benck SM, McAchran SE, Reed JD, Krueger CG, & Hopkins WJ (2013). Consumption of sweetened, dried cranberries may reduce urinary tract infection incidence in susceptible women -- a modified observational study. Nutrition journal, 12 (1) PMID: 24139545

McCall J, Hidalgo G, Asadishad B, & Tufenkji N (2013). Cranberry impairs selected behaviors essential for virulence in Proteus mirabilis HI4320. Canadian journal of microbiology, 59 (6), 430-6 PMID: 23750959

Lorenzo AJ, & Braga LH (2013). Use of cranberry products does not appear to be associated with a significant reduction in incidence of recurrent urinary tract infections. Evidence-based medicine, 18 (5), 181-2 PMID: 23416416

Ruel G, Lapointe A, Pomerleau S, Couture P, Lemieux S, Lamarche B, & Couillard C (2013). Evidence that cranberry juice may improve augmentation index in overweight men. Nutrition research (New York, N.Y.), 33 (1), 41-9 PMID: 23351409

Shidfar F, Heydari I, Hajimiresmaiel SJ, Hosseini S, Shidfar S, & Amiri F (2012). The effects of cranberry juice on serum glucose, apoB, apoA-I, Lp(a), and Paraoxonase-1 activity in type 2 diabetic male patients. Journal of research in medical sciences : the official journal of Isfahan University of Medical Sciences, 17 (4), 355-60 PMID: 23267397


For more information or classroom activities, see:

Seed dispersal mechanisms –

Scarification –

Different types of fruits –

Fimbriae and flagellae –