Is the bur of a cocklebur really a fruit?

This may seem like a simple enough question. Of course, a cocklebur is a fruit, right? Well, it's a bit more complicated than that.

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The common cocklebur (Xanthium strumarium) has prickly round egg-shaped structures (sometimes referred to as porcupine eggs). Each of these prickles has hooks on the end allowing them to stick to clothing and hair like velcro.

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Most people will see prickly structures and assume they are the fruits, but this is not the whole story. As it turns out, the outer prickly part of the "fruit" is actually made up of leaf tissue. Specifically, these are called phyllaries. But let's take a huge step back here before your eyes completely glaze over. In most flowering plants there are these things called calyces (singular: calyx). You've probably seen them before. They're the green structures that are under the petals, and when the flowers are in bud, they enclose the flower. Think of a rosebud. Those small green things enclosing the red petals right before it bursts into bloom, those are the calyces.

In the sunflower family (Asteraceae), to which cocklebur belongs, there are structures that look a lot like calyces. But there is something different about these. It turns out that what most people think is a flower is not actually a flower at all! Instead, it is a flower head with many flowers (often hundreds). So yeah, that sunflower flower you're so familiar with is actually an inflorescence (a stem with many flowers). And, the specialized calyx-like leaves are the phyllaries. Here's a picture of them in a species of dandelion (in this case Taraxacum erythrospermum):

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In the example above, notice that there are two types of phyllaries, a set that are all fused together (connate) and a set that isn't fused together (free). Not all members of the sunflower family have two sets phyllaries, but cocklebur does so try to remember this for later.

Before we dive headlong into cocklebur, there is one more thing that we need to address. The structures that most people think of as seeds in this family are actually fruits known as achenes or even more specifically as cypsela*. Think of a sunflower seed. The outer "shell" that you discard after cracking it open is the fruit tissue. The white part that you actually eat is the seed. Perhaps an illustration for all this will help sum up what we've learned. Here's an example in camphorweed (Heterotheca subaxillaris):

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Now, you should have enough information to start to understand cocklebur. I think it best to start with a photo:

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Try to get some sense of this on your own before moving to the next explanation.

Hopefully, you've taken some time to look closely at the photo. I want to show you another photo, this one cut open (longitudinal section). Try to figure out this one as well and then read the explanation:

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So what's going on here? As you might have guessed, the bur part of the cocklebur is actually composed of phyllaries, tough, fused phyllaries with hooks at the tip. Inside the phyllaries are two black fruits (cypsela or achenes), which each have a single seed in them. If you are particularly observant, you might have noticed the second set of unfused phyllaries at the base of the bur in the first photo. Here's a picture of a fruit and a seed out of a bur. The fruit (seed + fruit tissue) is on the left and a seed is on the right:

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So let's go one step further by asking the question: where are the flowers? Well, wherever you have fruits, you should have flowers. If there are two fruits in the cocklebur, that means there must be two flowers. In this case, there are only two flowers and they are reduced to only the female parts of the flowers. All that is visible from outside the phyllaries are the thin styles/stigmas that protrude out of the two largest spine-like phyllary tips at the apex of the burs. They can be seen below as narrow whitish structures coming out of the two largest hooked spines near the tip of the bur:

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These are the female (pistillate) flowers, but where are the males? Well, they are on separate heads that look like this:

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Those eggbeater shaped structures are the stamens (male flowers). Each yellowish strand represents an anther (the pollen-bearing structure). The white structure where all the yellow anthers originate is made up of 5 fused filaments (the stalks that the anthers sit atop). This is actually quite remarkable among members of the sunflower family as the anthers are typically fused together while the filaments are not fused.

So let's sum up. The bur of cocklebur is not technically a fruit but an inflorescence composed of an outer coating of tough, fused, hooked, modified leaves. The specific types of modified leaves (modified leaves are also known as bracts) are technically referred to as phyllaries. At the base of these tough phyllaries is another series of modified leaves that are not fused together, not hooked, and are more calyx-like. Inside the outer husk of the bur of fused phyllaries are two black fruits. Each fruit only has one seed. These fruits are technically referred to as achenes or cypsela. At the tip of the bur, we find the styles and stigmas of the female, pistillate, flowers. On different inflorescences, we find the male, staminate, flowers. The staminate flowers are eggbeater shaped and made up of unfused anthers (the pollen-bearing structures) and filaments (the stalks that the anthers sit on top of).

As it pertains to this project, it doesn't really matter that much if you say the fruits are shown in the observation field because the bur itself functions as a single fruit. However, if you really want to understand the biology, I find it interesting to learn how plants have modified their form in order to overcome limitations. The cocklebur though pesky and weedy really is amazing. It has literally modified its entire inflorescence to function as an entire fruit! I find this fascinating and I hope you do too, regardless of whether you consider the plant a friend or an enemy.

*Achenes differ from cypsela by whether or not there is a thin layer of stem tissue surrounding the fruit. Cypsela have this and are considered to have a inferior ovary while achenes don't and are considered to have superior ovaries.

Posted on August 14, 2019 06:00 AM by nathantaylor nathantaylor


well done, very nice!

Posted by jrichardabbott over 4 years ago

This is an outstanding lesson! Your photos are absolutely remarkable and make it very easy to understand everything that you are teaching! Thanks so much for taking the time to share this and to teach us!

Posted by suz over 4 years ago

That was very informative, and easy for a non-botanist like myself to follow.

Posted by star3 over 4 years ago

Very interesting! Thanks for sharing this.

Posted by lisa281 over 4 years ago

Glad you all like it! @star3 I'm especially happy to hear that as that is what I was aiming for!

Posted by nathantaylor over 4 years ago

Another great journal entry. The cocklebur fruit development is indeed a really neat thing too. :)

Posted by sambiology over 4 years ago

Thank you, Sensei

Posted by ellen5 over 4 years ago

Absolutely amazing and makes perfect sense! After reading this, I took a look at my Rough Cocklebur photos and can see what you are describing!

Posted by lulubelle over 4 years ago

Very interesting. So should we check the fruits AND flowers boxes when reporting these?

Posted by kitty12 over 4 years ago

Your explanations are so eye-opening. This one is amazing, thanks!

Posted by janetwright about 4 years ago

I'm a couple of years late, but thank you so much for composing this. Fantastic explanations.

Posted by little_metal_weirdo over 2 years ago

Thanks, @little_metal_weirdo, you caused me to read it again, just as good as the first time!

Posted by janetwright over 2 years ago

Wow, what an explanation! I've been on an Asteraceae bender recently and came across this species for the first time today, and had been wondering what was going on with the flowering and fruiting structures of this thing. This answered all my questions!

Posted by w-pearce-plants 5 months ago

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