seeds and fruits

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.

Image source: https://www.inaturalist.org/observations/2497162

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.

Image source: https://www.inaturalist.org/observations/30782793

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):

Image source: https://www.inaturalist.org/observations/21613871

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):

Image source: https://www.inaturalist.org/observations/30782793

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

Image source: https://www.inaturalist.org/observations/30782795

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:

Image source: https://www.inaturalist.org/observations/30782795

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:

Image source: https://www.inaturalist.org/observations/7893872

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:

Image source: https://www.inaturalist.org/observations/30782795

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

Image source: https://www.inaturalist.org/observations/16260070

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. less ↑