Path Rush

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Path rush (Juncus tenuis) is one of those plants that has really benefited from human expansion. Originally native to North America, it can now be found in numerous countries around the globe. It owes much of its success to both its ability to tolerate lots of disturbance as well as an ingenious seed dispersal mechanism. If you like to hike, there is a good chance you have encountered path rush somewhere along the way. There is also a strong chance that you have dispersed its seeds.

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Path rush is a relatively small species, topping out around 60 cm in height. Because it frequently grows where foot traffic is heavy, plants don’t always reach such stature. Like most rushes, it has round stems and surprisingly attractive flowers, though one would need a hand lens to fully appreciate their beauty. Flowering for path rush occurs during the summer and it is thought that wind is the main pollination mechanism for this species.

The darker vegetation running along the path is all path rush!

The darker vegetation running along the path is all path rush!

Following pollination, each flower is replaced by a tiny capsule filled with tiny seeds. Each seed is covered in a substance that turns into a sticky mucilage when wet. This mucilage is how path rush manages to move around the landscape so easily. The sticky seeds glom onto pretty much everything from fur to feathers, boots to car tires. This is why you most often find path rush on, well, paths! Its sticky seeds are carried far and wide by foot traffic. It is also why you can now find path rush growing well outside of North America.

Path rush enjoying a crack in the sidewalk.

Path rush enjoying a crack in the sidewalk.

Path rush frequents more habitats than simply paths too. The key to its success is soil disturbance. Anywhere the soil has been compacted and disturbed, path rush can find its niche. With little competition from surrounding vegetation, this tiny rush can grow into impressive colonies. Even cracks in asphalt can harbor a plant or two. Aside from its ability to tolerate soil disturbance, its tough, stringy foliage is not fed on by a lot of herbivores, which gives it yet another leg up on potential competitors. All in all, this is one tough little plant.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2]



Viper's Bugloss

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Throughout much of North America, brown fields, roadsides, and other waste places occasionally take on a wonderful blue hue. Often time the cause of this colorful display is none other than Echium vulgare, or as its commonly referred to, viper's bugloss. Viper’s bugloss is a member of the borage family and was originally native to most of Europe and Asia. However, humans introduced it to North America some time ago. It has since naturalized quite well and is even considered invasive in parts of Washington. No matter your views on this plant, the reproductive ecology of this species is quite interesting.

Viper's bugloss produces its flowers on spikes. Starting off pink and gradually changing to blue as they mature, the flowers ripen their male portions on their first day and ripen their female portions on the second day. This is known as "protandry." Plants that exhibit this lifestyle offer researchers a window into the advantages and disadvantages with regards to the fitness investment of each sex. What they have found in viper's bugloss is that there are clearly distinct strategies for each type of flower.

Male flowers are pollinator limited. They must hedge their bets towards increasing the number of visitors. Bees are the main pollinators of this species and the more bees that visit, the more pollen can be disseminated. Unlike female flowers, which are resource limited, male flowers can produce pollen and nectar quite cheaply. Because of this, male flowers produce significantly more nectar than female flowers to bring in more bees. As the anthers senesce and give way to receptive styles, things begin to change. The plant now has to redirect resources into producing seed. At this point, resources are everything. The plant produces considerably less nectar resources than pollen but the bees can’t know that without visiting.

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The other interesting aspect its reproductive ecology has to do with population size. Bees are notorious for favoring plants that are more numerous on the landscape. This makes a lot of sense. Why spend time looking for uncommon plants when they can go for easier, more numerous targets. This can be very detrimental to the fitness of rare plant species. However, plants like viper's bugloss don't seem to fall victim to this.

By looking at large and small populations, researchers found that pollination success pretty much evens out for viper's bugloss no matter how numerous it is in a given area. Large populations receive many more visits from bees but the bees spend less time on each flower. When viper's bugloss populations are small, flowers receive fewer visits but bees spend more time at each flower. This results is no significant difference in the reproductive fitness of either population.

Considering how efficient this plant is reproductively, it is no wonder it has done so well outside of its native range. Add to this its ability to grow in some of the worst soil conditions, it goes without saying that viper's bugloss is here to stay. If you find this species growing, certainly take time to get up close with the flowers. You will be glad you did.

Photo Credits: [1] [2]

Further Reading: [1] [2] [3]


Understanding the Cocklebur

Spend enough time in disturbed areas and you will certainly cross paths with a cocklebur (Xanthium strumarium). As anyone with a dog can tell you, this plant has no problems getting around. It is such a common occurrence in my life that I honestly never stopped long enough to think about its place on the taxonomic tree. I always assumed it was some sort of Amaranth relative. You can imagine my surprise then when I recently learned that this hardy species is actually a member of the family Asteraceae. 

Cocklebur doesn't seem to fit with most of its composite relatives. For starters, its flowers are not all clustered together into a single flower head. Instead, male and female flowers are borne separately on the same plant. Male flower clusters are produced at the top of the flowering stem. Being wind pollinated, they quickly dump mass quantities of pollen into the air and wither away. The female flowers are clustered lower on the stem and consist of two pistillate florets situated atop a cluster of spiny bracts. 

After fertilization, these bracts swell to form the burs that so many of us have had to dig out of the fur of our loved ones. Inside that bur resides the seeds. Cocklebur is a bit strange in the seed department as well. Instead of producing multiple seeds complete with hairy parachutes, the cocklebur produces two relatively large seeds within each bur. There is a "top" seed, which sits along the curved, convex side of the bur, and a "bottom" seed that sits along the inner flat surface of the bur. Studies performed over a century ago demonstrated that these two seeds are quite important in maintaining cocklebur on the landscape. 

You see, cocklebur is an annual. It only has one season to germinate, grow, flower, and produce the next generation. We often think of annual plants as being quite hardy but in reality, they can sometimes be a bit picky about when and where they will grow. For that reason, seed banking is super important. Not every year will produce favorable conditions so dormant seeds lying in the soil act as an insurance policy. 

Whereas the bottom seed germinates within a year and maintains the plants presence when times are good, the top seed appears to have a much longer dormancy period. These long-lived seeds can sit in the soil for decades before they decide to germinate. Before humans, when disturbance regimes were a lot less hectic, this strategy likely assured that cocklebur would manage to stick around in any given area for the long term. Whereas fast germinating seeds might have been killed off, the seeds within the seed bank could pop up whenever favorable conditions finally presented themselves. 

Today cocklebur seems to be over-insured. It is a common weed anywhere soil disturbance produces bare soils with poor drainage. The plant seems equally at home growing along scoured stream banks as it does roadsides and farm fields. It is an incredibly plastic species, tuning its growth habit to best fit whatever conditions come its way. As a result, numerous subspecies, varieties, and types have been described over the years but most are not recognized in any serious fashion. 

Sadly, cocklebur can become the villain as its burs get hopelessly tangled in hair and fur. Also, every part of the plant is extremely toxic to mammals. This plant has caused many a death in both livestock and humans. It is an ironic situation to consider that we are so good at creating the exact kind of conditions needed for this species to thrive. Love it or hate it, it is a plant worth some respect. 

Photo Credits: [1] [2] 

Further Reading: [1] [2]