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]



Gooey Seeds

Some seeds can get pretty sticky when water gets involved. Anyone that has ever tried to grow a Chia pet or put chia seeds into water will know what I mean. The seeds of chia (Salvia hispanica) are but one example of seeds that turn gooey with water. The question is, why do they do this? What role does sticky mucilage play in the reproductive cycle of plants around the globe?

It turns out that seed mucilage is an extremely useful trait for many plants. For starters, it can aid in dispersal of seeds. For some plants this simply means being sticky enough to attach to an animal that brushes up against ripe seeds. Mucilage can get stuck on everything from fur to feathers, and even scales. This is yet another form of seed dispersal known as epizoochory. Amazingly, mucilage has shown to be an effective trait for aiding in wind dispersal as well. Such is the case for a small mustard called Alyssum minus. This may seem counterintuitive as one would think that mucilage would weigh a seed down, not send it aloft. In this example, the mucilage forms a tiny wing that surrounds the seed after it has dried out. This wing made out of dried, papery mucilage significantly increased seed dispersal distances on windy days.

Chia seeds in water swell with mucilage, making them look more like frog eggs than seeds.

Chia seeds in water swell with mucilage, making them look more like frog eggs than seeds.

Following dispersal, the role of seed mucilage becomes even more important. Just as it can help seeds stick to potential seed dispersers, the mucilage can also help the seeds stick to the ground. This is especially useful for plants growing in sandy soils that move around a lot easier than more mesic soils. By sticking to the substrate, the mucilage helps the seed maintain good soil to seed contact, which is essential for successful germination. Without it, seeds would easily blow around and never rest in a place long enough to establish.

Adhering the soil also aids in water uptake for the seed. This is a prerequisite for any seed to successfully germinate. However, simply acting like a conduit for water to move from soil to seed isn’t the only advantage the mucilage provides. By swelling up with water, the mucilage acts as a tiny water reservoir, which buffers the seed from potential water stress. Again, this is especially useful for plants growing in xeric habitats. By keeping water around the seed longer than it would be if the seed was directly exposed to the environment, the mucilage speeds up germination and increases the chances of success for the resulting seedling.

Finally, seed mucilage can also protect seeds from predators. Seeds are tiny packets of concentrated nutrients and many animals don’t hesitate to gobble them up. By covering their seeds in sticky mucilage, plants are able to deter at least some potential seed predators like ants from moving and eating their seeds. Also, aside from gumming up the mouths of seed predators, the fact that the seeds stick to the substrate makes them difficult to move. With any luck, seed predators will tire of the chore and move on to easier meals.

Now if we think back to those Chia Pets, we can see why chia seeds are able to germinate on wet ceramic. Their mucilaginous coating not only enables them to adhear to the surface of the structure, it protects them from drying out by holding onto water. It kind of makes you look at those goofy gifts as a subtle way of displaying an interesting evolutionary mechanism in action. 

Photo Credit: [1]  [2]

Further Reading: [1] [2] [3] [4] [5]