Grass Defenses

Cut grass, sword grass, ripgut? All of these names have been applied to grasses. It may seem strange to attach such sharp adjectives to grasses but run through a prairie full of Leersia oryzoides or Spartina pectinata with shorts on and you will quickly learn why. As innocuous as they may look, grasses are well defended.

What looks like the mouth of a shark is actually a blade of grass. As you can see, it is covered in microscopic, razor sharp daggers. The daggers themselves are specialized structures called "phytoliths." Grasses manufacture phytoliths from silica that they absorb from the soil. Not all species produce these daggers. Some distribute phytoliths throughout their leaves, essentially packing themselves with tiny granules of glass. Their presence is an adaptation against herbivory. 

It's not hard to imagine how effective silica daggers can be. Run your finger along the stem or leaves of one of these grasses and you are likely to draw blood. Early settlers coined the term "ripgut grass" because the bellies of horses and other livestock would get seriously lacerated from running through it. Whereas this defense is rather straight forward, the other types of phytoliths are a little more subtle in their effectiveness. 

Silica is tough and chewing on leaves chock full of it can do a real number on your teeth. That is the main reason why the teeth of many grass grazers alive today grow continuously. If their teeth were like ours, the phytoliths within the blades of grass would wear them down to useless nubs. In fact, the evolution of phytoliths in grass is thought to have ushered in a new age of grazing mammals via the extinction of those that could not cope with these microscopic defenses. 

It's not just about teeth either. Insects feeding on blades of grass may be able to get past the phytoliths without an issue but the story changes once it makes it to the gut. Silica particles have been shown to interfere with digestion. Caterpillars feeding on grasses containing high amounts of silica in their leaves had decreased levels of digestion efficiency, which resulted in reduced growth rates. Unlike other plants, grasses can handle certain amounts of grazing because their growth tips are located beneath the soil rather than near the tips. As such, they can afford to gradually weaken the effectiveness of their predators. 

I have gained a new appreciation for grasses since moving to the prairies. This diverse order of plants has shaped the world we see today in a very big way. Because they don't have showy flowers, grasses are often overlooked as nothing more than turf. In reality, they are fascinating organisms supremely adapted to what the environment throws at them. One could only wish to be as hardy as a grass. 

Photo Credits: [1] [2]

Further Reading: [1] [2]