Seeds Muscle Their Way into the Soil
A biological motor has been found, of all places, on the seeds of wild wheat. A team of German and Israeli scientists watched wheat seeds and found they could dig themselves into the ground. How can a dry seed, with no muscles, nerves or circulatory system, accomplish such a feat? It all becomes clear when you look under the awning.
You’ve probably seen the long strands attached to the seeds of grasses like wheat and oats. These are called awns. They’re not just decorative; they are actively involved in seed dispersal. Once the seed drops to the ground, with awns still attached, a remarkable mechanism goes into action. As the humidity rises and falls throughout the day and night, the awns respond by bending or twisting.
How does the bending take place? At first, it seemed surprising anything would happen, because the tissues in cross section look uniform under an electron microscope. The authors, though, found a remarkable feature: a “huge acoustic impedance contrast” in cross section that affects the stiffness of the awn shaft from one side to the other. In cross section, the shaft resembles the shape of a mushroom with a cap. The cap portion had twice the Young’s modulus as the stem – a stiffness the equivalent of spruce wood. As humidity changes, the differential stiffness causes the entire awn to bend. By analogy, consider how a bimetal strip, like the coil in a thermostat, bends and straightens in response to temperature. Not only that, “silica tiles stiffen the epidermis and protect the structure as it interacts with the soil.”
So let’s follow the action in the wild. The seed, awns and all, falls to the ground. In real time, it might look like nothing is happening. The seed, after all, is dead; its tissues are removed from any source of nourishment or internal energy. A time lapse movie, however, shows the seed appearing to spring back to life. This time, it’s a robotic life exacting its energy from the air. The alternate bending and unbending of the awns gives a kind of “muscle” to the seed, propelling it along the ground – and even into the soil!
This mechanism for seed dispersal has been known for some time. What’s new is that the scientists found tiny silicified hairs on the outside of the awns that act like a ratchet – they force the motion to go one way. As a result, when oriented horizontally, the seed will swim like a frog along the ground. (They actually said this: “The movement is reversible; thus, the humidity cycle causes a periodic movement of the awns, which resembles the swimming stroke of frog legs.”) When oriented vertically, the seed acts like a power shovel. The awns open and close like the handles of a post hole digger. Meanwhile, those silicified hairs latch onto the soil particles, only allowing the seed to go down, not up. Thus, the seed works its way deeper and deeper into the soil – safely out of the reach of predators, fire and drought. “This suggests that the dead tissue is analogous to a motor,” they said. “Fueled by the daily humidity cycle, the awns induce the motility required for seed dispersal.”
This mechanism is optimized, they said, for the soil environment of the Fertile Crescent, where civilization first began to farm wheat thousands of years ago. In some kinds of domesticated wheat, the awns are no longer active. The authors speculated that the length of time since domestication has reduced the function of the awns without removing them entirely. Because humans now provide the muscle to plow the seeds into the soil, the awns have atrophied. Apparently “use it or lose it” applies to seed muscle as well as the animal kind.
In their summary, the authors suggested that humans might gain additional nourishment from wheat – food for thought, that is. The passive-muscle mechanism in wheat seeds might inspire, among other things, new ways to move weed killers where needed:
The understanding of this seed dispersal mechanism may help in developing new concepts in weed control. The microscopic mechanism found to provide motility to the seed may also serve as a model in biomimetic materials research. Indeed, a hydration-dependent bending movement was recently reported in an artificial system consisting of nano-silicon columns embedded in a hydrogel film. From a mechanistic point of view, we have discovered a device for movement that is composed of passive elements. Locomotion is provided by a volume containing nonoriented cellulose crystallites that shortens on drying and pulls the awn like a muscle. The energy source for this active movement is the daily cycle of air humidity.
Maybe someday artificial muscles in robotic devices will work without batteries, extracting the energy they need from the environment – all inspired by the slender filaments on the grass at your feet.
1Elbaum, Zaltzman, Burgert and Fratzl, “The Role of Wheat Awns in the Seed Dispersal Unit,” Science, 11 May 2007: Vol. 316. no. 5826, pp. 884-886, DOI: 10.1126/science.1140097.
A passive muscle driven by moisture in the air—amazing. Could a lowly grass figure out that it needed both a tissue differential with the right acoustic impedance to produce bending at the correct Young’s modulus, at the same time that it needed silicified hairs to act as a ratchet? Without both, this “frog” would swim in place and get nowhere. And what contractor laid the silica tiles? Silicon is not a normal part of plant tissue; it had to be guided into place by epidermal cells while the seed was growing. The fibrils in the awns, also, need to be arranged exactly right to produce the differential impedance. The arrangement of all the parts needs to be complete before the mechanism will work. Think of that – then think about the additional wonder that there are more motors, ratchets and machines at work, on a much smaller scale, inside every cell of the plant.
It was nice of the authors to spare us any evolutionary just-so stories about how this all came together by chance. Their only use of the E word was in reference to human history: “The short evolutionary time since domestication (about 10,000 years), probably allowed the complete loss of awns in several domesticated wheat lines, but not the alteration of the awn structure.” If so, this is a case of devolution, not evolution. They actually used the word design twice.* Anyone believing evolution could design this mechanism needs to eat more whole wheat to provide better nourishment to the brain.
The authors provided a couple of short time-lapse video clips to illustrate the bending action, but the best way to see this is to get a copy of the wonderful Moody Video production called Journey of Life. The filmmakers made an eye-popping time-lapse sequence of wild oat seeds, which propel themselves by a similar mechanism, but with twisting action instead of bending. You would swear you were looking at insects crawling along the ground instead of plant seeds. This and many other ingenious seed-dispersal mechanisms are wonderfully illustrated in this film (also recapped in Part 1 of the trilogy Wonders of God’s Creation).**
Plants may seem passive, anchored to the ground. In their own ways, though, they get around like world travelers: crawling, climbing, boating, ballooning, launching, helicoptering, hitchhiking and hunting (e.g., Venus flytrap), surprising us each time with their built-in ingenuity.
*Anecdote: One of the authors of the paper works for the Biotechnology Department of the Tel Hai Academic College in Upper Galilee, Israel. This is in the vicinity where a certain Teacher told some parables about wheat and sowing (e.g., Matthew 13). He also said, “I tell you the truth, unless a kernel of wheat falls to the ground and dies, it remains only a single seed. But if it dies, it produces many seeds.” (John 12:24). He was speaking in reference to the results His impending death would accomplish.
**Project: This article and the Moody films suggest a science project for your junior-high or high-school student. Many video camcorders have a time-lapse function (sometimes called interval timer). If you already own one, you have the most expensive part of a good science project. Look for backyard weeds and grasses with awns or other external structures; for instance, the seeds of filaree (Erodium cicurarium) work like little power drills. Suggest a hypothesis for how the shape of the seed contributes to its dispersal. Build a terrarium where you can control the cycles of temperature and humidity using electrical timers, and use the camcorder interval timer to record the action. Show your video clips with your display at the science fair. This seems like a sure way to attract the attention of the judges – and the envy of the other students. Better still, a demonstration of biological design might kindle some thoughts about a Designer.