Dionea Muscipula, also known as the Venus Fly Trap, is universally considered an interesting and eye-catching plant. Most people are fascinated by its ability to snap its lobes closed around prey, allowing it to then chemically dissolve the trapped animal and subsequently absorb nutrients from its body. What most people fail to realize is the incredible amount of biomechanics required for this plant to survive. Not only is this information useful and interesting to know, but it is crucial for scientists to better understand how plants can respond to physical stimuli.Continue reading “Secrets of the Rapid Snapping Mechanism of a Venus Fly Trap”
Scientists have studied plants for centuries. Charles Darwin in particular had studied climbing plants for years in an attempt to discover the secrets behind how they climb structures. Despite the years of study, very little is known about the mechanics that allow plants to climb. If the methods can be discovered, scientist might be able to mimic the power and flexibility that climbing plants have in man made structures, or be able to design structures that can better withstand the destruction plants tend to inflict upon buildings and roads.
There are five main types of climbing plants:
- Twining Plants
- Leaf Climbers
- Tendril Bearers
- Root Climbers
- Hook Climbers
Each type has a different mechanism for climbing that can be explored and replicated. To best understand how plants move, however, one my first understand a process called circumnutation.
Circumnutation is an action that all plants perform, some in a much more subtle way than others. In circumnutation, the plant’s stem bends and rotates around its central axis. Most non-climbing plants are circumnutating in a very small way that is difficult to notice. Climbing plants, however, will circumnutate much more dramatically with the top part of the stem parallel to the ground. The stem will then sweep around, as seen in the image to the right, in larger and larger circles until it contacts a support structure, at which point it will begin to climb.
The specifics as to how plants can move like this remains unclear. While it is known that cells in the stem change volume in a way to enable these bending an circular motions, it is unclear how the volume change occurs and how the specific cells are targeted for the change.
Twining plants are a great example of plants that circumnutate in an extreme way. They circumnutate until they contact their support structure, at which point they will climb by coiling around it in an even helical shape, as seen in the figure the left.
There have been several studies about the mechanics of twining plants. One theory as to how twining plants stay up on the supports was that the plants squeeze the structure as they climb. In order to prove this scientists allowed a twining plant to climb up a structure that had a strain gauge embedded in it to measure the amount of force felt by the structure. As it climbs the stem is put under tension and tightens around the structure, holding it in place.
To better understand the tension felt by the stem, scientists performed a test that compared how much force a vine wrapped around a support could sustain before failure. In this study a jar with 12 grams of water was tied to the top of a vine over a pully, and an empty jar was tied to the bottom of the vine over another pully. Water was added to the bottom jar until the vine broke. The vine was able to support 600 grams before breaking, much more than a piece of twine that was also tested and could only support 40 grams of water.
Leaf and Hook Climbers
Leaf and hook climbers are considered irritable plants. They respond to outside objects by growing thicker and bigger stems. Unlike twining plants that circumnutate with their main stem, leaf climbers circumnutate with stems called petioles that are off-shoots of the main stem. Hook climbers circumnutate with thorn-like hooks that grow off of the main stem and then wrap around the support structure.
When hook climbers experience an outside force, they break off at the spot where the hook meets the main stem. This point of failure allows the plant to live longer when they might otherwise snap and die.
Tendril bearers are similar to leaf climbers, as they circumnutate with petioles. These petioles will coil in mid-air in the shape of a spring, then coil in the opposite direction around the support structure. The spring gives the plant flexibility to move in reaction to outside forces like wind without breaking. It is unknown exactly what causes these strange coiling habits, although some scientists believe the direction of gelatinous fibers that make up the plant indicate the direction they will coil.
Unlike other climbing plants, root climbers do not use circumnutation to find and climb support structures. There are two main ways in which they grow: they fill in irregular spaces found in the support structure or they attach using glandular secretions that have an adhesive quality similar to cement. The mechanics of filling in irregular spaces are more familiar as they are similar to those of circumnutation. However there are many unknowns when it comes to glandular secretions, with the chemical composition having yet to be studied in depth.
Overall, there is still much to be studied about the mechanics of climbing plants. The different classifications often have different mechanisms behind their growth, and while much has been observed as to how certain plants climb support structures, little has been proved as to why they can do what they do, especially at the cellular level.
How far do you think a dandelion seed can drift from its base plant?
The Common Dandelion (Taraxacum officinale) primarily relies on wind flow to scatter its seeds. The dandelion seed has a fluffy structure that enables it to hold the most prolonged wind-based dispersal record. Commonly the seeds land 2 meters away from their mother plant. Still, in windy, dry weather favored by the dandelion, the seeds can fly up to 30 kilometers and even far (150 kilometers in some conditions). The vital point in this extraordinary adventure of the dandelion is flying with a constant velocity and having a short descent time, which means it should stay stable in the air for a relatively long time.
A study tried to illuminate the mystery behind the dandelion seeds’ ability to stay aloft in the air. Researchers attempted to mimic the dandelion flight using similarly structured silicon disks in a wind tunnel that simulated the airflow around the pappus. Pappus, the flying seed of the dandelion, consists of radially oriented filaments, each interacting with other adjacent ones, resulting in a reduction in the airflow. They compared the flight of natural seeds collected from one plant with several silicon disks with different porosities.
Their results showed that when pappus separates into the air, it forms an air bubble detached from its surface above it. This air bubble, known as a vortex, is unique to the dandelion seed. When the pappus is released in the air, it takes some time to reach a steady point, in which the vortex becomes symmetrical. An important feature affecting this symmetry is the porosity of the pappus, defined by the number of filaments and their dimension in each pappus— generally, the more the porosity, the steadier the flight. The best porosity for a stable vortex ring is greater than 84.97%, and the vortex ring begins to separate as the porosity falls below 77.42%. The experiment also indicated that the vortex is axisymmetric in low velocities, but it begins to lose its symmetry as the Reynolds increases.
This phenomenal plant has evolved techniques for mass germination, even in the absence of wind! Pappus’s hairy structure enables it to attach to an animal’s skin and soil particles, assuring the dandelion to have enough seed dispersion during the flowering season. More interestingly, the dandelion is known to have an informed dispersal in response to environmental fluctuations. When the plant experiences root herbivory, it intensifies the seed dispersal. The flight distance may vary from meters to kilometers to assure good germination in the absence of threatening conditions. Imagine a rainy day; the moist weather condition makes the pappus’ bristles come close together, reducing the possibility of separating from the plant. The moisture makes the current spot a suitable choice to stay. So even the detached seeds prefer to fall close to their base plant.
It worth mentioning that this remarkable plant also inspired areas of science. In their study of Mars exploration, researchers presented a primary model of a dandelion-inspired rover. Considering the harsh environmental condition on Mars, other excavators find the wind flow as an obstacle that results in damaged parts and incomplete missions. In contrast, the new dandelion-shaped rovers use the wind flow as an accelerating point to explore locations on Mars that other robots couldn’t access.
As Maple trees shed their fruits, it is hard not to be captivated by the view and stare in admiration. The free fall of maple seeds is simply graceful. Commonly referred to as helicopters, samaras are the fruit of Maple trees. Inside of each fruit one can find seeds that are used by the parent plant to produce new ones. The nickname helicopter refers to the similarity that exists between its motion as it falls to the ground and that of a helicopter. Indeed, a remarkable aspect of the samaras is the behavior they display as they fall. As the fruit of the Maple seed descends to the ground, it performs a rotating motion that mimics the rotor blade of helicopters in unpowered descent, a behavior that has intrigued scientists and has been the subject of many studies. The auto-gyration motion and flight mechanics of the samaras have been observed in order to explain why and how the fruit rotates on itself as it leaves the tree.
Importance of understanding the whirling motion of Samaras:
The use of seeds to produce new plants is called seed dispersal. Since trees are unable to move, they rely on different means for the dispersal of their seeds, such as the wind, water, animal, or human beings. Samaras being carried away by the wind are an example of wind dispersal. Seed dispersal is important for the survival of species. It allows the plant to spread in its environment. By allowing the seeds to fall at great distances from the parent plant, decreases the chances of interference of the growth of new plants with the development of the parent plant and allows it to colonize new environments. Studying the flight of samaras helps understand how this process can be enhanced. However, not only does it help with seed dispersal, but it may also have applications that can help advance the field of aviation and the conception of flying devices. (Sang Joon Lee et Al. 2014)
There are two principal categories of seeds that have the ability to use oncoming winds to travel away from the parent plant, pappose seeds, and winged seeds. Maple seeds fall within the second one. As the name indicates, winged seeds tend to have appendages that resemble wings and act as such. While pappose seeds use draft force to maintain their flight, winged seeds rely on lift force. While they share common traits, not all winged seeds rotate. The rotation of samaras is due to the location of its center of gravity which is found near the terminal end of the wings. The wings of the seeds of the Maple tree generate a stable leading-edge vortex (LEV) which accounts for the lift force slowing down the descent. Note that an LEV is a type of airflow that is halfway between a steady and a turbulent flow. Upon being detached from the tree and is released into the air, the very shape of the wing initiates the stable vortex. The mechanism used by samaras has been identified to be similar to the LEVs of insects such as flower flies.
If you think you’re shy, you should meet the plant known to botanists as Mimosa pudica! Also known as a touch-me-not, shame plant, or humble plant, M. pudica reacts rapidly to external stimuli – such as being touched, changes in heat, or changes in light intensity. The reaction generally includes the folding in of the plant’s leaves and the stem bending downward. These movements make the touch-me-not one of the most curious plants on the planet.
So how does the M. pudica react so quickly? The answer lies in the pulvini and changes in turgor pressure. Pulvini are the thickened bases of leaf stalks and leaves that act as joints for the plant. Because of the pulvini, M. pudica is able to fold in any direction. This manifests itself as what we see as “drooping” of the stem and folding of the leaves.
Turgor pressure refers to the force exerted by a fluid in a cell onto the cell wall. In other words, the fluid – in this case, water – pushes the cell membrane up against the cell wall. When the turgor pressure is high, the plant is more rigid, as a healthy touch-me-not is before stimulus. When the turgor pressure decreases – caused by the external stimulus – the pulvini and leaves droop, resulting in the “shy” appearance of the plant.
The final piece of the puzzle of the shame plant is how and why external stimulus results in this rapid change of turgor pressure. This is caused by potassium and chlorine ions – K+ and Cl-, respectively. When the plant experiences external stimulus, the ions move out of the cell through the ion channel. Because of the resulting increased ion concentration outside of the cell – and the decreased concentration inside of the cell – water also moves out of the cell. Because turgor pressure relies on the force of the water against the cell wall, the pressure quickly decreases. This results in M. pudica drooping and folding its leaves.
The uniqueness of M. pudica comes from its ability to react quickly. This rapid reaction is a result of the water channels known as aquaporins. Aquaporins are selective channels that allow water molecules to move outside of the cell without allowing the movement of other ions/molecules. This allows the water molecules to move outside of the cell at a rapid pace – about 2 seconds.
In summary, Mimosa pudica is a curious little plant with more to it than initially meets the eye. External stimulus results in the movement of ions from inside the plant cells to outside the plant cells. This change in ion concentration creates an imbalance, causing water to rapidly leave the cell through aquaporins. This decreases the turgor pressure, resulting in folding of leaves and the appearance of wilting through the use of the pulvini. Remember all of this next time you come across a touch-me-not; one little tap of its leaves will set off this entire chain reaction!
Sources and Further Reading:
Ahmad H, Sehgal S, Mishra A, Gupta R. Mimosa pudica L. (Laajvanti): An overview. Pharmacogn Rev. 2012;6(12):115-124. doi:10.4103/0973-7847.99945
Hagihara T, Toyota M. Mechanical Signaling in the Sensitive Plant Mimosa pudica L. Plants. 2020; 9(5):587. https://doi.org/10.3390/plants9050587
Sampath, Bhuvaneshwari. “Molecular Magic behind the ‘Touch Me Not’ Plant.” Science India, scienceindia.in/home/view_article/58.
Song, K., Yeom, E. & Lee, S. Real-time imaging of pulvinus bending in Mimosa pudica. Sci Rep 4, 6466 (2014). https://doi.org/10.1038/srep06466
Nash, Tainaya. “This Plant Moves When You Touch It, and the Video Is Wild.” House Beautiful, House Beautiful, 28 June 2019.
There you are, sitting in the park eating your spaghetti picnic on your favorite picnic blanket when your pollen allergy acts up. You let out a sneeze powerful enough to compete with Aeolus’ bag of wind, but now your spaghetti is all over your favorite picnic blanket. You immediately go to rinse it off, but your fine Italian sauce has thoroughly soaked in. If only nature had a solution to keep a surface clean. Enter: the lotus leaf.
The lotus leaf is renowned for its ability to stay clean in murky environments. This characteristic of the plant is regularly attributed to its superhydrophobic surface features and chemistry. A superhydrophobic surface is a surface which can maintain a contact angle with water above 150o and is correlated with a low free surface energy—which really means water pools and rolls off rather than soaking into the surface.
Modified from Zorba et al. 2008
A key attribute of the superhydrophobic surface is a hierarchical micro- and nanostructure. The microstructure is composed of plant cells grown in little mounds known as a “papillae” with small channels for air flow in between called “stomata.” The nanostructure is composed of hair-like wax crystal towers (epicuticular wax) built on the peaks of the papillae topography. The elevated wax towers combined with the stomata trap air and reduce the contact area of the water with the surface. The epicuticular wax chemistry reduces the adhesion to the towers themselves by being naturally hydrophobic.
Modified from Zorba et al. 2008
The tips of the wax towers create the largest repelling forces which form larger contact angles, while shorter towers can actually produce adhesive forces that reduce the contact angle. If the air is displaced and filled with water, the contact angle will decrease due to the water-water adhesion which “pulls” the droplet to the surface. Similarly, if the surface is damaged, the wax can be removed and decrease the surface’s hydrophobicity. The wax is naturally soft material and prone to mechanical damage increasing water adhesion and reducing the self-cleaning abilities of the leaf.
The papillae topography is the key to the robustness of the lotus leaf hydrophobicity. The papillae create natural valleys and creases which—like the tops—are still densely packed with wax hairs. When the surface is impacted, only the top of the papillae are exposed to the mechanical force so the wax tubules in the valleys are left undeformed and maintain their hydrophobic characteristics.
Photo by Chase Pellerin via Gear Patrol
Hydrophobic surfaces have many applications in everyday life, for example rain jackets and umbrellas perform their best when they are hydrophobic. Manufacturing processes rely on hydrophobic surfaces to reduce oxidation and stay clean in past-paced environments, and your favorite picnic blanket would be much less prone to spaghetti stains if it were hydrophobic. Nature has solutions to keeping surfaces clean; we just have to recognize them.
Plants come in all shapes and sizes, from the smallest blades of grass to trees so big that the tops can’t even be seen from the ground. But all plants are made from the same basic cell structures and components. So why is it that I can easily pick a flower, but could spend hours chopping at a tree and hardly make a dent? Trees are so much stronger than almost every other plant that they have become a staple in the construction industry. The key to the success of the tree is small differences in the structure.
Cell walls are made up of cellulose, pectin, and lignin. Cellulose, with a Young’s Modulus of 120-140 GPa, provides the most structure to the cell wall, and the cell walls of trees have high concentrations of cellulose, averaging 45%. The other components in-between the cellulose, also can greatly impact the stiffness of the cell wall. Along with the cellulose, the secondary wall of tree cells contain more lignin that pectin as a binding agent. Lignin is found in all plant cells, but the high concentrations found in tree cells are what set it apart from other plants. Lignin has a Young’s Modulus of 3 GPa, while pectin is a gelatinous component that provides little structure. A study conducted by Donaldson also showed the presence of lignin has increased the size of the microfibrils and cellulose matrix to make the cell wall less porous, further increasing stiffness. The high concentration of lignin in the cell wall is a defining characteristic of tree cells and greatly increases their stiffness and rigidity.
The arrangement of the lignin and cellulose in the cell wall also increases stiffness. As tree cells divide, different layers of the cell are formed. This process is described by Plomion et al. in their article on wood formation.
The first layer is the middle lamella, which is made up mostly of pectin, but lignin is added throughout the differentiation period to eventually increase stiffness. This layer is mostly used to adhere the cell together. The next layer is the primary wall, which can consist up to 70% lignin. The primary wall is very elastic, to allow the cell to continue to grow. In other plants, this layer can have a higher concentration of pectin. The lignin in this layer means the tree cell is less malleable, but also increases stiffness and strength. The final layer is the secondary wall, which is separated into three layers, S1, S2, and S3. Most plants only have only one secondary wall, so having three distinct layers increases the strength and stiffness of the tree. Each layer consists of cellulose microfibrils, which are arranged in parallel, but in a different orientation for each layer. As the cell grows, the secondary layer is packed with more lignin, increasing the stiffness of cell wall. The S2 layer is the thickest, and contains about 45% cellulose and 20% lignin. Again, the higher lignin content, instead of pectin, allows for more rigidity, strength, and stiffness in the layer that provides the most structure to the cell. All of the cellulose and lignin layers, closely packed together, create a very stiff cell wall unlike any other plant.
It’s the small changes to the structure of the cell wall, such as adding lignin and layers into the secondary wall, that make the difference in the structure between a flower cell and that of a tree.
Sources and Further Reading:
- Gibson Lorna J. The hierarchical structure and mechanics of plant materials J. R. Soc. Interface. 927492766 (2012) . http://doi.org/10.1098/rsif.2012.0341
- Cousins, W.J., Armstrong, R.W. & Robinson, W.H. Young’s modulus of lignin from a continuous indentation test. J Mater Sci 10, 1655–1658 (1975). https://doi.org/10.1007/BF00554925
- Donaldson, L. Cellulose microfibril aggregates and their size variation with cell wall type. Wood Sci Technol 41, 443 (2007). https://doi.org/10.1007/s00226-006-0121-6
- Christophe Plomion, Grégoire Leprovost, Alexia Stokes, Wood Formation in Trees, Plant Physiology, Volume 127, Issue 4, December 2001, Pages 1513–1523, https://doi.org/10.1104/pp.010816
Trees have the potential to be the largest organisms on Earth. The world’s tallest tree, dubbed Hyperion, is 380 ft tall and weighs over 1,600,000 lbs. Compared this to the world’s largest animal, a particularly massive blue whale which was 100 ft long and weighed 380,000 lbs, the simply massive size of this tree should be obvious. And unlike a whale, a tree is much less likely collapse and crush itself under its own weight. Trees need to be tall, even if doing so consumes a lot of resources, in order to compete for sunlight. So what lets trees get this big, and what limits their height?
There are two primary rules that govern tree sizes. The first is mechanics, the way the trunk of the tree is built and how it responds to weight. The wood of the biggest trees has a very high strength to weight ratio, which enables a tree to carry its own massive weight without collapsing. The layout and structure of this wood is analyzed at length in the journal by M. Ramage, but in summary, tall trees have internal cells called tracheids. These tiny circular tubes are 2-4 mm long and around 30 μm wide and provide support to the tree and allow water to flow throughout it, without adding as much weight.
The high strength to weight ratio of wood allows trees to support themselves at incredible heights. Using B. Blonder’s research about the scaling of trees, it can be shown that trees are so strong and yet comparatively light weight that a tree would not actually collapse under its own weight until it was almost 15 Empire State Buildings tall. Obviously no tree is this tall, meaning some other factors must limit their height, but the incredible strength of wood should now be clear!
The second primary rule that governs tree size is hydraulics, and it restricts the height a tree can reach. Hydraulics defines a tree’s ability to move water from its roots to its upper leaves in order to perform photosynthesis. The taller a tree gets, the more difficult this process becomes until the tree becomes incapable of growing any taller. G. Koch’s article, The Limits to Tree Height, explores how this hydraulic system works and how it restricts the heights a tree can reach. Xylem, tiny internal pipes that run from the roots to the tops of the tree, and carry water in a long continuous column this whole length. The longer this column becomes, the more difficult it is to maintain and the greater suction pressure that occurs at the highest leaves. Koch studied how properties in leaves changed the higher up they could be found, determining that the efficiency of Photosynthesis decreased, the pressure at the end of the xylem increased, and the size of leaves decreased. At great heights, the status of leaves seemed remarkably similar to those of a tree undergoing a severe drought.
Koch determined that these changes with height would eventually hit a maximum limit which they could not exceed, a limit that was determined to occur between 122-130 meters. So while the efficient properties of wood allow trees to reach incredible heights, their restricted ability to move water limits just how tall they can grow.
Sources and Further Reading:
- Ramage M., Burridge H., Busse-Wicher M., et al. The wood from trees: The use of timber in construction. Renewable and Sustainable Energy Reviews 68, 333-359 (2017)
- Blonder B. The size of trees: exploring biological scaling (2010)
- Koch, G., Sillett, S., Jennings, G. et al. The limits to tree height. Nature 428, 851–854 (2004)
When something bad happens, do you tell your friends? Do you warn the people around you?
Similar to human beings telling one another about risks and dangers, plants can communicate with one another about environmental stressors. One stressor for plants is physical contact with other plants, such as their leaves touching. The presence of other plants means competition for resources and the possibility of invasive species. Biologists at the Swedish University of Agricultural Sciences conducted a study on the ability of plants to communicate with neighboring plants through chemical signals excreted through their roots to indicate to neighboring plants that they contacted another plant above ground.
In their experiment, the Root Choice Test, maize plants were placed in individual chambers where the roots had two possible paths for further growth. One path led to a solution made from the soil of a plant whose leaves had been touched by the leaves of another plant every day for a week. The other solution had soil from a plant that had not been disturbed.
Despite there being an equal chance of growth in either direction, the maize plants chose to grow in the solution of the untouched plant significantly more often. In some cases, a plant would begin to grow roots in the direction of the touched plant’s soil, but then changed growth direction to the other solution. There were no scenarios wherein a plant would switch from the undisturbed plant solution to the touched plant solution.
These results demonstrate a robust system of communication between plants. Plants possess the ability to recognize when their leaves touch another plant’s leaves and can transform this information into a chemical signal released by their roots. Other plants can understand these signals and modify their behavior as a result.
Plants can also communicate through physical connection including underground fungal networks. These networks are made up of fungal root hairs called hyphae that insert themselves into the cell membrane of a plant’s roots. Hyphae are made up of long, individual, filamentous branches whose structural characteristics, such as cell wall thickness and branching, are determined by its function. These vast systems of interconnected fungi plants act like telephone wires for communication.
Plants in these networks are able to exchange nutrients, become more resistant to disease, and grow larger. Healthy areas of the fungal network have also been shown to respond defensively when other areas of the network have been under bacterial attack. This shows that the fungal networks and the plants they connect exchange warnings similar to plants communicating though chemical signals in the soil.
Research into plant communication could one day lead to producing desired changes in plants by artificially mimicking their communication. Farmers could create chemical or physical signals that instruct their crops to grow more effectively. As research into this topic continues, perhaps one day, plant communication will be as well understood as any spoken language.