Childhood memories! The science version of "The Foot of Ivy"

Produced by: Science Popularization China

Produced by: Zhang Lijun (Wuhan Botanical Garden, Chinese Academy of Sciences)

Producer: Computer Network Information Center, Chinese Academy of Sciences

Do you still remember Mr. Ye Shengtao’s article "The Feet of Ivy" in the elementary school textbook?

The article describes creepers as follows: "It turns out that creepers have feet, and the feet grow on the stems. On the back of the stems where the petioles grow, six or seven branch-like filaments extend out, each of which is like a snail's tentacle. The filaments are tender red, just like new leaves. These are the feet of creepers."

The suction cup of Parthenocissus tricuspidata (Photo credit: Yuan Minghui)

Mr. Ye Shengtao observed the creeper from the perspective of a writer. In fact, the feet of the creeper he mentioned are suction cups.

With its suction cups, Parthenocissus tricuspidata is a leader in the climbing plant world. If you don't believe it, you can observe carefully and you will find that it will make the exterior walls of buildings full of vitality and greenery in a short time.

Ivy covers the exterior walls of buildings (Photo source: veer Gallery)

Suction cups - the "feet" of creepers

Parthenocissus, also known as creeping wall parthenocissus, ground brocade, flying centipede, belongs to the genus Parthenocissus of the grape family, and is a deciduous perennial woody climbing vine. As a climbing plant, climbing up with the help of other objects is a survival skill of plants such as Parthenocissus.

In the large family of climbing plants, there are various ways of climbing. Some climb by bending and rising through branches; some transform their flowers, leaves and stems into tendrils; some plants evolve suction cups, adventitious roots or thorns to fix themselves and achieve the purpose of attaching upward; and some smarter plants have multiple functions mentioned above at the same time, just to realize their dream of "flying over eaves and walking on walls".

The tendrils of ivy are transformed from stems. When they encounter a climbable surface, the top and tip of the tendril will develop into suction cups, which they use to climb upwards in search of growth space.

Tendrils and suction cups are like the "hands" and "feet" of the creeper. Using both its hands and feet, the creeper can firmly cling to rocks, walls or trees.

Tendrils and suckers of Parthenocissus tricuspidata (Photo credit: taken by the author)

In the field of botany, the first person to observe creeper was Darwin, the pioneer of evolution theory.

While observing the movement of the tendrils of Virginia creeper (also known as Parthenocissus tricuspidata), he found that some plants' tendrils would retract on their own after wrapping around branches or sticks, and when they touched the flat surface of wood or wall, they would bend all the tendrils toward it, spread out sparsely, and make the sides of their hooked tips touch it.

After about two days, the top of the tendril will swell to form a "small pad" that can adhere firmly, which is what we call a "suction cup."

Tendrils that are not attached to anything will wither and fall off after a week or two. Darwin proposed that the suction cups of creepers will not develop actively unless they are stimulated by something, such as temporary contact with some object.

Image source: Darwin, "The Movements and Habits of Climbing Plants"

2.8 million times! Ivy suction cup with super strong adsorption power!

In daily life, we often use suction cups, such as clothes hangers on the wall or storage trays in the bathroom. These vacuum suction cups use air pressure for adsorption and can generally withstand weights of several kilograms.

But compared with the adsorption power of creepers, the suction cups are a bit "small witch meets big witch". Darwin found that a mature creeper branch with a tree age of more than 10 years had only one suction cup in contact with the base. Under the condition of hanging a two-pound weight (gravity is about 8.9 Newtons) on the branch, the suction cup can still adhere firmly to the surface of the base without falling off.

Scientists have made detailed measurements of the suction force of Parthenocissus tricuspidata. The average mass of a mature suction cup is about 0.0005 grams, the average contact area with the substrate is only 1.22 square millimeters, and the adhesion force is 13.7 Newtons.

Through calculation, it is found that a single suction cup can support 260 times its own weight, including the weight of the stem, leaves, branches and tendrils; and the maximum pulling force that the suction cup can bear is 2.8 million times its own weight. This is a very amazing data.

As we all know, geckos have strong adhesion capabilities and can climb and even hang upside down on various walls. However, mature suction cups can withstand adhesion forces that are 112 times that of a gecko's feet.

In addition, based on a rough estimate of the contact area and adsorption force of the suction cup, a fingertip of a "bionic palm" designed to imitate the suction cup can support a 114 kg person through adsorption.

A fingertip of a hand made of suction cup material attached to a base can support a 114 kg person (Image source: Reference 1)

One suction cup is already very powerful, but the adhesion system of the ivy has multiple suction cups at the same time, plus the spiral structure of the tendrils as a "plug-in", allowing the ivy to climb vertically on the wall without fear of gravity and withstand strong winds and heavy rains.

How do suction cups grow? How do they stick?

The scientists propose that the development of a full-fledged sucker from a tendril tip is a complex process that relies on changes in the sucker's morphology and structure.

They found that the search branches on the creeper tendrils have a strong ability to recognize the substrate and can sense whether the substrate surface allows them to adhere firmly. After the immature sucker is stimulated by contact, a series of complex cell division and expansion processes occur, and at the same time, the cells of the epidermis and subepidermis accumulate a sticky substance and secrete it from the epidermal cells through the cell wall.

This mucus will rupture the epidermis, and the secreted mucus will make the suction cup sticky to the support, eventually bonding the suction cup and the substrate together.

In a completely adhered suction cup, the flowing mucus acts like "double-sided tape", occupying all the gaps inside the epidermal cells and the gaps between the epidermal cells and the substrate.

Searching branches on creeper tendrils (Image source: Reference 5)

Through microscopic observation, the suction cup of Parthenocissus tricuspidata is clearly divided into two parts: the central area and the peripheral area.

The peripheral area is the main area where the sucker secretes mucus and the epidermal cells extend. The uneven depressions on the surface of the base are either occupied by flowing mucus or filled with epidermal cells, forming a perfect bite between the sucker and the base, thus maintaining super strong adhesion.

Scanning electron microscopy experiments also revealed some new and unique microstructures of the suction cups. These sponge-like porous structures facilitate the flow and transport of mucus and significantly enhance the adhesion between the suction cups and the substrate.

Image source: Reference 1

Once the suckers are firmly attached, the tendrils begin to curl, thicken and become woody, giving the creeper's tendrils and suckers considerable holding power.

In addition, scientists have observed that the suction cups of ivy are arranged alternately along the main axis of the tendril, which not only follows the symmetry-asymmetry rules in architecture, but is also consistent with the principle of stable adsorption in surface physical chemistry.

The alternating arrangement of the creeper suckers along the main axis of the tendril is a classic example of structural mechanics. Even more amazing is that the geometric correlation between the suckers, tendrils and stems is surprisingly similar to the distribution of branch pipes and main pipes in urban pipe networks.

Distribution of creeper suckers (Photo source: veer gallery)

Model diagram of the arrangement of Parthenocissus tricuspidata suction cups (Image source: Reference 2)

Pipeline branch arrangement (Image source: veer gallery)

What is the "glue" secreted by the suction cup?

In his early years, Darwin also conducted solubility experiments on the sticky secretions in the creeper suckers. He collected some mature suckers from the plaster wall and soaked them in hot water for several hours. After that, he washed them with ethanol and acetic acid and found that the small stones adhering to the suckers were still stubborn and difficult to fall off.

However, after the suction cups were soaked in ether for a day, the stone particles began to loosen, and in mild essential oils (mainly peppermint and thyme), the stone particles were completely loosened after only a few hours.

The results of the solubility experiment seem to indicate that the adhesive component in the suction cup of Parthenocissus tricuspidata is a resin adhesive.

Later, scientists discovered through staining experiments that the sticky substance secreted by the ivy suckers is most likely an acidic sticky polysaccharide; and modern immunocytochemical methods further showed that the sticky properties of the suckers' secretions are mainly related to the rhamnosegalacturonan polysaccharide that has been debranched.

In 2012, scientists used HPLC/MS to preliminarily separate 21 organic components in the creeper suckers, most of which contained nitrogen, sulfur, and oxygen. Compounds containing these elements can basically produce polarity. Therefore, it is speculated that hydrogen bonding may be a weak adsorption force generated by the suckers during the climbing process.

In 2016, polarized light microscopy revealed that the cortical cells of the sucker contained crystals, which were commonly found in mature cells. Acid solubility tests revealed that the crystals were needle-shaped crystals composed of calcium oxalate, known as raphes. The needle-shaped crystals were embedded in bundles by an organic matrix. Large amounts of water can dissolve the crystal bundles and separate individual crystals.

Structural characteristics of calcium oxalate crystals (Image source: Reference 4)

Calcium oxalate crystals provide important mechanical support, enhancing the stability of the sucker on the substrate. In addition, the needle-shaped crystals may help protect the sucker from being eaten by herbivores or insects.

The stability of the crystals is very strong. Some people have found that after the stone on which the ivy grew had experienced 130 years of changes in nature, there were still crystals deposited on the surface of the base or the periphery of the suction cup.

Hypothesis of the adhesion mechanism of the suction cup

1. Interface reaction hypothesis

The scientists hypothesized that the adsorption mechanism is an interfacial reaction leading to the "anchoring" of the suction cup and the formation of negative pressure by nitrogen-oxygen inhalation.

The mucus secreted by ivy is a weakly acidic substance. A slow chemical reaction will occur at the contact surface between the suction cup and the substrate. This reaction is difficult to detect with the naked eye and ordinary analytical methods.

The chemical products of the interfacial reaction act as micro-fillers at the molecular level, which can significantly enhance the adhesion between the suction cup and the substrate.

This interfacial chemical reaction causes the sucker to "anchor" on the substrate surface. In addition, as the sucker grows and develops, the tip of the tendril is constantly stimulated by contact, and secretions are continuously produced, so that some air is enclosed in the sucker. During the growth and development process, photosynthesis consumes the nitrogen enclosed in the sucker.

At the same time, oxidation reactions of certain reducing secretions consume the oxygen contained in the cupula.

Photosynthesis and oxidation reactions consume almost all of the gas contained in the suction cup, which results in a negative pressure inside the suction cup, thereby increasing the adhesion strength between the suction cup and the substrate.

In addition, weak interaction forces such as adsorption, intermolecular force, electrostatic force, capillary force and van der Waals force also play an auxiliary role in adhesion.

2. Multi-level system enhancement hypothesis

Other scientists believe that, overall, the suction cup has a structure similar to reinforced concrete, in which cell wall fibers are like steel bars, cell matrix and mucus act as cement, and crystals are like stones.

In the natural environment, the mucus-filled epidermis of the suction cup dehydrates and solidifies to form a primary protective layer; the crystal-rich cortical tissue greatly increases the mechanical strength of the cortex, which is the secondary protection system; the vascular column with well-developed xylem is the tertiary protection system, which will play a role when the first two protection systems fail.

This multi-level system-enhanced structure can ensure the stability and durability of the suction cup adhesion to the greatest extent. In nature, it can last from 10 to 130 years or even longer.

In short, the adsorption principle of the suction cup is far more complicated than we imagined.

To reveal the microstructure and adhesion mechanism of the suction cup, more in-depth research is needed. The power of nature can not only create beauty, but also miracles.

References:

1. He Tianxian, Yang Wenwu, Deng Wenli. Review of the latest research results and research progress of Parthenocissus tricuspidata, a vine plant with super adhesion. Progress in Natural Science. 2008, (11): 1220-1225

2. He Tianxian. Study on the adhesion of Parthenocissus tricuspidata suction cups. South China University of Technology. 2012. Doctoral dissertation.

3. Zhang Li. Isolation, purification, structural characterization and adhesion properties of Parthenocissus tricuspidata sucker polysaccharide. South China University of Technology. 2014. Doctoral dissertation.

4. Yang Xiaojun. Functional morphology and structure of adhesion system of climbing plants. South China University of Technology. 2016. Doctoral dissertation.

5. Steinbrecher T, Beuchle G, Melzer B, et al. Structural development and morphology of the attachment system of Parthenocissus tricuspidata. Int J Plant Sci 2011, 172: 1120-1129.

6. Deng WL. Tendril, adhesive disc and super adhesive effect of climbing plant. Available from Nature Precedings (2008)