1. Tendrils Ignore Raindrops but Bend When Touched by Light Threads
Tendrils have no hands or eyes, yet they seem to know when they are touched. Charles Darwin, known for his work on evolution, investigated this sensitivity through experiments. In a type of passion flower, Darwin found that placing a very light thread loop gently on a tendril caused it to bend. [7]
It even bent when touched by a fragment of thin platinum wire, which he noted weighed about 1/50 of a grain. Calculating with 1 grain as approximately 65 milligrams, this is about 1.3 milligrams. Since a 1-yen coin weighs 1 gram, this is roughly one-thousandth of that weight. [7]
However, the tendrils did not bend when water droplets were flicked with a brush, even when water was blown strongly enough to swing the whole tendril. [7]
It is thought there is a mechanism where they react to very light objects that remain in contact, but not to raindrops. However, these results are from the passion flower family Darwin studied and cannot be applied to all plants. So, when does a tendril start moving after being touched? [7]
2. Tendril Bending Starts in Minutes, but Coiling Takes Half a Day
Darwin also measured the time from touch to bending. For a species called *Tacsonia manicata* in the passion flower family, he noted that after rubbing a tendril, it bent slightly in about 7 minutes, bent clearly in 10 minutes, and became hook-shaped in 20 minutes. [7]
When a tendril touches something, the tip begins to wrap around it at the same time that the part closer to the base twists into a spiral to pull the plant body closer. [7]
The movement of the spiral shrinking is much slower. Darwin wrote that in a member of the gourd family, shrinking begins about 12 to 24 hours after it grasps an object. [7]
This is the same family as bitter melon and cucumber, but it is an observation of a different species. It can be read as grabbing in minutes to 20 minutes, and shrinking in about half a day. However, because these two timeframes come from different species, they cannot be combined into one single schedule. So, what force creates the spiral in a tendril that has no muscles? [7]
3. What Force Drives Tendril Coiling Without Muscles?
According to an explanation from the National Institute for Basic Biology, in tendrils of the *Cucumis* genus (which includes cucumbers), gelatinous fibers hardened by lignin (the substance that makes wood hard) form inside the soft tissue on the back side. It is proposed that the asymmetric shrinking of these fibers on the back and front sides causes the tendril to coil. [7]
In the August 31, 2012, issue of *Science*, a research group from Harvard University reported results from studying cucumber tendrils. They found that a ribbon of cells, called g-fibers (two cell layers thick), runs along the entire length of the tendril. The asymmetric shrinking of this ribbon causes the coiling. [7]
One side of the ribbon layer contains more lignin and is considered harder. [7]
The researchers believe the source of the force that creates spirals without muscles lies in the difference in how this ribbon shrinks. So, what happens when you pull this spiral from both ends? [7]
4. Why Pulling a Coiled Tendril Tightens It, Unlike a Phone Cord
In a cucumber tendril with both ends fixed, a left-handed spiral connects to a right-handed spiral in the middle. Harvard University news notes that Darwin called this point where the winding direction reverses a "perversion." [7]
The more surprising result is what happens when pulled. A telephone cord unwinds and stretches when pulled. However, when the spirals of a cucumber tendril are pulled from both ends, they do not unwind; instead, the number of coils increases. [7]
It is reported that the spring is soft when pulling begins but becomes suddenly hard when pulled strongly. [7]
It is also written that older tendrils resist pulling more strongly than younger ones. [7]
However, these results are about cucumber tendrils and extracted fiber ribbons. It is not known if the same applies to bitter melon. So, are these coiling tendrils choosing what they touch? [7]
5. Some Tendrils Avoid Beans Infested with Spider Mites
There are also reports of tendrils choosing what they touch. In a 2019 study from Kyoto University published in *Scientific Reports*, the tendrils of Japanese bindweed did not coil around bean plants that had spider mites; they let go. [7]
Morning glories, however, coiled around beans regardless of whether spider mites were present. [7]
It is thought that Japanese bindweed senses the webs spun by spider mites. [7]
However, this is an experiment with Japanese bindweed, not bitter melon or cucumber. Tendrils seem to be organs that change their movement depending on what they touch, rather than just being "springs." So, what can we check with common tendrils? [7]
6. How to Gently Observe Tendrils on Cucumbers, Peas, and Other Plants
If there are plants with tendrils nearby, you can observe them gently. Plants with tendrils include cucumbers, bitter melons, peas, and grapes. [7]
Find a tendril that has not yet coiled and record the time with a clock. Lightly place a thin thread on the tip first, and record changes at 10 minutes, 20 minutes, 1 hour, and half a day. Do not pull or tear the tendril. [7]
On a tendril that has already coiled into a spiral, look for the point where the winding direction changes. If the movement is different from Darwin’s passion flowers, it may be a clue to differences between species. [7]
For further reading, the National Institute for Basic Biology’s plant dictionary blog and the Kyoto University research summary are available in Japanese. [7] [7]