1. Does Earth's Rotation Decide Which Way Morning Glories Wrap?
Morning glories always wrap in the same direction. Because we live in the Northern Hemisphere, it is tempting to think they wrap in alignment with the direction of Earth's rotation. However, according to a Q&A from the Japanese Society of Plant Physiologists, plants in the same region wrap in both directions: some wrap in the same direction as the rotation, and others wrap in the opposite direction. Morning glories in Australia reportedly wrap in the same way as those in Japan. This suggests that Earth's rotation is not the cause [1].
It is considered certain that the direction is determined by genetics. However, at the time of that answer, research had not yet advanced on how those genes function [1]. Currently, the morning glory's wrap is called "right-handed" (or right-winding). This follows the same counting method used for screws [2]. Setting aside the mystery of direction for a moment, let us look at the "mechanism of the body" that allows the vine to wrap.
2. How Does a Morning Glory Vine Tip Move in the Air Before Touching a Support?
When wrapping, the vine does not suddenly hug the support. Long before it touches the support, the tip of the stem moves slowly in the air, tracing circles or ellipses. This movement is called circumnutation. In a 2005 paper by Kitazawa and colleagues, the amplitude of this movement in the flower stem of wild-type *Arabidopsis thaliana* was measured at about 20 mm, with a period of about 120 minutes (one full circle taking about 2 hours) [3]. These are not values for morning glories, but measurements that change depending on conditions, so they are only rough guides. This means it takes about 2 hours to draw a small circle. So, what clues cause this rotating movement to occur?
3. Why Can't Weeping Morning Glories, Which Don't React to Gravity, Wrap Around Supports?
A mutant variety that does not react to gravity with its stem and does not grow upward is called "weeping morning glory." Interestingly, the root's reaction to gravity is normal [3]. The paper reports that weeping morning glories could not perform either circumnutation or the reaction of wrapping around a support [3].
So, where in the stem is gravity being sensed? The paper concludes that cells called the endodermis in the stem sense gravity, and that these cells are essential for both circumnutation and wrapping. The weeping mutation is reported to result from the loss of function of a gene called SCARECROW [3][4]. The force to wrap is paired with a "gravity sensor" inside the stem.
It is important to note here that only the stem is defective in these plants.
4. How Does the Johnsson Two-Oscillator Model Explain the Vine's Circling Movement?
There is a concept called the Johnsson two-oscillator model for how circumnutation occurs. The paper references the idea that the reaction to gravity works as a "feedback oscillation" driven from the outside, combining with the rhythm the plant has inside its body to create movement [3]. The image is that a tilted stem tries to correct itself toward the direction of gravity, overshoots, and then corrects again to the other side. This repetition creates the circular movement.
This is not a confirmed explanation, but one of the models being considered. The paper also states that there is still no direct evidence that the reaction to gravity drives circumnutation [3].
5. How Can You Investigate Twist Direction Yourself with Screws and Jar Lids?
Try examining the "twist direction" of screws or jar lids you have at home. Those that move in a clockwise direction when advancing along the screw are called right-handed [2]. Comparing them shows that "direction" is determined by naming conventions. The overall picture of wrapping and the history of how these names changed are also covered in another note on the same site, "vine-twining-mechanism" (how vine plants wrap). If you want to read the original source, the paper from [3] is available in full text on PMC. It is in English, but you can gather information from the figures and abstract.