Despite the veins in a leaf appearing like a nervous system, our woody neighbors do not have a nervous system—but that doesn't mean they can't feel your touch upon their many hands.

Quite the contrary, scientists have established using sophisticated microscopy that plants register the beginning and end of every touch by sending slow waves of calcium signals to their cells.

Conducted at Washington State Univ., the scientists used 84 experiments from twelve members of tobacco and thale cress species that had been specially bred with calcium sensors.

Previous research has shown that when a pest like a caterpillar bites a plant leaf, it can initiate the plant's defensive responses such as the release of chemicals that make leaves less tasty or even toxic to the pest. An earlier study also revealed that brushing a plant triggers calcium waves that activate different genes.

Using a glass rod the width of a human hair, they gently probed the leaves' individual cells under a microscope to see what the response was.

"It is quite surprising how finely sensitive plants cells are—that they can discriminate when something is touching them. They sense the pressure, and when it is released, they sense the drop in pressure," said Michael Knoblauch, WSU biological sciences professor and senior author of the study in the journal Nature Plants.

"It's surprising that plants can do this in a very different way than animals, without nerve cells and at a really fine level."

It took just 30 seconds for the waves to reach out and begin signaling, pretty fast for tree-time.

The authors believe these waves are likely due to the change in pressure inside the cell. Unlike animal cells with permeable membranes, plant cells also have strong cellular walls that cannot be easily breached, so just a light touch will temporarily increase pressure in a plant cell.

Scientists like Peter Wohlleben, the famous German "tree herder" are prepared to make some pretty jaw-dropping claims about the awareness of plants—that they can see, talk, recognize relatives, and even teach.

The current study was able to differentiate the calcium waves between touch and letting go, but how exactly the plant's genes respond to those signals i.e. what the plant is capable of initiating in response to touch remains to be seen.

You can see the reaction of the calcium signals under the microscope in the .gif file at the WSU press page. 

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