The Frequency of Thirst

The graduate student had left a sticky note on the equipment rack that said DO NOT TOUCH in three different colors of marker. Soren read it twice, then looked at Maya.

Maya was already not touching anything. She was standing very still in front of a row of tomato plants, her head tilted, doing the thing she did when something wasn't adding up yet.

"He said we could wait in here," Soren said. "He didn't say we could use the ultrasonic detector."

"I know what he said."

"You have that look."

"I don't have a look."

She had a look. Soren got out his notebook.

They were there because of a school project on plant biology, and Dr. Reyes had said yes, fine, wait in the greenhouse while I finish the department meeting, it will take twenty minutes. That had been forty minutes ago. The fluorescent lights buzzed. The air smelled like warm dirt and something green and sharp Soren couldn't name.

The equipment rack held a laptop, a small gray box with a dial on it, and a microphone on a flexible arm that was pointed at the tomatoes. A cable ran from the gray box to the laptop. The screen showed a graph with a flat green line running across it. The y-axis read kHz. The numbers went up to one hundred.

"He's recording something," Soren said.

"From the plants," Maya said. It wasn't a question.

"Plants don't make sounds."

"The machine is pointed at the plants, Soren."

He looked at the microphone. He looked at the flat green line. He wrote: plants, ultrasonic detector, why.

The tomatoes were ordinary. Some of the lower leaves were yellowing. One plant near the end of the row looked different from the others, its leaves curled slightly inward at the edges, the soil around it pale and dry. Someone had put a small paper flag in that pot. The flag had a number on it. Seven.

Maya walked to plant seven and crouched down.

"This one hasn't been watered," she said.

"The flag probably means something."

"It means they're doing something to it on purpose." She looked up at the microphone, then at the laptop screen, then back at the plant. "Soren. Look at the graph."

He looked. The green line wasn't flat near plant seven. Near plant seven it was jumping. Small spikes, irregular, like a heartbeat that didn't know it was a heartbeat. Fifty kilohertz. Sixty. Back down. Up again.

"That's coming from the plant," Maya said.

"It can't be coming from the plant."

"Nothing else is in that part of the graph."

"Plants don't," he started, and then stopped, because the line jumped again at sixty-three kilohertz, and plant seven's leaves were curled, and the soil was dry, and the microphone was pointing directly at it.

He wrote it down. He wrote it down very carefully.

"We can't hear it," Maya said. She wasn't talking to him. She was talking to the plant. "It's too high. Above twenty kilohertz, we just, nothing. It's not there for us."

Soren checked the y-axis again. He thought about the range of human hearing. He thought about dogs, and how people always used that as the example, the thing beyond what you could know was happening. But this wasn't a dog whistle. This was a tomato plant. A tomato plant with dry soil and curled leaves, spiking to sixty kilohertz in a quiet room.

"What is it doing, though," he said. "What is the sound actually from."

Maya shook her head. "I don't know."

That was the part that stopped him. Maya always had a theory. She was already somewhere in the answer before he'd finished the question. He looked at her face and she was genuinely, completely uncertain, and somehow that made the spikes on the graph more real than anything else in the room.

"The water," he said slowly. "Plants move water up through, there are tubes. Xylem. If there's not enough water, maybe the tubes, if a bubble forms in a tube and then pops."

"Like when you crack your knuckles?"

"Sort of. But tiny. And sixty thousand times a second kind of tiny."

Maya looked at the plant. The plant did not look back. The graph jumped at forty-eight kilohertz and came down.

"It's not calling for help," she said. "It doesn't know we're here. It doesn't know anyone's here."

"No."

"It's just, it's just doing it. The whole time. Every plant that's ever been thirsty, anywhere, has been doing this, and we just," she opened her hand and closed it, "walked right through it."

Soren wrote that down too. He wasn't sure exactly what he was writing. He wrote: doing it whether or not anyone hears it. He looked at the words for a moment.

The door to the greenhouse opened and Dr. Reyes came in pulling off his lanyard and already talking about the project, about methodology, about what they would be allowed to observe. He was apologizing for being late. He was looking at his phone.

"Sorry, sorry, department meetings, you know how it is, I hope you weren't too bored in here, nothing exciting to look at yet, we're still in early data collection, the real results are,"

"Plant seven is spiking," Soren said.

Dr. Reyes looked up. He looked at the laptop screen. He walked to it quickly and leaned in, and his whole body changed, the phone going into his pocket, the lanyard dropped on the rack.

"You could tell from across the room?"

"The microphone was aimed at the dry one," Maya said. "And the graph was moving."

Dr. Reyes looked at them with an expression Soren didn't have a word for. Not surprise, exactly. More like: of course.

"How much do you know about cavitation?" he asked.

"Almost nothing," Soren said.

Dr. Reyes pulled over two stools. He sat down himself and looked at the screen, where plant seven was still spiking in a frequency no human ear would ever find on its own.

Maya reached out one finger and almost touched the dry, curled leaf, and then didn't.