The tree appeared sometime during the night, and by 0700 was big enough to trip Memnis as he stepped out of his room. Shugga and I were up the trellis, which, we well knew, was the best place to see without being seen.

Memnis’ armoured plates made a delightful racket, but Shugga snorted and Memnis’ hearing is so good that his eye swivelled toward us instantly. Scant minutes after he’d fallen over the tree, Shugga and I were stood before him; Shugga with his spines drooping forlornly, and me with my arms crossed over my chest.

“What is this?” Memnis demanded, pointing at the tree. Shugga was having trouble recalibrating his voicebox after the snort, so the answer was left to me.

“A pin oak, sir,” I said. As if hearing its name, the tree shivered and scootched a few centimetres taller.

“And how did it come to be here?”

Shugga paused in his recalibration, his spines stiffening in surprise. Didn’t Memnis know? I opened my mouth, and my lips shaped a few different words as my brain tried to decide what to say. Nothing came that wouldn’t get us into more trouble than we were already in.

Luckily, about this time the tree got tired of standing outside Memnis’ door. One thick root snaked along the floor, thin feeler-rootlets tip-tapping as they went. There was a woody moan, and the trunk swayed elegantly between Memnis and us. The tree’s crown was leafy enough that we used its cover to scamper around the corner and onto the promenade.

Station admin had sent a team to the promenade to negotiate with the oak, but in spite of universal translators and the presence of several hydroponics experts, no progress was made.

The oak (a word I hadn’t known until last week) that had sprouted from Shugga’s science fair project had grown so large by now that it completely eclipsed the blue-white glow of Earth. Its roots had grown thick and hairy, criss-crossing the sidewalks and terraces, and completely disrupting the trolley system so anyone who still had a job to get to had been forced to walk. A few commuters were irritated by the inconvenience, but many seemed to have forgotten where they were going, and were wandering among the cathedral-roots as if bewitched. The roots of the oak had even snaked down—so the newsfeed told us–into the operating mechanisms of the station. There, they took up positions near the control panels. Engineers perched atop knobby roots to find that the tree had, somehow, locked them out of the controls and entered new commands into the computer.

Station admin had sent a team to the promenade to negotiate with the oak, but in spite of universal translators and the presence of several hydroponics experts, no progress was made.

Shugga, feeling responsible, had offered our services to clear up the many acorns that fell from the oak. It had been tiring work scooping them up into the compost container station staff had provided, so we’d eventually just swept them into one of the air returns. We’d felt quite smug about our ingenuity until the first sapling had heaved the vent wide and – its branches spreading as we watched – pulled itself out of the ductwork.

Shugga’s voicebox had coughed a bit and said that that wasn’t in his hypothesis at all, and wondered if he was going to lose marks because of it.

Given that the original oak had, by that point, engulfed the entire school, I figured the principal had bigger worries on its mind.

I knew that trees were meant to root in soil, but whatever irradiating Shugga had done to his project seemed to have changed the trees needs. We walked along the Promenade, stepping over roots that twined around trapped trolleys and light stands. Trees twined their branches around pipes, bikes, even the chairs and tables outside a café that was still open, despite the disruption. Citizens lounged in the crooks of branches and turned various parts of their dermis to the sunlight that filtered greenly through leaves grown big as dinner plates.

There was a shout behind us. Memnis! The security captain clambered over a root, hoisting his armour out of the way with a clang. Then the entire station shook. Shugga registered shock as a puff of sweet-smelling pheromone. We three stood, Memnis even checking his armour to see if it had somehow caused the shaking. From far away, there came a roar.

“That’s engine noise,” Shugga’s voicebox coughed.

There was a shift and a groan from the very fabric of the station itself. Tree branches trembled with excitement. I put a hand out to one trunk to support myself, and the tree leaned into me, as if to help.

“What have you done?” Memnis shouted. Light shifted around us as, gradually, the station began to lift itself out of Earth orbit.

In the promenade, the oak’s crown twisted, as if to look out the wide windows at the retreating planet. Then, it twisted the other way, as if to see where we were going.