Cautiously the ColU extended a manipulator arm to prod a nearby growth, right at the junction point of its three sprawling ‘leaves’. The leaves fluttered and shook and rose up, and Yuri saw, yes, it was one end of a kite, and a nearby triple leaf was the other end, a big, fat clumsy kite with the characteristic six vanes of its kind. The vanes trailed tendrils, grubby threads anchored to the ground. Disturbed, the kite shook itself free of the grasping tendrils, flapped and whirled its vanes, and took off, clattering noisily away through the air until it was out of reach of the ColU. It came to a relatively open patch of ground where it settled again and spread out its vanes, snapping them open with what looked like irritation.

A cloud cleared, and the light of Proxima beat down almost vertically on the ground cover. There was a creak of shifting leaves, they barely moved, but Yuri somehow sensed the kites were basking.

‘Those trailing threads are some kind of roots,’ the ColU said. ‘Made up of chains of small, jointed stems.’

‘It’s the light, isn’t it?’ Mardina said. ‘It’s all about access to the light.’

‘Yes. We’re seeing a local adaptation to the position of the star, and maybe the winter. Life on this world always competes to grab as much of the light flow of the parent star as it can. Up in the north, approaching the terminator, you can best do that by becoming a tree, growing tall and angling your big leaves at the unmoving sun. Here, Proxima is almost directly overhead. You don’t need to go to all the expense of growing a trunk; you can just cover the flat ground and let the light beat down on you. And as long as you maintain that cover, nothing else can take root and grow over you.’

‘And if it does,’ Beth said, ‘you can just hop up and fly away to somewhere safer.’

‘That’s true, Beth,’ the ColU said. ‘But it seems wasteful to maintain a whole animal metabolism just for that. I believe this sessile mode is an adaptation for the star winter. When the energy is low, all you want to do is lie there and soak it up. When the “summer” returns and there’s more energy around then you can take up flying again. I suspect it’s not just a behavioural adaptation but a genetic one; in changing conditions a new epigenetic expression can deliver rapid adaptations. These winter kites are probably quite different in form from their grandparents, the summer kites. And when the summer returns, the form will switch back again . . .’

There was a distant rumble of thunder, from the south. Yuri, distracted, turned to look at the substellar point, that tremendous weather system, wondering if such a distant storm could throw thunder this far.

‘Yuri Eden?’

‘Sorry. Yeah, summer and winter kites. But what’s that got to do with Mattock being kept warm at night in his hovel?’

‘Ah, yes. Another clever feature of this life mode. The surfaces of these leaves are a few degrees warmer than their surroundings; they reradiate some waste heat. And this patch of Arduan greenery is so extensive that it’s actually created a local hotspot. It’s a typical ecological feedback loop: the more the plants grow, the more ground they cover, the more they generate the heat that helps them grow.’

‘But is it a big hotspot?’ Yuri asked. ‘Will we be able to grow our crops here?’

‘I believe so,’ the ColU said. ‘Perhaps we could even encourage the hotspot to spread. Get the kites to breed, or even corral wild ones. The star winter cannot last for ever. Perhaps we can weather it here . . .’

As Mardina questioned it further, Yuri found his attention drawn again to the south.

Beth grabbed her father’s arm. ‘Dad? You aren’t listening.’

‘Hmm? Sorry, honey. I keep looking at the big weather system over there. It’s just – Beth, we’ve come so far since you were born. This endless trek. You know, the ColU has figured we’ve crossed thousands of kilometres. I mean, the shuttle from the Ad Astra could have covered that in a few minutes. But we had to do it on foot. Carrying our babies.’

She snorted. ‘And now those babies are carrying you.’

‘And we did it knowing that we wouldn’t even find any food to eat; every time we moved we didn’t just have to grow our own food, we had to create the very soil to do it. There can’t have been a trek like it in human history before.’

‘Except that everybody else we found was doing the same thing.’

‘That’s true. Everybody heading to the substellar, to escape the winter. Everybody heading for that—’ he pointed south ‘—the centre of everything. The navel of the world. Right under the sun . . . We’re getting so close. I’m finding it hard to care much about these kites.’

‘Let’s go and see what we can see.’ She linked her arm in his, as she used to when she was younger, and they walked away, leaving Mardina and the ColU and the ground-dwelling kites.

Father and daughter walked together along the riverbank, heading south. They continued to try to avoid treading on the kite-leaves that plastered the ground. The green cover started to break up maybe a kilometre from the ColU.

Then Yuri stopped dead. There was something on the ground, right at his feet.

‘Dad? You’ve gone quiet again.’

He looked at her. Then he pointed down, at what he’d found in the dirt, at their feet.

A tyre track. Not from a ColU, so not created by Mattock’s people. It snaked away from here, following the river, heading straight for the substellar point.

‘There’s somebody in there,’ Yuri breathed.

CHAPTER 49

The sighting of that track changed everything for the wanderers. They had to follow it, of course.

They spent a year at the Mattock Confluence, as they called it, resting, raising a crop, preparing. A whole year.

Then they began their trek to the substellar point. The weather grew warmer yet, until it passed the norm they remembered from before the star winter, and they shucked off their cold-weather clothes and raised warm-climate crops, and moved on, heading steadily south.

The trek took them two more years.

CHAPTER 50

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For the last few hundred kilometres, the land rose steadily. The river valleys they had followed since the Mattock Confluence became narrower, with steeper walls and beds of tumbled, broken rock: they were gouges cut through country that was increasingly hilly, and at times mountainous. Forest crowded the valleys, clumps of squat, sturdy, wind-resistant, fast-growing trees with wide leaves turned up hungrily to the perpetually cloudy sky. The character of the country was quite different from the plains that seemed to cover much of the continent that dominated the starward face of Per Ardua, the plains across which they had trekked to get here.

They climbed further, and found lakes nestling in the hills, fed by streams tumbling from the still higher ground ahead, choked by stem beds. And on the slopes above that there was little but a smear of Arduan lichen, with a few mobile bands of builder-like motiles or kites working the rare stem beds. The ColU speculated that the life up here, sparse as it was, was taking advantage of the relatively clement conditions of the star winter. Without the drop in temperature brought about by the big reduction in the star’s heat output, this high country would be unliveable for all but heat-loving extremophile-type life forms.

And on they climbed, into this strange, fractured upland. The valleys became narrower, steeper-walled, the river flows more energetic. They had to walk single file at times, and in the narrowest valleys they had trouble with their baggage train.

Yuri’s ColU was put to work guiding its lobotomised fellows, which were being used as trucks, dragging their pallets of food and precious topsoil behind them. It had developed a system of communication and control with trailing fibre-optic cables, which periodically got hung up on rocks or stem clumps, and Beth and Freddie organised parties of children to help out.