Azelio winced. ‘Yes. If it’s enough to keep the plant growing when it otherwise wouldn’t, that would make the results meaningless. You don’t want the settlers to find out after half a year that it was only contamination that made it look as if they could survive here.’

He carried the freed plant over to the row of holes he hadn’t yet made. ‘What happens if I try to put it in the wrong one?’ he mused. ‘Is that possible?’

Ramiro aimed his coherer at the nearest of the holes, then watched as Azelio knelt down, a trowel in one hand and the wheat plant in the other. He lowered the plant until its roots were in the hole, then he started adding soil from the surrounding mound. Some of the soil was scooped in with pressure from behind, in the ordinary manner. Some appeared to pursue the trowel, the way the dust sometimes pursued Ramiro’s feet. What decided between the two? Azelio’s own actions had to be consistent with the motion of the soil, but which determined which? Maybe there was no answer to that, short of the impossible act of solving in the finest detail the equations that Agata was yet to discover, revealing exactly which sequences of events were consistent with the laws of physics all the way around the cosmos.

In any case, the laws of physics seemed to allow the plant to end up firmly bedded in Esilian soil. Azelio tried to shake his trowel clean, but each time he flicked it as many specks of dirt rose up from the ground to stick to the blade as parted from it.

‘I guess that’s now my Esilian trowel. Do you want to do the next one?’

Ramiro said, ‘I wouldn’t trust myself to get the roots clean.’

‘I’ll deal with that,’ Azelio replied. ‘You can do the planting.’

‘All right.’

When Azelio had prepared the second plant, Ramiro accepted it and took it to the next hole. He knelt on the ground; Azelio passed him the trowel then stood back to provide a steady light.

Ramiro gazed down at the neat mound of soil beside the hole. If he’d had a camera here during the dust storm he might have watched the mound rising up, as speck after speck fell into place from the turbulent air. But if an Esilian wind had scattered it, who had given it its shape? If he refused to do it himself, would Azelio be compelled to take his place? But why would one of them be compelled and not the other?