So the lichen were prospering, and along with the species that Sax could identify, with the help of photos on his wristpad’s little display screen, were many more that seemed not to correspond to any listed species. He was curious enough about these nondescripts to pluck a few samples, to take back and show to Claire and Jessica.

But lichen was only the beginning. On Earth, regions of broken rock newly exposed by retreating ice, or by the growth of young mountains, were called boulder fields, or talus. On Mars the equivalent zone was the regolith — thus effectively the greater part of the surface of the planet. Talus world. On Earth these regions were first colonized by microbacteria and lichen, which, along with chemical weathering, began to break the rock down into a thin immature soil, slowly filling the cracks between rocks. In time there was enough organic material in this matrix to support other kinds of flora, and areas at this stage were called fellfields,/el! being Gaelic for stone. It was an accurate name, for stone fields they were, the ground surface studded with rocks, the soil between and under them less than three centimeters thick, supporting a community of small ground-hugging plants.

And now there were fellfields on Mars. Claire and Jessica suggested to Sax that he cross the glacier, and hike downstream along the lateral moraine, and so one morning (slipping away from Phyl-lis) he did so, and after half an hour’s hiking, stopped on a knee-high boulder. Below him, sloping into the rocky trough next to the glacier, was a wet patch of flat ground, twinkling in the late-morning light. Clearly meltwater ran over it most days — already in the utter stillness of the morning he could hear the drips of little streams under the glacier’s edge, sounding like a choir of tiny wooden chimes. And on this miniature watershed, among the threads of running water, were spots of color, everywhere, leaping out at the eye — flowers. A patch of fellfield, then, with its characteristic millefleur effect, the gray waste peppered with dots of red, blue, yellow, pink, white…

The flowers were mounted on little mossy cushions or florettes, or tucked among hairy leaves. All the plants hugged the dark ground, which would be markedly warmer than the air above it; nothing but grass blades stuck higher than a few centimeters off the soil, tie tiptoed carefully from rock to rock, unwilling to step on even a single plant. He knelt on the gravel to inspect some of the little growths, the magnifying lenses on his faceplate at their highest power. Glowing vividly in the morning light were the classic fellfield organisms: moss campion, with its rings of tiny pink flowers on dark green pads; a phlox cushion; five-centimeter sprigs of bluegrass, like glass in the light, using the phlox taproot to anchor its own delicate roots … there was a magenta alpine primrose, with its yellow eye and its deep green leaves, which formed narrow troughs to channel water down into the rosette. Many of the leaves of these plants were hairy. There was an intensely blue forget-me-not, the petals so suffused with warming anthocyanins that they were nearly purple — the color that the Martian sky would achieve at around 230 millibars, according to Sax’s’calculations on the drive to Arena. It was surprising there was no name for that color, it was so distinctive. Perhaps that was cyanic blue.

The morning passed as he moved very slowly from plant to plant, using his wristpad’s field guide to identify sandwort, buckwheat, pussypaws, dwarf lupines, dwarf clovers, and his namesake, saxifrage. Rock breaker. He had never seen one in the wild before, and he spent a long time looking at the first one he found: arctic saxifrage, Saxifraga hirculus, tiny branches covered with long leaves, ending in small pale blue flowers.

As with the lichens, there were many plants that he couldn’t identify; they exhibited features from different species, even gen-uses, or else they were completely nondescript, their features an odd melange of features from exotic biospheres, some looking like underwater growths, or new kinds of cacti. Engineered species, presumably, although it was surprising these weren’t listed in the guide. Mutants, perhaps. Ah but there, where a wide crack had collected a deeper layer of humus and a tiny rivulet, was a clump of kobresia. Kobresia and the other sedges grew where it was wet, and their extremely absorbent turf chemically altered the soil under it quite rapidly, performing important work in the slow transition from fellfield to alpine meadow. Now that he had spotted it he could see minuscule watercourses marked by their population of sedges, running down through the rocks. Kneeling on a thinsulate pad, Sax clicked off his magnifying glasses and looked around, and as low as he was, he could suddenly see a whole series of little fellfields, scattered on the slope of the moraine like patches of Persian carpet, shredded by the passing ice.

Back at the station Sax spent a lot of time sequestered in the labs, looking at plant specimens through microscopes, running a variety of tests, and talking about the results to Berkina and Claire and Jessica.

“They’re mostly polyploids?” Sax asked.

“Yes,” Berkina said.

Polyploidy was fairly .frequent at high altitudes on Earth, so it was not surprising. It was an odd phenomenon — a doubling or tripling or even quadrupling of the original chromosome number in a plant. Diploid plants, with ten chromosomes, would be succeeded by polyploids with twenty or thirty or even forty chromosomes. Hybridizers had used the phenomenon for years to develop fancy garden plants, because polyploids were usually larger — larger leaves, flowers, fruits, cell sizes — and they often had a wider range than their parents. That kind of adaptability made them better at occupying new areas, like the spaces in and under a glacier. There were islands in the Terran Arctic where eighty percent of the plants were polyploid. Sax supposed that it was a strategy to avoid the destructive effects of excessive mutation rates, which would explain why it occurred in high-UV areas. Intense UV irradiation would break a number of genes, but if they were replicated in the other sets of chromosomes, then there was likely to be no genotypic damage, and no impediment to reproduction.

“We find that even when we haven’t started with polyploids, which we usually do, they change within a few generations.”

“Have you identified the triggering mechanism that causes it?”

“No.”

Another mystery. Sax stared into the microscope, vexed by this rather astonishing gap in the bizarrely rent fabric of biological science. But there was nothing to be done about it; he had looked into the matter himself in his Echus Overlook labs in the 2050s, and it had appeared that polyploidy was indeed stimulated by more UV radiation than the organism was used to, but how cells read this difference, and then actually doubled or tripled or quadrupled their chromosome count…

“I must say, I’m surprised at how much everything is flourishing.”

Claire smiled happily. “I was afraid that after Earth you might think this was pretty barren.”

“Well, no.” He cleared his throat. “I guess I expected nothing. Or just algae and lichen. But those fellfields seem to be thriving. I thought it would take longer.”

“It would on Earth. But you have to remember, we’re not just throwing seeds out there and waiting to see what happens. Every single species has been augmented to increase hardiness and speed of growth.”

“And we’ve been reseeding every spring,” Berkina said, “and fertilizing with nitrogen-fixing bacteria.”

“I thought it was denitrifying bacteria that were all the rage.”

“Those are distributed specifically in thick deposits of sodium nitrate, to transpire the nitrogen into the atmosphere. But where we’re gardening we need more nitrogen in the soil, so we spread nitrogen-fixers.”