Bob Amherst shivered a little despite the heated interior of the autobus, but grinned none the less as he made out the frosty towers of Hydropole. He was always glad to return to the polar city, if only for the pleasure of staring up at buildings piled story upon story like those of his native Syracuse on a gray planet some half a billion miles sunward.Hydropole, south polar city of Jupiter's third major moon, Ganymede, was a chilly town at all seasons with its thirty degree Fahrenheit mean, and its variation of only ten degrees. But it was certainly the only settlement on the satellite that was worthy of the title of city.Amherst had served four terrestrial years on the watery planet as collector for Cree, Inc., moving from town to town gathering the precious medicinal moss, to take it finally to Hydropole, the rocket port, for transshipment to Earth.He was one of the hundreds of such collectors for the giant company, each with his own route, each picking his own way from town to town, riding his hipp (the sea-horse of Ganymede, Hippocampus Catamiti) through the wild torrents of the afterfloods, past mountains whose locked valleys were apt to spill countless millions of tons of water upon him with no warning save the crash of the bursting mountain walls.Only in Hydropole was there safety. Situated on the south pole, it escaped the great wash of water which, due to the strong gravitational pull of Jupiter, every three months encircled the tiny moon.As a result, only in, and for a few miles around Hydropole, was there vegetation. Save for the strange moss, cree, which clung so close to the rocky crevices of the mountain that even the raging tides could not pry it loose, not a living plant broke the great, gray expanse of rock.So, on Ganymede, all life revolved about the blue moss, cree. Ages back, the Nympus, natives of Ganymede, had carried it deep underground where, piled layer after layer on the solid rock around the doomed villages, it served as earth. There, with seeds garnered from the small area about Hydropole, they grew the small variety of food on which they lived.Above ground the moss had a deep, blue color. As litmus paper, colored by the Earth lichen, rocella tinctoria shows the presence of acid or alkali by its color change, so Ganymedian cree reacted to the ammoniated atmosphere of the planet. The air underground, however, artificially produced, had little ammonia content, and there the moss was red. Indeed, even the mountain cree, after being washed by the hydrogen containing waters of the flood, for a short time showed red.Up to a short time ago, the gatherers had had only a limited time in which to pick the moss. Red cree lacked the medicinal quality of the blue in which, partly because of its chemical reaction to the ammoniated air and partly due to the latent eggs it harbored, lay the curative power so much in demand on Earth. Now, however, Carl Kent had evolved a formula by which cree picked red might be endowed with the healing power of the blue. So, in the area around his small trading station in Aquia, red as well as blue cree was gathered.The autobus turned silently down the wide street of Hydropole. Robot-guided, insulated from noise and cold, it was certainly preferable to traveling by hipp. But hipp travel was unavoidable from here on. The trip to Aquia verged on the wet side of the planet-the side from which burst the mighty floods. So, added to steep, rocky drops, impassable by autobus, were the dank, muddy flats which only the hipp could traverse.