It was a gigantic hexagon some two thousand feet across, suspended in the blackness and turning with slow deliberation. In the naked vacuum of space it showed stark and clear. He could easily pick out the lines of the main timbers where the sheathing material had been seamed to them.
Lowstation was a real antique — one of the first of the wood-framed space stations to be built after the Nar had acquired the code for the poplar genome from Original Man. It had started life as an equilateral triangle with thousand-foot sides. Even then, it had given good service with temporary environmental shells attached at the three points. Five identical triangles had been knocked together out of additional thousand-foot timbers floated in from the nursery wheel that grew them. Then the six wedges had been fitted together, and Lowstation was in business — still skeletal, to be sure, but starting to enclose itself a little at a time.
In those days, they had still bothered to square off the timbers, in imitation of man, and that had been the biggest part of the job. Even so, assembling large space structures from ready-made framing elements that could theoretically be miles in length was still easier and cheaper than lofting flimsy plastic girders into orbit from the planetary surface.
Poplar had not been much used as a building material on that long-vanished planet of man. Hardwood had better lateral strength and made better posts and beams. But poplar had one advantage: It grew fast and it grew straight, provided that it didn’t have to compete for its little piece of sky.
It grew even faster in the low simulated gravity of a space wheel, and it grew just as straight: pointing itself at the hub along the lines of force. In a closed spoke of such a wheel, provided with air, water, and sunlight, its ultimate height was limited only by the radius of the wheel.
Tensile strength is more important than rigidity in space construction, anyway, and a poplar trunk certainly could not be pulled apart by the usual one-g forces; plastic girders, assembled in segments, were more likely to do that. In fact, the poplar’s relative elasticity, adjusting to longitudinal stress, was an advantage.
Later, the Nar learned to grow curved beams for arches and the curved perimeters of space wheels. You simply enclosed a wedge of a space wheel and moved a tubbed sapling in gradual stages along a straight floor that made a chord along the curved perimeter. The poplar, always yearning for the hub, would keep changing its direction of growth.
Man, it was known from the records, had made wood-framed space stations and other structures the norm by the end of his twenty-first century. Even the larger ships used for intrasystem voyaging were wooden vessels fitted out with rocket units. For man, the space-grown poplar had brought about an era of cheap interplanetary expansion. And for the Nar, in the early stages of their own space age, it had done the same.
But man had never taken the next obvious step. To teach the poplar tree to live and grow in vacuum.
He would have been surprised to learn what the Nar had done with his gift.
“Please fasten your harnesses,” the Nar attendant said gently from his pedestal in the center of the transfer vehicle’s oval cabin. “We will begin to decelerate shortly.”
An obedient rustle of movement ran around the circlet of seats lining the cabin. A few adventurous souls who had been swimming in midair — to the annoyance of their fellow passengers — hauled themselves back to their seats along the lines they had been clipped to.
The attendant waited until everybody had settled and announced: “I’m afraid that there will not be much to see during most of the approach, since we will of course be decelerating floor first.”
There was a groan of disappointment from the passengers. The attendant, with a sympathetic swirling of arms, continued. “But our pilot has promised to invert us briefly at some point between deceleration thrusts so that, for a few minutes at least, you should have a fine overall view of the tree through the overhead dome.”
Bram turned to Kerthin in the seat beside him. “Won’t that be marvelous?” he asked.
She nodded without answering. She had regained her color, Bram was glad to see. The trip in the ferry up to Lowstation had been the worst part of it for her. The paneled corridors and reassuring solidity of the orbital facility had acted as a tonic. The Nar personnel had gone out of their way to make things comfortable for the human party during their wait for transportation — two loads of earlier arrivals had already been delivered to the higher orbit where the tree had parked itself — and the window-filling view of the Father World from the passenger lounge had been spectacular. Kerthin had ignored the splendid scenery and instead had given her nervous attention to every human drifting in and out of the lounge, as if searching for the reassurance of a familiar face. Bram or Trist had waved at several casual acquaintances, and some of them had come over to chat briefly with the foursome, but Kerthin had remained preoccupied and aloof. She had glanced up sharply once at some people returning from the rest room — Nen had seen it and reminded her that there would be no facilities on board the transfer vehicle — and later had excused herself to use Lowstation’s amenities herself. When she rejoined Bram, she seemed more relaxed, and Bram wondered if she had been sick all this time. At any rate she seemed more herself now and showed signs of looking forward to her visit to the tree.
“Ready to fire,” the Nar attendant murmured from his central post, keeping watch with all five eyes on his ring of human charges.
There was the gentlest of nudges, then the stars outside the plastic dome turned as the craft flipped over. A moment later there was a small steady push from the floor, and Bram felt a pound or two of weight return to him.
There were four or five firings at intervals, then a period of coasting, while Bram wished that someone had thought to install a window in the floor. Finally the Nar attendant spoke again. “We’re less than a thousand miles from the tree now, and if you’ll keep your eye on the overhead dome, you’ll be able to see it in a minute.”
Bram glanced at Kerthin. She was immersed in a printout of Shaw’s Saint Joan, which she had brought along for the ride. She appeared not to notice the tiny jolt when the attitude jets began to roll the vehicle over on its back again. Bram got her attention with a nudge of his elbow, and she put the printout down and looked up.
The great living starship that was the tree swam majestically into view through the overhead bubble and came to a halt as the transfer craft steadied itself.
Bram gaped unabashedly. Beside him, he heard Kerthin catch her breath.
The star-traveling organism resembled not at all any tree ever seen on land or even the giant trees grown in space stations. Vacuum-poplars spread outward, not upward.
Its form was that of two umbrella-shaped masses held apart by a trunk that was relatively too short to be seen between them at this shallow angle of approach. The trunk would, Bram knew, be some eighty miles long and twenty or more miles thick. But it was dwarfed by the twin wheels of growth that it had given rise to after it had done its essential work.
The immense silvery crown of leaves, some three hundred miles in diameter, was a flattened dome — almost a disk — whose irregularities were canceled by distance to make it as round and symmetrical as any artificial object.
The foliated root system at the other end of the hidden stem was an almost exact match in size and shape to the crown. A living system like a vacuum-poplar had exquisite feedback. It had to. Spinning at one gravity at its twin rims — slowing its spin as it grew outward in order to maintain a constant one-g level — it had regulated its growth over the centuries for balance and symmetry.
It was hard to detect the spin at the tree’s present enormous diameter, but spin it must, Bram knew, unless it wanted to lose its shape. A space poplar started life as a seedling on one of the billions of snowballs in the cometary halo at the outskirts of the system, feeding on water ice and the inevitable carbon and nitrogen compounds. When it had used up its comet — rarely more than a few miles in diameter — it was still a fairly orthodox looking
tree, with leaves at one end and roots at the other. Long shapes tumble in space, and the random rotation encouraged the tree to continue to grow straight — much like the trees grown in space stations — except that centrifugal force pulled it in both directions. The roots foliated, modified themselves to take advantage of random traces of water and organic molecules floating around in the shell of unborn comets. The true leaves, with the help of a little biological engineering by the Nar, learned to conserve water rather than respire it.
And like all plants, it reached out for water in the only way it could.
The leaves were dark on one side, silvery and reflective on the other. With the cunning phototropism of plant life, the leaf system used available starlight to change the tree’s direction of spin. Instead of rotating end over end, the tree began, over a period of years, to twirl on its axis. That encouraged the branches and roots to grow out laterally — to spread its sails and enable the tree to move by the pressure of starlight. At the same time, the axial rotation put a stop to the tendency of the trunk to grow taller and instead made it grow thicker — better able to support the opening umbrellas.
Now the tree was a true spacefarer — able to follow the tenuous currents of life-giving vapor wherever they led, engulfing new cometary cores and sucking them dry, building its own tissues in the process.
The cometary halos of neighboring stars are contiguous. The vacuum-poplars were seeding themselves outward. Already, thinly spread forests of them grew around Juxt One, with little help from the Nar, and they had been found in the cometary shells of stars as distant as twenty light-years.
When the Nar needed a new starship, all they had to do was tag a likely specimen, tow it to planetary orbit, and outfit it with living quarters, a parasitic ecology; and simple biological controls to make it deploy its foliage as commanded.
Man, for all his technical superiority, had never brought his own space-grown trees to their full potential. With his shorter life span, man had to hustle between the stars. The Nar could afford a leisurely sail, pushed by the gentle pressure of starlight.
“I … I’ve seen pictures of trees,” Kerthin breathed in wonder, “but I never expected …”
“It’s hard to grasp,” Bram agreed. “Lowstation looked pretty big to me. But it would just be a speck next to that thing. You wouldn’t notice it at all. You could hide it inside a twig at the end of the branches.”
As they watched, the moon-sized disk underwent a startling change. It was growing darker on one side. A wave of dark green rippled across its face until, within a few moments, it was neatly bisected by a geometrically straight line — silver on one side, green on the other.
“We’ve all been very fortunate to see that,” the Nar attendant said. “Our timing is lucky. We’ve just seen the tree decide to make a small attitude correction.”
“What happened?” Kerthin said.
“The leaves,” Bram said. “It turned its leaves over on half the reflecting surface. Over the next few days, it’ll get a small push that’ll —” He checked the cloud-marbled surface of the Father World below. “— line up the trunk vertical to its orbit. I guess it doesn’t like tidal forces.”
“You are correct,” the Nar attendant said. “The star trees are uncomfortable in the vicinity of large planetary gravitational fields and try to avoid them unless they are forcibly guided.”
Bram turned mildly pink. He hadn’t meant to be overheard.
“How do they do that?” somebody asked from the other side of the cabin.
“We can deceive the tree with various synthetic hormones,” the attendant said. “There’s a pumping station in the trunk. There are also simpler means. We can throw bait — release water vapor ahead of it. Or use artificial light.”
“Poor tree.” Someone laughed. “The carrot leading the stick.”
“I’ll have to learn that trick,” Nen said, twisting around in her seat to face Bram and Kerthin. “About the hormones, I mean.”
“Don’t pay any attention to her,” Trist said. “She’s been doing it for years. She’s got all the hormones she needs.”
“Where the hormones —” Nen started.
“There moan I,” Trist finished for her. “How about you, Bram?”
The line had been shamelessly stolen from some ancient author; Trist had used it years ago in the bachelor lodge during his ribald period. Bram smiled helplessly at Kerthin, apologizing for Trist, but Kerthin wasn’t smiling. Bram tried to think of something lighthearted to say before the mood got too chilly.
“We’re about to turn over for our final approach now,” the Nar attendant said. He must have received a signal from the pilot through his glove.
A murmur of disappointment rose from the passengers. Trist raised an eyebrow at Bram and turned around in his seat to adjust his safety harness.
The two-tone crown of the tremendous tree began to slide from view. The line of green, Bram noticed, was keeping pace with the ponderous rotation as millions of leaves turned over ahead of it and turned back again behind it.
Kerthin seemed just as fascinated as he was. She kept her eyes on the colossal shrub until it was gone. She licked her lip and turned to Bram with moisture glistening on it. “If only we had one of those,” she said.
“We?”
“The human race. We could migrate to another star and start out again on our own. Away from the Nar.”
“We’d have to travel at least a hundred light-years to get to a habitable planet that wasn’t already settled by the Nar,” Bram said reasonably. “At no more than about seven percent of the speed of light. And when our descendants got there, fourteen hundred years later, they’d probably find that the Nar had arrived there ahead of them.”
She gave him a strange look. “Maybe there’s someplace closer,” she said.
“We’ll land on the trunk,” the attendant was saying. “It would be too dangerous to attempt to dock on the outer branches. At one hundred and fifty miles from the axis of rotation, we’d acquire one gravity immediately, at the moment of contact. I’m sure you can all appreciate the wisdom of avoiding that kind of brush. But the trunk is only twenty miles in diameter at its thickest, so that no point along its surface is more than approximately ten miles from the center of rotation. For all practical purposes, gravity — if we can call it that — is negligible anywhere along its length. We’ll be landing at about the midpoint, by the way, so you needn’t worry about our colliding with either canopy. We’ll have miles and miles of leeway for maneuvering.”
“How are we getting to the tip, then?” somebody grumbled. “Do we have to transfer to another vehicle?”
“No, we’ll stay inside this one all the way,” the attendant said in a voice that, despite the nonhuman way in which it was produced, suggested smugness.
“But how?” another passenger cried.
“You’ll see when the time comes,” the attendant replied. “If you’ll all think about it, I’m sure you’ll realize that there’s a very simple method by which we can acquire the one-gravity force of the outer rotation period gradually and then dock at our final destination. I think I can promise you an interesting experience.”
“How? Tell us!” several cries went up. But the attendant was enjoying his little moment of suspense.
“Like children,” Kerthin said. “Guessing games and treats. That’s what they’ve brought us to.”
“He’s only trying to make it more fun for us,” Bram said.
The passenger boat continued to drop.
Dropping was the way Bram’s perceptions interpreted the motion now, as did the perceptions of the people around him. The up-down sense had been reinforced by the feeble but regular bursts of the braking jets under their feet.
So the astonishing horizon that now began to rise through the clear viewdome became the distant edge of a flat plain — a green and silver forest that stretched for hundreds of miles before it was cut off by the black knife-edge of space.
He knew it wa
s an illusion. He was looking down a vertical wall of greenery, not across a forest landscape. The g forces were outward from the center of that vast canopy of leaves. But the seat of his pants kept contradicting his common sense.
The horizon marched toward him as the passenger boat continued its descent. There was enough curvature across the tree crown to do that. Now the down-sloping plain was hidden behind the skyline. He appeared to be looking uphill toward the shallow arc of the crest. But the passenger boat was not going to land among those mighty branches. It would fall past the fringes of the crown at a safe distance.
The mound of silvery green rose up and filled the universe. They had dropped past the edge. Now Bram could see the flat underside of the umbrella. Close up, the circumference wasn’t the geometrically perfect curve he had seen from afar. Great twisting growths poked into space, any one of them big enough to have swallowed up a small city in its foliage. But the tree averaged out the mass.
The boat dropped another forty miles and, with a long, finely tuned burn of its thrusters, hovered. Bram drank in the tremendous sight.
Above, the roof of foliage stretched for three hundred miles. Below, the tangled root system formed what at this distance looked like an equally flat and solid floor, Its modified structure drank in and conserved every molecule of moisture that escaped from the miserly canopy above and sent signals to the living light sail to keep it on course in its search for water ice. it dwarfed all but the largest comets. When it caught a comet in its fibrous net, it shed its adventitious leaves within the circle of contact and sent out root hairs to melt and drink every last drop. And the tree celebrated another minute fraction of growth.
“It’s a world!” Kerthin exclaimed in awe. “But it’s alive!” Her hand clutched Bram’s.
Trist’s head bobbed in her direction. “Think of them, spreading slowly outward between the stars,” he said cheerfully. “Acquiring their own ecology. Parasites like us, for example. They could make planets obsolete.”
The Genesis Quest Page 15