Forty-five minutes later still, Calhoun heard stirrings outside. He went out. The two men on guard gazed off into the city. Something moved there. It was a ground-car, running slowly and without lights. Calhoun said undisturbedly:
"Whoever was running the cattle fence found out their gadget wasn't working. Their lights flickered, too. They came to see what was the matter at the landing-grid. But they've seen the lighted windows. Got your blasters handy?"
But the unlighted car turned and raced away. Calhoun only shrugged.
"They haven't a prayer," he said. "We'll take over their apparatus as soon as it's light. It'll be too big to destroy, and there'll be fingerprints and such to identify them as the men who ran it. And they're not natives. When the police start to look for the strangers who were living where the cattle-fence projector was set up.... They can go into the jungles where there's nothing to eat, or they can give themselves up."
He moved toward the door of the control building once more. Allison said desperately:
"They'll have hidden their equipment. You'll never be able to find it!"
Calhoun shook his head in the starlight.
"Anything that can fly can spot it in minutes. Even on the ground one can walk almost straight to it. You see, something happened they didn't count on. That's why they've left it turned on at full power. The earlier, teasing uses of the cattle fence were low-power. Annoying, to start with, and uncomfortable the second time, and maybe somewhat painful the third. But the last time it was full power."
He shrugged. He didn't feel like a long oration. But it was obvious. Something had killed the plants of a certain genus of which small species were weeds that destroyed Earth-type grasses. The ground-cover plants—and the larger ones, like the one Calhoun had seen decaying in a florist's shop which had had to be grown in a cage—the ground-cover plants had motile stems and leaves and blossoms. They were cannibals. They could move their stems to reach, and their leaves to enclose, and their flowers to devour other plants, even perhaps small animals. The point, though, was that they had some limited power of motion. Earth-style sensitive vines and flycatcher plants had primitive muscular tissues. The local ground-cover plants had them too. And the cattle-fence field made those tissues contract spasmodically. Powerfully. Violently. Repeatedly. Until they died of exhaustion. The full-power cattle-fence field had exterminated Mayan ground-cover plants all the way to the end of the east-bound highway. And inevitably—and very conveniently—also up to the exact spot where the cattle-fence field had begun to be projected. There would be an arrow-shaped narrowing of the wiped-out ground-cover plants where the cattle-field had been projected. It would narrow to a point which pointed precisely to the cattle-fence projector.
"Your friends," said Calhoun, "will probably give themselves up and ask for mercy. There's not much else they can do."