A hand closed around the Grad's neck. Long, strong fingers overlapped the thumb, tightening like a steel collar. Clave's voice was a hot snarl in his ear. "You'll tell me now!"

The Grad froze. Clave had gone crazy.

"Tell me what happened!"

"The tree came apart."

"Why?"

"Maybe the fire set it off, but it was ready. Clave, everything in the Smoke Ring has some way of getting around. Some way tos~y near the median . . . middle, where there's water and air. Where do you think jet pods come from?" The hand relaxed a little, and the Grad kept talking. "It's a plant's way of getting around. If a plant wanders out of the median, too far into the gas torus region-"

"The what?"

Alfin asked, "What on Earth is going on?"

"Clave wants to know what happened. Alfin, can you steer this thing and pick up some more of us? Here-" He passed across his store of jet pods.

Alfin took them. He took his time deciding what to do with them, and the Grad ignored him while he lectured. "The Smoke Ring runs down the median of a much bigger region. That's the gas torus, where the molecules. . . the bits of air have long mean-free-paths. The air is very thin in the gas torus, but there's some. It gets thicker along the median. That's where you find all the water and the soil and the plants. That's what the Smoke Ring is, just the thickest part of the gas torus, and that's where every living thing wants to stay."

'Where it can breathe. All right, go on."

"Everything in the Smoke Ring can maneuver somehow. Animals mostly have wings. Plants, well, some plants grow jet pods. They spit seeds back toward the median where they can grow and breed, or they spit sterile seeds farther into the gas torus, and the reaction pushes the plant back toward the median. Then there are plants that send out a long root to grab anything that's passing. There are kites-"

"What about the jungles?"

"I . . . I don't know. The Scientist never-"

"Skip it. What about the trees?"

"Now, that's really interesting. The Scientist came up with this, but be couldn't prove it-"

The hand tightened. The Grad babbled, "If an integral tree falls too far out of the median, it starts to die. It dies in the center. The insects eat it out. They're symbiotes, not parasites. When the center rots, the tree comes apart. See, half of it falls further away, and half of it drops back toward the median. Half lives, half dies, and it's better than nothing."

Clave mulled that. He said, "Which half?"

"East takes you out, out takes you west, west-"

"What are you doing?"

"I'm trying to remember. We were too far in toward Voy, so our end

-" It only hit him then. The revelation blocked his throat.