"One last thing," he said. "Remember that plants evolved on planets. No matter what we do, we can't convince the plant that it's still on a planet. Light's the easiest. As far as we know, it will grow indefinitely under our artificial light. Artificial gravity is different. I don't know the difference, and neither do the physicists, but the plant does. It can live in the ship just so long and then has to be taken out for a rest. There are other things that affect it, vibration, noise, and maybe more. You know how I have to keep after the pilot to dampen his drive. All these things change the cycle it has to have." "Agreed," she said impatiently, meaning mostly that she didn't care. "Let's go out and look at it." The plant was a machine and the machine was a plant. It occupied a large space in the center of the ship. And it wasn't wasted space; properly cared for, the plant could supply food for the crew indefinitely. The plant machine evolved from earlier attempts to convert raw material and energy into food. Originally algae were used because they were hardy and simple to control. But the end product had to be processed and algae did not produce the full scale of nutrients in the proper proportion for the human diet. Certain cells of more highly evolved plants were far more efficient in the conversion of raw materials into proteins, vitamins and the like. Originally, inedible parts were produced too, the stalk, which might or might not be used for food, and the leaves and roots. On a planet with plenty of room, this made little difference. But on an overcrowded planet, or one with a poisonous atmosphere, and especially on a ship where space was at a premium, normal methods could not be used. In the plant machine were certain cells which had been selected because of their ability to produce a variety of nutrients. The inedible parts of the plant were replaced by machinery. Instead of roots to draw water and minerals from the soil, there were pumps and filters. Instead of stems to elevate that material to the leaves, there were hoses. Instead of entire leaves to perform photosynthesis, there were only those cells most efficient at the process. There were no seeds, tubers, roots, nor fleshy stalks to store the food. Collecting trays replaced them. There was no waste space; nothing was produced that couldn't be eaten. There was an additional problem of reconciling the various cell fluids and different rates of growth. In part, that was accomplished by the plant machinery; the rest depended on the plant mechanic. His job was akin to that of a factory manager. In a sense, the plant machine was nothing more than a highly organized and complex factory, of which the productive units were the actual cells. * * * * *