The engineering of plants has a long history on this continent. Fields, forests, orchards, and prairies are the result of repeated campaigns by amateurs, tradesmen, and scientists to introduce desirable plants, both American and foreign, while preventing growth of alien riff-raff. These horticulturists coaxed plants along in new environments and, through grafting and hybridizing, created new varieties. Over the last 250 years, their activities...