Tracer experiments reveal that roots cannot distinguish potassium (needed in large amounts) from other elements which are chemically similar but quite different in size. Once inside the plant, only potassium can be metabolized and similar but heavier elements (rubidium, cesium) are useless. This is like an absentminded builder who buys brick, boulders, and gravel indiscriminately for his wall and then finds he can use only part of his materials. The process called photosynthesis whereby green plants use energy from the sun to convert simple compounds from air and soil into complex, energy-rich substances has been termed the most important chemical reaction in the world. It is the basis for man’s entire food supply and, except for nuclear energy, all significant fuel as well. Tracer techniques have multiplied the research efforts on photosynthesis tremendously. When only chemical tests were available, food manufacturing in green leaves had to progress for hours before scientists could measure the products. But with tracers and other new techniques they have narrowed the experimental time to minutes and finally to seconds. Today they know that a green leaf has formed sugars more complex than fructose, “fruit sugar,” after exposure to light for only _one second_! When the incredible complexities of photosynthesis are finally unraveled, radioactive tracers, especially radioactive carbon-14, will have provided the significant clues. Plant Diseases and Weeds How Can We Combat Plant Diseases? At one time to stop epidemic spread of plant diseases was virtually impossible; farmers had to abandon fields and crops. Such catastrophes caused by microbes have changed the course of history. For example, the Irish famines of the 1840’s resulted from the potato blight and caused mass emigrations from Ireland. In this country today plant diseases result in losses estimated at $3 billion a year. So far, the most economical means of reducing the ravages of plant diseases has been to breed resistant plant varieties. Although such a variety may cost $100,000 to develop, its cost is usually repaid within a year or two. But the victory is only temporary. Although plants are bred to resist the pathogen (fungus) of the moment, Nature is constantly changing the microbial population by mutation and hybridization. Within a few years virulent strains of fungi which can attack the “resistant” variety increase to such an extent that the new variety must be replaced. For crops that provide high per-acre income such as some vegetables and vine and tree fruits, chemical control of fungous diseases is economically possible; in fact, it is a real necessity. But such treatment is too costly for most field crops, unless some cheap seed treatment or fertilizer additive can be found.