[Illustration: have not reached above ground parts in 5 minutes]
[Illustration: but have, as indicated by Geiger counter, reached these
parts in 20 minutes.]
[Illustration: ]
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.