the animals had reached maximum growth they expected that the growth
factor would not be necessary. Why had their animals declined in weight?

The second paper that interested McCollum was by Wilcock and Hopkins.
These authors carried out experiments similar to those of the paper just
cited but using corn protein (zein) in place of gliadin. This protein had
already been shown to be deficient in a chemical constituent known as
tryptophan. Animals fed on the zein mixture died in a few days but the
inexplicable thing was that when the missing tryptophan was added to the
diet the animals lived a little longer but finally declined and died. Why?

McCollum wished to answer this "Why?" These experimenters had complied
with every known law of nutrition and yet their mixtures failed to nourish
the animals. What was lacking? Earlier work at the Station by Professor
Babcock suggested an interesting line of attack and in collaboration with
Professors Hart and Humphries, McCollum began a series of studies that
have become classic contributions to the vitamine hypothesis and brought
this worker into the field as one of the most important contributors to
the subject. His initial experiments may be briefly summarized as follows:
Young heifer calves weighing 350 pounds at the start and as nearly alike
in size and vigor as could be obtained were selected as experimental
animals. These were divided into groups and fed with rations so made up as
to be alike in so far as chemical analysis could determine, but differing
in that the sources of the ration were divided between three plants. One
group was supplied with a ration obtained entirely from the wheat plant. A
second group derived their ration solely from the corn plant. A third from
the oat plant and a fourth or control group from a mixture of oat, wheat
and corn. By chemical analysis each group received enough of its
particular plant to produce exactly the same amount of protein, fat and
carbohydrate and all were allowed to eat freely of salt. All groups ate