April, 1915.] The Inheritance of Size in Tomatoes. 
477 
were observed. All of this information was carefully recorded 
in the accession book, together with any unusual features which 
the fruit may have possessed. 
A system of careful labelling was devised and each pot was 
labelled with an aluminum label by means of which the plant 
might be identified. The key to the labels was kept in the acces¬ 
sion book so that at any time the exact pedigree and descriptions 
of ancestors of any particular fruit could be readily found. The 
danger of losing the identity of any plant or fruit was thus reduced 
to a minimum. 
HISTORICAL REVIEW. 
Mendel (1860-70) formulated his epoch-making law of her¬ 
edity as a result of experiments on the inheritance of qualitative 
characters in garden peas. His results led him to believe that 
each character depended upon a single determiner or factor, for 
he worked on simple characters belonging to different parts of the 
plant. When two plants differing with respect to one unit 
character were crossed, the segregation in the F-2 generation was 
computed and found to be in the ratio of 3 to 1. Where there was 
a difference of two characters between the parents, the F-2 segre¬ 
gation resulted in the ratio of 9 to 7. The possibilities, which 
would occur when there was a difference of three characters 
between the parent plants, were computed and the results obtained 
by breeding came close to the theoretical explanation. 
Mendel’s law of heredity was rediscovered and rescued from 
obscurity (about 1900) by De Vries, Correns and Von Tschermak. 
Following the lead of these three pioneers of heredity, hundreds 
of other scientists did experimental work along the same lines, 
until the validity of this law with its three fundamental principles 
of independence of unit characters, dominance and segregation 
has been amply proven. 
Net until within the last decade, however, was it discovered 
that the expression of some qualitative characters require the 
presence of more than a single, separately inherited determiner 
or factor. Bateson’s work in 1908 with two strains of sweet 
peas (Lathyrus), Bour’s investigation with the snapdragon 
(Antirrhinum) and Castle’s experiments with guinea pigs have 
shown that the qualitative character—color—may depend upon 
the interaction of at least two gametic factors. East in 1910 (14) 
found two factors for the production of yellow color in the endo¬ 
sperm of maize. Emerson in 1911 (21) discovered two yellow 
colors in the endosperm of maize that seemed to be unlike in 
appearance. Nilsson-Ehle in 1909 (39) crossed a white and 
browned-glumed wheat and found two factors necessary for the 
production of the brown-glumed condition, as the F-2 generation 
segregated into the ratio of 15 brown to 1 white head, which was 
