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. 



Not 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 detenniner 

 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 (2l) 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 



