April, 1915.] The Inheritance of Size in Tomatoes. 
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based on linear dimensions, to show complete segregation of size 
characters, varying in the Mendelian fashion from the larger to 
the smaller parent. He assumes a cross between two tomatoes 
with the linear dimensions 4x4x4 and 9x9x9 respectively, and gets 
an F-l hybrid which is 6x6x6. He assumes factors for length, 
width, breadth and shape. Shape modifies the dimensional 
factors, while each of the three dimensional factors modifies the 
other two, from which it can be seen that this is a multiple factor 
hypothesis. If all the tomato fruits were perfect spheres, this 
explanation would be more tenable; but, as noted before, the 
extreme irregularity of shape causes any explanation, founded on 
linear dimensions, to be liable to considerable error. 
The results presented in this paper, showing apparently such 
unusual dominance of the red currant size factors, cannot be 
interpreted by Groth’s hypothesis. However, a Mendelian 
explanation has been worked out which agrees fairly well with 
the facts. This explanation is given in the following paragraph, 
as it seems to be the best possible interpretation of these results 
at the present time. 
As noted before, Nilsson-Ehle in his work on tri-hvbrid red 
wheat found in the second generation 63 grains of varying redness 
to one white wheat grain. From this he reasoned that the red 
grains possessed three independent color factors each of which 
was able to give the red color to the wheat. In the F-2 tomato 
generation 44 plants have been grown and the segregation of size 
characters has been so incomplete as to warrant the assumption 
of at least four size factors. The small size factors of the red 
currant seem to be incompletely dominant over the large size 
factors of the yellow pear, because, when an equal number of 
large and small size factors are present, as in the F-l generation, 
the geometrical mean between the parents is realized. As the 
number of small size factors increases or decreases from the number 
present in the F-l generation so will the weight of the resulting 
fruit vary more or less from the geometrical mean. This varia¬ 
tion will not be large, as the small size factors, however few, are 
incompletely dominant over any number of large size factors. 
There should be occasional returns to both parent sizes, the 
frequency depending upon the number of factors concerned. If, 
with further experiments, no such original parental size is ever 
attained, there is evidently more than multiple factors involved. 
SUMMARY. 
1. A more accurate representation of the size of tomato 
fruits can be obtained from their weights than from their linear 
dimensions. 
2. The size of fruit of the F-l generation of the currant-pear 
cross is the geometrical mean between the parental sizes. 
