180 E. C. MACDOWELL 



From quantitative differences in form we come to quantitative 

 differences involving size alone. The possible application to 

 size inheritance of this theory of multiple factors, so strongly 

 indicated by color characters, was soon realized by Nilsson-Ehle 

 ('07). He found that crosses between races of wheat or oats 

 with long and short stalks gave intermediate hybrids whose off- 

 spring had stalks ranging all the way from the length of the long- 

 stalked race to that of the short-stalked race. The length of 

 the head of a certain wheat acted in crosses as though it were 

 dependent upon a strong dominant shortening factor and two 

 weaker factors for length, that lacked dominance. Spillman ('02) 

 in crosses involving the length of wheat heads, and Emerson 

 ('11), in certain crosses involving the height of corn plants, re- 

 port intermediate first generations followed by second genera- 

 tions in which the ranges of variability included, or even ex- 

 ceeded, the extremes of the parental races. East and Hayes ('11) 

 present crosses of maize involving ear length, number of rows per 

 ear, size of grain; Emerson ('10) sizes of beans; Tammes ('11) 

 sizes of Linum seeds and petals; in all these cases the same char- 

 acteristic results were found, namely, a constant and intermedi- 

 ate first generation and a second generation with wide variabil- 

 ity. Philips ('12) gives a preliminary report on size crosses in 

 ducks which indicates a like increase in the variability of the 

 second generation birds. 



Certain structural characters have been found that seem to 

 depend upon two or more similar units of inheritance. In oats 

 the presence and absence of ligulae, and the arrangement of the 

 spikelets in the head — either on all sides of the rachis or only 

 on one side — give strong evidence of multiple factors, definite 

 ratio being obtained (Nilsson-Ehle '09). 



Such physiological characters as winter hardiness, rust re- 

 sistance and flowering times of cereals have been found by Nils- 

 son-Ehle to show increased variabilty in crosses. Tammes ('11) 

 found similar results in the opening or remaining closed of ripe 

 Linum capsules. They partly open in the first generation; in 

 the second generation some remain closed, others fully open, while 

 most are half opened. Tschermak ('11) and Leake ('11) pre- 



