HOWARD AND HOWARD. 3 



3. The influence of environment is of considerable importance 

 in the study of inheritance when the factors differ from each other 

 by small amounts. The number of homozygotic forms which are 

 possible is expressed by the formula 2 W , where n is the number of 

 factors. Thus, if there are four factors, 16 homozygotic forms are 

 possible. It is obvious that when the total number of factors in 

 any one character is large the types of wheat will differ from each 

 other very slightly. Nevertheless these differences will be inherited. 

 In studying this inheritance, however, environmental differences 

 supervene, and these latter may be greater than the inherited 

 differences. For example, in the study of inheritance of height 

 in wheat the environmental differences may be greater than and 

 so mask the inherited variations. 



4. The existence of so many factors and the occurrence of 

 natural crossing 1 ' 3 are sufficient to account for the complexity of 

 botanical varieties in wheat and for the existence of a large number of 

 types which breed true and which differ from each other very slightly. 

 Even a small number of factors by recombination among themselves 

 is sufficient to produce a large number of types. Many of the 

 characters of wheat have already been shown to owe their existence 

 to several factors. Further work will doubtless increase the num- 

 ber of these factors. (In the investigations described in the present 

 paper relating to felted and smooth chaff and to bearded and beardless 

 wheats it is shown that at least four and possibly more factors are 

 involved in each of these characters.) The various combinations 

 of these factors among themselves would give a relatively enormous 

 number of different wheats. 



5. The complexity of the characters in wheat renders it more 

 than ever necessary that all studies in inheritance should be made 

 with pure lines, that is, with cultures grown from a single parent 

 plant. Such cultures, however, even if they breed true in all visible 

 characters, may not necessarily be homozygotic, especially in 

 physiological characters. In the present state of knowledge, 



I Kiessling, Fuhling's land. Zeitung, Bd. 57, 1908, a. 737. 



Nilsson-Ehle, I.e., 1909, s. 16. . 

 » Howard and Howard. Mem. Dept. Ayr. in India (Rot. series), Vol. Ill, No. 6, 1910. 



