220 GENETICS IN RELATION TO AGRICULTURE 



although vigorous and normally developed, are totally sterile. The mule 

 is a familiar example, many others could be given, but they will be con- 

 sidered elsewhere along with the problem of sterility in species hybrids. 

 For the present we shall consider one of the simpler cases in which the 

 species hybridized, although differing very markedly in morphological 

 characters, produce hybrids which appear to be fully fertile. 



Species Hybrids in Antirrhinum. Baur crossed the wild Antirrhinum 

 molle with the common garden snapdragon, Antirrhinum majus, and 

 Baur and Lotsy have made extensive studies of the progenies obtained in 

 successive generations of this cross and of other species hybrids in 

 Antirrhinum. Antirrhinum majus and A. molle differ strikingly in a 

 large number of morphological characters. The size proportions and 

 general characteristics of the common snapdragon of the garden are fa- 

 miliar to everyone. It is a strong growing erect herbaceous plant, about 

 three feet high producing spikes of large zygomorphic flowers. Under 

 the careful attention of commercial seedsmen it has produced a very large 

 number of varieties which differ in the form and color of the flowers, in 

 height and in other characteristics. Antirrhinum molle on the other 

 hand is a low growing prostrate plant which is profusely branched and 

 produces flowers about one-third as large as those of majus, but very like 

 them in form and general appearance. The species differs from majus 

 also in being apparently totally self-sterile, so that with respect to their 

 genetic constitution plants of molle are normally heterozygous to some 

 extent. Since molle occurs in nature in a number of slightly different 

 forms its self-sterility must not be lost sight of in interpreting the results 

 of hybridization between it and majus. F\ of the reciprocal crosses 

 molle X majus and majus X molle are completely self -fertile, and identical 

 in every respect. Minor differences did occur but they were of such a 

 nature that they could be accounted for as a result of the slight degree of 

 heterozygosis of the particular plant of molle which was used as a parent. 



Baur employed a peloric majus for crossing with molle in order that he 

 might follow a known factor difference throughout the investigation. 

 The FI plants in this experiment bore zygomorphic flowers, a fact which 

 indicated a corresponding behavior as regards dominance for the factor 

 for zygomorphic flowers in molle and majus. Six FI plants differing 

 slightly in their characters were selected as parents for the F 2 generation. 

 Lotsy grew the progeny of five of these, obtaining from them 624 F 2 

 plants. The general conclusions which Baur and Lotsy have drawn 

 from a study of these F 2 plants is that the extreme range of forms dis- 

 played, so great that no two plants resembled each other in all their 

 characters, is a result of Mendelian segregation and recombination of 

 characters. The diversity, however, was so great as to preclude the 

 application of any exact factor analysis to the case. 



