SPECIES HYBRIDIZATION 229 



every character, although to varying extents in different characters. 

 All, however, have found it relatively infertile, although among some 

 hundreds Lotsy, in one experiment, discovered one plant which possessed . 

 a rather unusual degree of fruitfulness. 



Although the condition of intermediate character expression includes 

 by far the majority of species hybrids, there are some notable exceptions 

 which very closely duplicate the set of characters of one parent almost to 

 the exclusion of those of the other. This fact was recognized even by 

 the older investigators, for Gartner states that any condition may be 

 obtained from that of strict intermediacy to a condition so closely re- 

 sembling one parent as to be distinguished from it only by increased 

 vigor and partial sterility. Gartner found examples of dominance of 

 one parent particularly striking in some Nicotiana crosses. Thus N. 

 paniculata X N. langsdorffii is reported to give a hybrid form almost 

 indistinguishable from N. langsdorffii and N. suaveolens X N. macrophylla 

 is predominantly N. macrophylla in its character expression. Later in 

 this chapter we shall describe crosses between N. sylvestris and a series 

 of varieties of N. tabacum which constantly yield hybrids resembling the 

 particular tabacum variety used in crossing. Curious instances of such 

 predominance of one type are reported for triple hybrids. Thus N. 

 rustica X N. paniculata pollinated with N. angustifola gives plants closely 

 resembling N. angustifola; if the same hybrid is pollinated with N. 

 glutinosa it produces plants closely resembling N. glutinosa. 



There are authentic instances of species crosses which do not give 

 equivalent results in reciprocal crosses. It is a common observation 

 that some species crosses may be made in one way only. Crosses be- 

 tween wheat and rye are sometimes successful when wheat is the female 

 parent, but the reciprocal cross has never been obtained. But usually 

 when a cross is possible in both directions the reciprocal hybrids are prac- 

 tically indistinguishable. Among exceptions to this rule are crosses 

 between Digitalis purpurea and D. lutea, strikingly different species, 

 which constantly give hybrids resembling the female parent. In Oeno- 

 thera such results are particularly common, and de Vries and others have 

 investigated a number of such cases. A typical case is that of 0. biennis 

 and 0. muricata which give strongly patroclinous hybrids in reciprocal 

 crosses. The fact, however, that these hybrids breed true in further 

 generations introduces a complication which places us on our guard 

 against the operation of some as yet undiscovered factors. We can under- 

 stand why reciprocal crosses should give different results, when there 

 are differences in chromosome number or content in the two sexes as is 

 generally the case among animals, but in plants it is more difficult to 

 assign a reason for this type of behavior aside from a few cases in which 

 apogamy is known to occur. It is, therefore, necessary for us to accept 



