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. Gértner 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 O. brennis 
and O. 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 
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