PLANT HYBRIDIZATION BEFORE MENDEL 187 



will cross, whether the progeny are likely to be fertile or not, and 

 the general appearance of the hybrid with respect to the parents. 

 Briefly summarized, these are as follows : 



1. "That plant-forms, which stand systematically near together, can 

 form crosses with one another." (4c, p. 191.) 



from which it follows conversely that systematically nearly 

 related plant forms may cross, the limit for crossing in general 

 not exceeding the genus, and very often not going beyond the 

 same section of the genus, and sometimes remaining within the 

 species, different natural orders and genera differing in this re- 

 gard. 



2. "plant-forms cross with much more difficulty and, on reciprocal 

 fertilization, give a much scantier number of fertile seeds, the less they 

 are sexually interrelated. This sexual affinity is not the same in signifi- 

 cance as systematic affinity, which makes itself evident through external 

 differences in form, color and habit, nor as that of the inner relationship, 

 which is based upon the chemical and physical constitution." {ib., p. 193-) 



Varieties and species cross with the greater difficulty, and in 

 reciprocal crosses produce the smaller number of fertile seeds, the 

 less closely related they are sexually. This "sexual affinity" is 

 taken by Nageli not to be identical with systematic relationship 

 as determined by morphological characters, color or habit, nor 

 with the inner chemical or physical constitution. Just what "sexual 

 affinity" thus implies is not entirely evident. Nageli illustrates 

 the fact by the case of two plants, A and B, in which A can be 

 fertilized by B, but not B by A, quoting Gartner's case of Nico- 

 tiana pantculata X -^- langsdorfii in which, out of 79 flowers, 

 66 set fruit ; whereas, of 44 flowers of the reciprocal cross, not 

 one set seed. Nageli remarks (p. 196), regarding sexual affinity: 



"As pertains to the latter, one knows nothing concerning the nature 

 of it. Possibly it might be conditioned through external (mechanical) 

 causes ; more probably it is connected with local chemico-physical consti- 

 tutions lying in the sex organs. 



3. "The fertility of hybrids is so much the less, . . . the farther the 

 propagating forms are removed from one" another in sexual relationship. 

 Species-hybrids are thus, in general, less fertile than variety-hybrids." 

 {ib., p. 200.) 



4. "The rule that the sexual affinity is so much the greater the nearer 

 the parental forms are externally and internally related holds good only 

 up to a certain limit, within which the fertility diminishes in both re- 

 spects." {ib., p. 207.) 



The closer the sexual affinity, the easier cross-fertilization oc- 



