242 GENETICS IN RELATION TO AGRICULTURE 
The behavior is truly remarkable when viewed in the light of modern 
Mendelian conceptions. The number of character differences between 
the two forms is very considerable, and the recovery of the parental 
forms with almost unimpared fertility is so frequent that subsidiary 
assumptions must be made to account for them on a Mendelian basis. 
Goodspeed and Clausen, therefore, have developed the conception of 
Mendelian reaction systems for an explanation of these phenomena. 
According to this conception the normal functioning of a gametic or zy- 
gotic set of factors depends upon the harmonious interrelations which 
the factors maintain with one another. The uniform resemblance of the 
F, hybrids of N. tabacum varieties with N. sylvestris to the N. tabacum 
varieties is held in these cases to indicate that the N. tabacum set of 
factors is dominant as a Mendelian reaction system to the set of factors 
contributed by N. sylvestris. The fact that these hybrids so completely 
resemble the N. tabacum parent indicates that the elements of the N. 
sylvestris system are throughout unable to interact normally with those 
in the opposed N. tabacum system. It is for this reason that a reces- 
sive factor which is practically completely swamped in F;, intervariety 
crosses in N. tabacum, expresses itself so strongly in the F hybrids with 
N. sylvestris for, if the corresponding element of the N. sylvestris system 
were unable to interact with the elements of the dominant reaction 
system, then it is clear that although the factor is dominant, the corre- 
sponding character cannot possibly express itself in the individual. 
The haploid number of chromosomes in. these Nicotiana species and 
varieties is probably twenty-four. Consequently the recombination 
series is given by the expansion of the expression (1 + 1)?4. Only one 
gamete in 16,777,316 would carry only N. tabacum chromosomes and 
the same proportion would hold for gametes carrying only N. sylvestris 
chromosomes. This is on the assumption that no crossing-over occurs 
in the formation of gametes in the F; hybrid. If crossing-over should 
occur normally the proportion of pure N. sylvestris or N. tabacum gametes 
would then be correspondingly reduced. The further assumption is 
also tacitly made that there are some factor differences between N. 
tabacum and N. sylvestris in every chromosome, which is in all probability 
correct when we consider the striking differences between the two species. 
Accordingly the results of the back-cross with N. sylvestris which gives 
a relatively high percentage of what are apparently pure N. sylvestris 
plants are exceedingly significant. Developing the reaction system 
hypothesis, 1t would appear that, if the N. tabacum and N. sylvestris 
systems display a high degree of mutual incompatibility, any gamete 
containing elements derived from both systems would give a reaction 
system subject to profound disturbances incident upon the inharmonious 
relations set up between the N. tabacum and N. sylvestris elements. If 
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