92 Self- Incompatibility in Hermaphrodite Plants 



This indicates most clearly, as I have pointed out above, that 

 incompatibilities, and especially self- incompatibilities, are acquired, and 

 may, under special conditions, give way to the original primitive condition 

 of full self-fertility. The whole set of conditions favours the conception 

 that compatibility depends on similarity of gametes. Acquired differ- 

 ences which result in self- incompatibility have marked selective value 

 only to the advantages in variability resulting from limiting sexual 

 reproduction to crossing between individuals or races. 



Further -evidence of variability in the relations of incompatibility 

 is seen in the fact that self- fertility is more pronounced in N. alata 

 than it is in N. Forgetiana. It is interesting to note that East and 

 Park consider that these conditions and differences can be so fully 

 disregarded in judging heredity that each species can be called fully 

 self-sterile, and be described as homozygous for a single unit factor 

 (or possibly multiple factors) solely concerned with the hereditary 

 transmission of self-incompatibility. 



East and Park have made the most extensive studies of reciprocal 

 crosses that have thus far been reported. Their facts show a considerable 

 variation in the reciprocal relations of two individuals, but they believe 

 that this is solely due to experimental error and to differences in 

 maturity of the individuals, and that reciprocal matings should give the 

 same results provided end-season conditions are not involved. They 

 decide, therefore, that the condition of compatibility or incompatibility 

 between sex organs (including the gametophytic generation) is deter- 

 mined for a plant as a whole rather than for sex organs as such. This 

 view is decidedly at variance with the results which Sirks, and also 

 the writer have found, as noted above. 



East and Park consider that inbreeding or breeding from self-fertile 

 plants increases the amount of cross-incompatibility; the marked or 

 very general cross-incompatibility of a progeny being ascribed to 

 increased homozygosity. This assumption seems to have some evidence 

 in its support, but it has by no means been rigorously tested and 

 adequately proven. 



The point of view of East and Park is that incompatibilities, both 

 self and cross, are not fundamentally phenomena of sex differentiation, 

 but are properties of plants as wholes predetermined by line -stuffs. The 

 emphasis is placed on a Mendelian description in terms of hereditary 

 units. They recognize that characters and factors representing them 

 are very generally variable, but prefer to regard the marked variations 

 in self-fertility as a " pseudo " fertility of no genetic significance and to 



