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THE AMERICAN NATURALIST [Vol. XLIX 



of chance altogether or must we assume a differential rate 

 of growth increasing directly with the constitutional dif- 

 ferences existing between the somatic cells and the vari- 

 ous gametes 1 If we assume that any constitutional dif- 

 ference between the two calls forth the secretion of the 

 direct stimulus to growth, chance fertilization by gametes 

 of every type different from that of the mother plant will 

 ensue; if there is a differential rate, selective fertilization 

 will occur. This question can not be decided definitely at 

 present, but two different lines of evidence point toward 

 chance fertilization. 



1. Flowers from a single plant pollinated by different 

 males show no decided difference in rate of fertilization. 



2. Color differences are transmitted in expected ratios. 

 Further, it will be recalled that beginning with the F 2 



generation, sister plants crossed together have given us 

 our F 3 and F 4 populations, and that these F 3 and F 4 popu- 

 lations apparently have given a constantly increasing per- 

 centage of cross-sterility. This is what should be ex- 

 pected under the theory that a small difference in germ 

 plasm constitution is as active as a great difference in 

 causing the active stimulation to pollen tube growth. 

 Breeding sister plants together in succeeding generations 

 causes an automatic increase of homozygosity as is well 

 known. This being a fact, cross-sterility should increase. 

 Such an increase in cross-sterility has been observed in 

 the F 3 and the F 4 generations, but it would not be wise to 

 maintain dogmatically that it is significant. 



There are various questions, including the important 

 one of the origin of self-sterility, that can not be discussed 

 at this time. In conclusion, therefore, let us turn once 

 more to the phenomenon of self-sterility in Ciona intes- 

 tinalis. It seems to me that the hypothesis outlined above 

 has few, if any, drawbacks when applied to self- sterility 

 in plants. The question there, as far as we have gone, is 

 one of pollen tube growth, and the theory that the secre- 

 tion of the direct stimulant can be called forth only by a 

 gamete that differs in its constitution from the somatic 



