stout: pollinations in cichorium intybus 419 



that full fertility is attained in self-fertilization between plants of 

 strains that are most pure. 



It must be noted that Burck does not further identify the type 

 of sterility which he considers develops in cultivated varieties as a 

 result of crossing. It is conceived to be due essentially to dis- 

 similarity of gametes, although there may be, as it appears, also a 

 decreased vigor. The question whether it may assume such 

 marked types of self- (and cross?) incompatibilities as are seen in 

 Eschscholtzia, Reseda, etc., is not considered. In fact this type of 

 sterility was not discussed. Proper experimentation in such cases 

 should show whether such sterility is in evidence. It is clear, 

 however, that Burck is not considering a type of extreme impo- 

 tence. 



In marked opposition to the views of Burck and in general 

 agreement with the conceptions of Darwin is the doctrine empha- 

 sized by Shull and more especially by East and Hayes ('12), that 

 heterozygosity gives an increase of both vigor and fertility in 

 proportion to the number of "heterozygous factors in the organ- 

 ism" and that inbreeding although "not injurious in itself" (p. 33) 

 "tends to isolate homozygous strains which lack the physiological 

 vigor due to heterozygosity" (p. 37). 



The results of East and Hayes ('12) appear in two types of 

 experiments; (a) those involving chiefly interspecific and inter- 

 varietal crosses in tobacco, which is usually highly vigorous and 

 fertile in continued normal self-fertilization, and {b) those involving 

 intervarietal crosses in corn, which is normally cross-fertilized 

 because of the marked proterandry that exists. 



In the interspecific crosses in tobacco 42 crosses were attempted 

 (East and Hayes, '12, table 5) : 9 of these failed to produce viable 

 seed (the parent species were cross-incompatible) ; of the 33 differ- 

 ent hybrid progenies grown, 14 showed decreased vigor, 2 were 

 equal to parent species, and 1 7 were of increased vigor. It appears 

 from the discussion that the percentage growth of hybrids, as 

 given m the table just summarized, is based on the average of the 

 parent species. Of the 14 hybrid progenies with decreased vigor 

 one is reported as fertile and 2 as slightly fertile; of the 17 showing 

 increased vigor 8 were fertile, 3 slightly fertile and 8 sterile. 

 There was increased vigor in only 17 cases, but there is no apparent 

 reason why, if it is simply heterozygosity that increases vigor, 

 more of the combinations should not show increased vigor. 



